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presentation format of questions and answers has been used in this Bulletin to foon the factors which are pertinent to a basic understanding and application

overcurrent protective devices. Relevant Sections of the National Electrical Code ®

eferenced and analyzed in detail. Each Section is translated into simple, easily understoanguage, complemented by one-line diagrams giving sound, practical means of applyovercurrent protection, as well as affording compliance with the National Electrical Cod

This Buss Bulletin is helpful to engineers, contractors, electricians, plant maintenan

personnel, and electrical inspectors. It also should prove to be a valuable training aid ormal and informal instruction.

ContentsClick Item Below to View P

90-2 Covers the Scope of the N.E.C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

110-3(b) Covers Requirements for Proper Installation of Listed and Labeled

Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110-9 Covers the Requirements for Proper Interrupting Rating of Overcurrent

Protective Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

110-10 Covers the Proper Protection of System Components

from Short-Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

110-22 Covers the Proper Marking and Identification of

Disconnecting Means . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

210-22(c) Covers Ratings of Overcurrent Devices on Branch Circuits Serving

Continuous and Non-Continuous Loads . . . . . . . . . . . . . . . . . . . . . . . . . .

215-10 Covers Requirements for Ground-Fault Protection of

Equipment on Feeders. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

230-65 Covers the Short-Circuit Rating of Service Entrance Equipment. . . . . . . 1

230-82 Covers Equipment Allowed to be Connected on the Line Side

of the Service Disconnect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

230-95 Covers Ground-Fault Protection for Services. . . . . . . . . . . . . . . . . . . . . . 1

240-1 Covers the Scope of Article 240 on Overcurrent Protection . . . . . . . . . . 1

A



240-3 Covers the Protection of Conductors Other Than Flexible Cords

and Fixture Wires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

240-4 Covers Proper Protection of Fixture Wires and Flexible Cords . . . . . . . . 14

240-6 Covers Standard Ampere Ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

240-8 &

380-7 Covers Protective Devices Used in Parallel . . . . . . . . . . . . . . . . . . . . . . . 14

240-9 Covers Thermal Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

240-10 Covers Requirements for Supplementary Overcurrent Protection. . . . . . 14

240-11 Covers the Definition of Current-Limiting Overcurrent

Protective Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

240-12 Covers System Coordination or Selectivity . . . . . . . . . . . . . . . . . . . . . . . 16

240-13 Covers Ground Fault Protection of Equipment on Remote Structures. . . 17

240-21 Covers Location Requirements for Overcurrent Devices

and Tap Conductors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

240-40 Disconnecting Means for Fuses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

240-50 Covers Plug Fuses, Fuseholders, and Adapters . . . . . . . . . . . . . . . . . . . 19

240-51 Covers Edison-Base Fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

240-53 Covers Type S Fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

240-54 Covers Type S Fuses, Adapters, and Fuseholders . . . . . . . . . . . . . . . . . 20

240-60 Covers Cartridge Fuses and Fuseholders . . . . . . . . . . . . . . . . . . . . . . . . 20

240-61 Covers Classification of Fuses and Fuseholders. . . . . . . . . . . . . . . . . . . 20

240-83(c) Covers Marking–Interrupting Rating of Circuit Breakers

and Series Ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

240-100 Covers Feeder Overcurrent Protection Over 600 Volts, Nominal. . . . . . . 20

250-1 Covers the Requirements for Proper Grounding and

Bonding of Electrical Installations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

250-51 Covers the Requirements for an Effective Grounding Path . . . . . . . . . . . 21

Contents (cont.)

Click Item Below to View Pa



250-70 Covers Bonding Requirements and Short-Circuit Withstand. . . . . . . . . . 21

250-75 Covers Bonding Other Enclosures and Short-Circuit Withstand

Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

250-95 Covers Sizing of Equipment Grounding Conductors . . . . . . . . . . . . . . . . 21

310-10 Covers Temperature Limitation of Conductors. . . . . . . . . . . . . . . . . . . . . 22

364-11 Covers Protection at a Busway Reduction. . . . . . . . . . . . . . . . . . . . . . . . 22

384-16 Covers Panelboard Overcurrent Protection . . . . . . . . . . . . . . . . . . . . . . . 22

430-1 Covers Scope of Motor Article . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

430-6 Covers Ampacity of Conductors for Branch Circuits and Feeders . . . . . 22

430-32 Covers Motor Overload Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

430-36 Covers Fuses Used to Provide Overload and

Single-Phasing Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

430-52 Covers the Sizing of Various Overcurrent Devices for

Motor Branch Circuit Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

430-53 Covers Requirements for Connecting Several Motors

or Loads on One Branch Circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

430-71 Covers an Introduction to Motor Control-Circuit Protection . . . . . . . . . . . 25

430-72(a) Covers Motor Control-Circuit Overcurrent Protection. . . . . . . . . . . . . . . . 25

430-72(b) Covers Motor Control-Circuit Conductor Protection. . . . . . . . . . . . . . . . . 25

430-72(c) Covers Motor Control-Circuit Transformer Protection. . . . . . . . . . . . . . . . 27

430-94 Covers Motor Control Center Protection . . . . . . . . . . . . . . . . . . . . . . . . . 28

440-5 Covers Marking Requirements on HVAC Controllers. . . . . . . . . . . . . . . . 28

440-22 Covers Application and Selection of the Branch Circuit Protection

for HVAC Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

450-3 Covers Protection Requirements for Transformers . . . . . . . . . . . . . . . . . 28

450-3(a) Covers Protection Requirements for Transformers Over 600 Volts . . . . . 29

450-3(b) Covers Protection Requirements for Transformers 600 Volts or Less . . . 30

Contents (cont.)




450-6(a)(3) Covers Tie Circuit Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

455-7 Covers Overcurrent Protection Requirements for Phase Converters. . . . 30

460-8(b) Covers Overcurrent Protection of Capacitors . . . . . . . . . . . . . . . . . . . . . 30

501-6(b) Covers Fuses for Class I, Division 2 Locations . . . . . . . . . . . . . . . . . . . . 31

517-17 Covers Requirements for Ground Fault Protection and

Coordination in Health Care Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

520-53(f) Covers Protection of Portable Switchboards on Stage . . . . . . . . . . . . . . 31

550-6(b) Covers Overcurrent Protection Requirements for

Mobile Homes and Parks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

610-14(c) Covers Conductor Sizes and Protection for Cranes and Hoists . . . . . . . 32

620-62 Covers Selective Coordination of Overcurrent Protective Devices

for Elevators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

670-3 Covers Protection of Industrial Machinery . . . . . . . . . . . . . . . . . . . . . . . . 32

700-5 Covers Emergency Systems – Their Capacity and Rating . . . . . . . . . . . 33

700-16 Covers Emergency Illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

700-25 Covers Emergency System Overcurrent Protection

Requirements (FPN). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

701-6 Covers Legally Required Stand-by Systems –

Their Capacity and Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

702-5 Covers Optional Standby Systems –

Their Capacity and Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

705-16 Covers Interconnecting Electrical Power Production Sources –

Their Interrupting and Withstand Rating . . . . . . . . . . . . . . . . . . . . . . . 34

725-23 Covers Overcurrent Protection for Class 1 Circuits . . . . . . . . . . . . . . . . . 34

760-23 Covers Requirements for Nonpower Limited Fire Protective

Signaling Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Contents (cont.)




Overcurrent Protection And The 1996 National Electrical Code ®

NE96Questions & Answers To Help You Comply

Bussmann

National Electrical Code ® and N.E.C.® are registered trademarks of the National Fire Protection Association (NFPA), Inc., Quincy, 02269. This bulletin does not reflect the official position of the NFPA.

Great care has been taken to assure the recommendations herein are in accordance with the N.E.C® and sound engineering principles. Bussmcannot take responsibility for errors or omissions that may exist. The responsibility for compliance with the regulatory standards lies with the user

Copyright January 1996 by Cooper Industries, Inc., Bussmann Division.

Printed U.S.A.



What is the importance of Section 110-3(b)?

Some equipment is listed, subject to specific conditions ofinstallation or operation. In such instances, the conditions shouldbe adhered to for safe protection.

What is the protection requirement of an air conditioner when its nameplate specifies Maximum Fuse Size Amperes?

Fuse protection in the branch circuit is mandatory to meet therequirements of the U.L. Listings and the National Electrical Code.

Note that the U.L. Orange Book “Electrical Appliance andUtilization Equipment Directory,” April 1995, requires the followingfor central cooling, air conditioners: “Such multimotor and

combination load equipment is to be connected only to a circuitprotected by fuses or a circuit breaker with a rating which does notexceed the value marked on the data plate. This marked protectivedevice rating is the maximum for which the equipment has beeninvestigated and found acceptable. Where the marking specifiedfuses, or “HACR Type” circuit breakers, the circuit is intended tobe protected only by the type of protective device specified.” U.L.Standard 1995 also covers this subject.

What about a motor starter heater table (such as that shown below)which specifies Maximum Fuse?

Heater Full-Load Current Max.

Code of Motor (Amperes) Fuse

Marking (40°C Ambient)

XX03 .25- .27 1

XX04 .28- .31 3

XX05 .32- .34 3XX06 .35- .38 3

XX14 .76- .83 6

XX15 .84- .91 6

XX16 .92-1.00 6

XX17 1.01-1.11 6

XX18 1.12-1.22 6

Above Heaters for use on Size 0

Like an air conditioner, use of fuse protection is mandatory. Also,the fuse must provide branch circuit protection and be no largerthan the specified size [430-53(c)]. The chart shown, for example,is typical for starter manufacturers and may be found on the insideof the door of the starter enclosure. (See starter manufacturer forspecific recommendations.)

4

110-3(b) Covers Requirements for Proper Installation of Listed and Labeled Equipment

90-2 Covers the Scope of the N.E.C.

8RY461M3-A

230

230

37

60

207

-—-—

60

60

140

Typical Nameplate of a Central Air Conditioning Unit.

LISTED SECTION OF CENTRAL COOLING AIR CONDITIONER

ADME

812H

COMPRESSOR

FAN MOTOR

MINIMUM CIRCUIT AMPACITY

MAXIMUM FUSE SIZE AMPS

MINIMUM OPERATING VOLTAGE

FACTORY CHARGED WITH REFRIGERATORSEE CONTROL PANEL COVER FOR AOF SYSTEM REFRIGERANT

*COMPRESSOR RATED IN RLA

ELECTRICAL RATINGSVAC PH CYC LRA

FOR OUTDOOR USE

UL TYPE NO.®

CIRCUITBREAKERBRANCHCIRCUIT NON-FUSED

DISCONNECT

AIR CONDITIONERMARKED WITH"MAX" FUSE

Violates N.E.C. & Listing Requirements


BRANCHCIRCUITFUSEDDISCONNECT

FUSEDFEEDERCIRCUIT

Conforms to N.E.C. & Listing Requirements


NON-FUSEDDISCONNECT

FUSEDBRANCHCIRCUIT


BRANCHCIRCUITFUSEDDISCONNECT

CIRCUITBREAKER



What does this Section mean?90-2(b) covers installations that are not covered by requirementsof the N.E.C. However, the fine print note states that it is the intent

of this section that utility installed utilization equipment located onprivate property is subject to the National Electrical Code.



5

What violation exists when a “series-rated” panelboard with a “42/10”system rating has the potential to see a fault current less than 4 ft. fromthe loadside circuit breaker?

U.L. 489 Series Rating tests allow a maximum of 4 ft. of rated wireto be connected to the branch circuit breaker. Whenever the

potential for a fault exists closer than 4 ft. from the circuit breaker,i.e., where the #12 wire leaves the enclosure, or a maintenanceman is working on the equipment “hot”, a violation of 110-3bexists, as does a potentially hazardous condition. In this situation,the interrupting capacity of the circuit breaker does not equal itsmarked interrupting rating!

110-3(b) Covers Requirements for Proper Installation of Listed and Labeled Equipment

Can a series-rated system recognized by a testing laboratory be used in afacility with motors on the load side of the main circuit breaker?

No, this would be a violation of 110-3b, since the series-rated

system is tested only with a short-circuit source on the line side ofthe main circuit breaker. Should motors be used on the load sideof the main circuit breaker, the main circuit breaker will not “see”this contribution, but the branch circuit breaker will, thus changingthe opening characteristics of the combination, and violating therecognition.

The recommended solution would be to specify a fully-ratedsystem, with all devices meeting the requirements of Section110-9.

#12 CuWIRE

10KA.I.R.20A CB's

200A Panelboard

Branch Circuit

Fault <4' from BranchCircuit Breaker

200A42KAA.I.R.

40,000 Amperes Available

AS TESTED VIOLATION COMPLIANCE

Series RatedRecognizedCB-CB System

Series Rated RecognizedCB-CB with Short-CircuitCurrent from more thanone source

Fully-Rated System

M M

What is the importance of Section 110-9?

Equipment designed to break fault or operating currents musthave a rating sufficient to withstand such currents. This articleemphasizes the difference between clearing fault level currentsand clearing operating currents. Protective devices such as fusesand circuit breakers are designed to clear fault currents and,therefore, must have short-circuit interrupting ratings sufficient forfault levels. Equipment such as contactors and switches haveinterrupting ratings for currents at other than fault levels. Thus, theinterrupting rating of electrical equipment is now divided into twoparts. Current at fault (short-circuit) levels and current at operatinglevels.

Most people are familiar with the normal current carrying ampere rating of a fuse or circuit breaker; however, what is a short-circuit interrupting rating? It is the maximum short-circuit current that an overcurrentprotective device can safely interrupt under specified test

conditions.

What is a circuit breaker’s interrupting capacity? It is the highest short-circuit current at rated voltage that the devicecan safely interrupt.

Note: Several proposals were submitted to add a definition forinterrupting capacity to the 1993 Code. These proposals wererejected. However, the industry is beginning to understand whenthere is a difference between a circuit breaker's interrupting rating and it's interrupting capacity . This difference is due, in large part,to the industry standards that allow added wire impedance duringthe interrupting rating tests.

An AdHoc Committee will be appointed to investigate thisissue.

The following definition of Interrupting Capacity is paraphrased

from the IEEE Standard Dictionary of Electrical and ElectronicTerms:

Interrupting Capacity (CB): Actual test Ip and IRMS the circuitbreaker sees during the U.L. tests for standard circuit breakerapplications. This value should not be exceeded.

What happens if a fault current exceeds the interrupting rating of a fuseor the interrupting capacity of a circuit breaker?It can be damaged or destroyed. Severe equipment damage andpersonnel injury can result.

In this circuit, what interrupting rating must the fuse have?

At least 50,000 amperes. (Class R, J, T, L and CC fuses have anInterrupting Rating of at least 200,000 amperes. The interrupting rating of a fuse and switch combination may also be 200,000amperes. . .well above the available short-circuit current of 50,000amperes. The interrupting rating of Class G fuses is 100,000amperes; K1 and K5 fuses can be 50,000, 100,000, or 200,000amperes.)

In this circuit, what interrupting rating must the circuit breaker have?

Available fault current–50,000 amperes

Available fault current–50,000 amperes

110-9 Covers the Requirements for Proper Interrupting Rating of Overcurrent Protective Devices



110-9 Covers the Requirements for Proper Interrupting Rating of Overcurrent Protective Devices

6

Given the full-load transformer secondary amperage and percentimpedance of a transformer, how can you compute the level of short-circuit amperes that can be delivered at the secondary terminals(Assuming an infinite, unlimited, short-circuit current at the primary)?

ISCA = (F.L.A.) x 100

%Z x .9††

Given: 1.3% impedance from nameplate of 500 KVA transformerwith a 480V secondary601 Full-Load Amperes (from Table below)

ISCA=

601 x 100= 51,368 Amperes

1.3 x .9

What are typical values of transformer short-circuit currents?

Short-Circuit Currents Available from Various Size TransformersVoltage+ KVA Full- % † Short-

and Load Impedance †† Circuit

Phase Amperes (Name plate) Amperes

25 104 1.58 11,574371 / 2 156 1.56 17,351

120/240 50 209 1.54 23,122

1 ph.* 75 313 1.6 32,637

100 417 1.6 42,478

167 695 1.8 60,255

150 416 1.07 43,198

225 625 1.12 62,004

300 833 1.11 83,383

500 1388 1.24 124,373

120/208 750 2082 3.5 66,095

3 ph. 1000 2776 3.5 88,127

1500 4164 3.5 132,190

2000 5552 5.0 123,377

2500 6950 5.0 154,444

1121 / 2 135 1.0 15,000

150 181 1.2 16,759

225 271 1.2 25,082300 361 1.2 33,426

277/480 500 601 1.3 51,368

3 ph. 750 902 3.5 28,410

1000 1203 3.5 38,180

1500 1804 3.5 57,261

2000 2406 5.0 53,461

2500 3007 5.0 66,822

† Three-phase short-circuit currents based on "infinite" primary.* Single-phase values are L-N values at transformer terminals. These figures are based

on change in turns ratio between primary and secondary, 100,000 KVA primary, zerofeet from terminals of transformer, 1.2 (%X) and 1.5 (%R) multipliers for L-N vs. L-Lreactance and resistance values, and transformer X/R ratio = 3.

†† U.L. listed transformers 25KVA or greater have a ±10% impedance tolerance. “Short-Circuit Amperes” reflect a worst case scenario.

+ Fluctuations in system voltage will affect the available short-circuit current. Forexample, a 10% increase in system voltage will result in a 10% increase in theavailable short-circuit currents shown in the table.

Some value greater than or equal to 50,000 amperes. Seediscussion on circuit breaker interrupting rating in Section 110-10for a further evaluation. (Faults within four feet of the breaker couldcause complete destruction of the breaker if it is applied where theavailable fault current approaches the tested interrupting capacityof the breaker.)

There is an addition to 110-9 that requires the overcurrentdevice to have a sufficient interrupting rating for phase voltage andphase-to-ground voltage.

What is the significance of this addition?Certain molded case circuit breakers have lower single-poleinterrupting ratings than their multi-pole A.I.R. For example, acircuit breaker marked 65,000 A.I.R. may have a single-poleinterrupting rating of 8,600 amperes. Engineers must be aware ofthe lower line-ground (L-G) ratings and the available L-G faultcurrent at the point of application.

Does an overcurrent protective device with a high interrupting ratingassure circuit component protection?No. Choosing overcurrent protective devices strictly on the basisof voltage, current, and interrupting rating alone will not assurecomponent protection from short-circuit currents. High interrupting

capacity electro-mechanical overcurrent protective devices,especially those that are not current-limiting, may not be capableof protecting wire, cable, starters, or other components within thehigher short-circuit ranges. See discussion of Sections 110-10 and240-1 for the requirements that overcurrent protective devicesmust meet to protect components such as motor starters,contactors, relays, switches, conductors, and bus structures.

Note: Breaking current at other than fault levels.

The rating of contactors, motor starters, switches, circuit breakersand other devices for closing in and/or disconnecting loads atoperating current levels must be sufficient for the current to beinterrupted, including inrush currents of transformers, tungstenlamps, capacitors, etc. In addition to handling the full-load currentof a motor, a switch and motor starter must also be capable ofhandling its locked rotor current. If the switch or motor starter has a

horsepower rating at least as great as that of the motor, they willadequately disconnect even the locked rotor current of the motor.

It is necessary to calculate available short-circuit currents at variouspoints in a system to determine whether the equipment meets therequirements of Sections 110-9 and 110-10. How does one calculate thevalues of short-circuit currents at various points throughout a distributionsystem?There are any number of methods. Some give approximate values;some require extensive computations and are quite exacting. Asimple, usually adequate method is the Buss Point-To-Point procedure presented in Buss Bulletin SPD, Selecting Protective Devices. The point-to-point method is based on computation of thetwo main circuit impedance parameters: those of transformers andcables. Of these two components, the transformer is generally themajor short-circuit current factor for faults near the serviceentrance. The percent impedance of the transformer can vary

considerably. Thus, the transformer specification should always bechecked. As shown in the illustration of a typical transformernameplate, “%” impedance is specifically designated. Bussmann'sTRON® Software includes BUSSPOWER™, which calculates three-phase short-circuit currents.

H2

H1 H3X1 X3

X2H0X0

0° ANGULAR DISP.

X3X2X1H0X0H1H2H3

%Zor

PercentageImpedance

COOPERPower Systems Division

THREE PHASE

VOLTAGE

RATING

CATNO%IMP

TRANSFORMER 60 HERTZ65°CRISE

BIL-KVFULL-WAVE

WDG.MAT LV HV

GALOIL

CLASSOA

LV ENCLOSURE LBS.

KVA 500

12470GRD. Y/7200480Y/277

PCWN 416124-500-L1

LBS. TOTAL

LBS. OIL

LBS.

LBS.

TANK & FITTINGS

UNTANKING

1.3HV

SER.



What is the importance of Section 110-10?The design of a system must be such that short-circuit currentscannot exceed the withstand ratings of the components selectedas part of the system. Given specific system components and levelof “available” short-circuit currents which could occur, overcurrentprotective devices (mainly fuses and/or circuit breakers) must be

used which will limit the energy let-thru of fault currents to levelswithin the withstand ratings of the system components. (Current-limitation is treated under 240-11 of this Bulletin).

What is component short-circuit withstand rating? It is a current rating given to conductors, switches, circuit breakersand other electrical components, which, if exceeded by faultcurrents, will result in “extensive” damage to the component. Therating is expressed in terms of time intervals and/or current values.Short-circuit damage can be heat generated or the the result ofelectro-mechanical force of high-intensity, magnetic fields. (Forfurther details, see Buss Bulletin EDP-3, Engineering Dependable Protection).

Conductor Protection

How is the component withstand rating of conductors expressed?

As shown in the Table below, component withstand of conductorsis expressed in terms of maximum short-circuit current vs. cycles (or time).

Table 1—Copper, 75° Thermoplastic Insulated Cable Damage Table*(Based on 60 HZ).Copper Maximum Short-Circuit Withstand Current

Wire Size in Amperes

75° For For For For

Thermoplastic 1/2 Cycle** 1 Cycle 2 Cycles 3 Cycles**

#14 2,400 1,700** 1,200** 1,000

#12 3,800 2,700** 1,900** 1,550

#10 6,020 4,300 3,000 2,450

#8 9,600 6,800 4,800 3,900

#6 15,200 10,800 7,600 6,200

#4 24,200 17,100 12,100 9,900

Footnotes—*Reprinted from ICEA. **From ICEA formula (All circuit examples of conductor

protection related to the use of wire are specified in Table 1.)

In this circuit, what is the maximum permissible available short-circuitcurrent?

2700 amperes. Since the protective device is not current-limiting,the short-circuit current must not exceed the one cycle withstandof the #12 conductor, or 2700 amperes.

In this 20 ampere circuit with a non-current-limiting protective device,what would be the smallest size conductor that would have to be used?

No. 4 wire. Since the protective device is not current-limiting, thewire selected must withstand 12,000 amperes for one cycle.

110-10 Covers the Proper Protection of System Components from Short-Circuits

7

In this circuit, what type of protective device must be used?

It must be current-limiting. When the available short-circuit currentexceeds the withstand rating of the wire, a protective device suchas a current-limiting fuse, properly selected, will limit fault current toa level lower than the wire withstand rating (3,800 amperes for 1 / 2cycle). (See Section 240-1.) For instance, a LOW-PEAK YELLOW™

LPN-RK20SP fuse will limit the 12,000 amperes available short-circuit to less than 1000 amperes and clear in less than 1 / 2 cycle.

Protection of Motor Controllers, Contacts and Relays

In this circuit, what kind of fuse must be used to provide adequateprotection of the starter?

A current-limiting fuse, such as the Buss LOW-PEAK YELLOW orFUSETRON® dual-element fuse. Such a fuse must limit faultcurrents to a value below the withstand rating of the starter andclear the fault in less than 1 / 2 cycle.

What is Type 2, motor starter protection?U.L. has developed a short-circuit test procedure designed toverify that motor controllers will not be a safety hazard and will notcause a fire.

Compliance to the standard allows deformation of theenclosure, but the door must not be blown open and it must bepossible to open the door after the test. In the standard short-circuit tests, the contacts must not disintegrate, but welding of thecontacts is considered acceptable. When testing with fuses,

damage to the overload relay is not allowed, and it must perform inaccordance with the calibration requirements. Tests with circuitbreakers allow the overload relay to be damaged with burnout ofthe current element completely acceptable.

There is an IEC (International Electrotechnical Commission)Standard that offers guidance in evaluating the level of damagelikely to occur during a short-circuit with various branch circuitprotective devices. IEC Publication 947, "Low Voltage Switchgearand Control, Part 4-1: Contactors and Motor Starters", addressesthe coordination between the branch circuit protective device andthe motor starter. It also provides a method to measure theperformance of these devices should a short-circuit occur. IECdefines two levels of protection (coordination) for the motor starter:

Type 1. Considerable damage to the contactor and overloadrelay is acceptable. Replacement of components or a completelynew starter may be needed. There must be no discharge of partsbeyond the enclosure.

Type 2. No damage is allowed to either the contactor oroverload relay. Light contact welding is allowed, but must be easilyseparable.

Where Type 2 protection is desired, the controller manufacturermust verify that Type 2 protection can be achieved by using aspecified protective device. U.S. manufacturers have recentlybegun having both their NEMA and IEC motor controllers verified tomeet the Type 2 requirements outlined in IEC 947-4. As of thiswriting only current-limiting fuses have been able to provide thecurrent-limitation necessary to provide verified Type 2 protection.In many cases, Class J, Class RK1, or Class CC fuses arerequired, because Class RK5 fuses and circuit breakers aren't fastenough under short-circuit conditions to provide Type 2 protection.

PROTECTIVE DEVICE(1 cycle opening time;not current-limiting)

#12 Cu(75°C thermoplasticinsulated Cu)Available

Short-CircuitCurrent

Short-Circuit

PROTECTIVEDEVICE(20A, 1 cycle opening time;not current limiting)

?12,000Aavailablefault current

Short-Circuit

PROTECTIVEDEVICE

#12 Cu12,000Aavailablefault current

Short-Circuit

25,000Aavailablefault current

Size 1 Starter(Tested by UL with 5000A available)

Short-Circuit



8

Protection of Transfer Switches†

In this circuit, what protection must the fuse give the 100 ampere transferswitch and what kind of fuse must be used? (Test standards require 100ampere transfer switches to be tested at a minimum of 5000 amperes).

The fuse must limit fault currents to less than 5000 amperes. Itmust be a current-limiting fuse such as Buss LIMITRON KTS-R100or LOW-PEAK YELLOW LPS-RK100SP or LPJ100SP.Transfer switch withstand is also addressed in NFPA Publication 110 "Emergency andStandby Power Systems". Section 4-2.2 requires that the transfer switch be capable ofwithstanding the effects of available fault circuits.

Protection of Circuit Breakers

There are several key concepts about the protection of circuitbreakers that need to be understood.

1. The user should be aware of the potential problemsassociated with series-rated circuit breakers. The engineer

can not always "engineer" the installation as beforebecause,

2. A molded case circuit breaker's interrupting capacity maybe substantially less than its interrupting rating, and

3. Some molded case circuit breakers exhibit "dynamic"operation that begins in less than 1 / 2 cycle. This makesthem more difficult to protect than other static electricalcircuit components.

The most practical and reliable solution is to specify a fully-rated fusible system.

Molded Case Circuit Breakers—U.L. 489 and CSA5 Test ProceduresU.L. 489 requires a unique test set-up for testing circuit breakerinterrupting ratings. Figure F illustrates a typical calibrated testcircuit waveform for a 20 ampere, 240 volt, 2-pole molded casecircuit breaker, with a marked interrupting rating of 22,000amperes, RMS symmetrical.

Figure F

Figure G illustrates the test circuit as allowed by U.L. 489.

Figure G

110-10 Covers the Proper Protection of System Components from Short-Circuits

Standard interrupting rating tests will allow for a maximum 4 ft.rated wire on the line side, and 10 in. rated wire on the load side ofthe circuit breaker. Performing a short-circuit analysis of this testcircuit results in the following short-circuit parameters, as seen bythe circuit breaker.

• Actual short-circuit RMS current = 9900 amperes

RMS symmetrical• Actual short-circuit power factor = 88%• Actual short-circuit peak current = 14,001 amperes

Following is an example of a partial table showing the actual IP andIRMS values to which the circuit breaker is tested.

240V–2-Pole MCCB INTERRUPTING CAPACITIES (KA)

CB 10KA 14KA 18KA 22KA

RATING Ip Irms Ip Irms Ip Irms Ip Irms

15A 7.2 5.1 8.7 6.1 9.3 6.6 9.9 7.0

20A 8.9 6.3 11.4 8.1 12.6 8.9 14.0 9.9

25A 10.7 7.5 14.2 10.1 16.5 11.7 19.9 13.5

30A 10.7 7.5 14.2 10.1 16.5 11.7 19.9 13.5

40A 11.7 8.3 16.0 11.3 19.2 13.6 22.7 16.1

50A 11.7 8.3 16.0 11.3 19.2 13.6 22.7 16.1

60A 12.5 8.8 17.3 12.2 21.3 15.1 25.6 18.1

70A 13.0 9.2 18.1 12.8 22.6 16.0 27.4 19.480A 13.0 9.2 18.1 12.8 22.6 16.0 27.4 19.4

90A 13.2 9.3 18.3 12.9 23.0 16.3 27.9 19.7

100A 13.2 9.3 18.3 12.9 23.0 16.3 27.9 19.7

These values are known as the circuit breaker’s interruptingcapacities.

Protection of Bus Structures

In the circuit below, what must be the busway short-circuit bracing?

100,000 amperes, because the overcurrent device is not current-limiting.

In this circuit, what would the busway short-circuit bracing have to be?

36,000 amperes (as shown in the Minimum Bracing Table). With anavailable short-circuit current of 100,000 amperes, the LOW-PEAKYELLOW™ KRP-C1600SP fuse will only let-thru an equivalent of36,000 amperes, RMS symmetrical.

P.F. = 20%IRMS = 22,000 Amps

IRMS = 22,000A

Ip = 48,026A

Time

A m p s

10,000Aavailablefault currentat 480V

100A TRANSFER SW.

LPS-RK100SP

S.C. P.F. = 20%S.C. Avail. = 22,000A

RLINE XLINERCB XCB

20A

RLOAD XLOAD

RS

XS

SOURCE: 4' Rated Wire (#12 Cu)

Note: For calculations, RCB and XCB are assumed negligible.

10" Rated Wire (#12 Cu)

NON-CURRENT-LIMITING DEVICE

1600A BUSWAY100,000Aavailablefault current

KRP-C1600SP FUSE(Current-limiting)

1600A BUSWAY100,000Aavailablefault current

†



9

110-10 Covers the Proper Protection ofSystem Components from Short-Circuits

Minimum Bracing Required for Bus Structures at 480V.(Amperes RMS Symmetrical)Rating*

Busway Fuse Available Short-Circuit Amperes RMS Sym.

25,000 50,000 75,000 100,000 200,000

100 100 3,400 4,200 4,800 5,200 6,500225 225 6,000 7,000 8,000 9,000 12,000

400 400 9,200 11,00 13,000 14,000 17,000

600 600 12,000 15,000 17,000 19,000 24,000

601 601 11,000 14,500 17,000 18,000 24,000

800 800 14,200 17,500 20,000 23,000 29,000

1200 1200 16,000 22,500 26,000 28,000 39,000

1600 1600 22,500 28,500 33,000 36,000 46,000

2000 2000 25,000 32,000 37,000 40,000 52,000

3000 3000 25,000 43,000 50,000 58,000 73,000

4000 4000 25,000 48,000 58,000 68,000 94,000

*Fuses are: 100-600 Ampere—LOW-PEAK YELLOW Dual-Element Fuses—LPS-RK_SP(Class RK1) or LPJ_SP (Class J); 800-4000 Ampere—LOW-PEAK YELLOWTime-Delay Fuses—KRP-C_SP (Class L). (LOW-PEAK YELLOW fuses arecurrent-limiting fuses.)

110-22 Covers the Proper Marking andIdentification of Disconnecting Means

What does this new Section require?

Labeling ConsiderationsN.E.C. Sections 110-22 and 240-83 require special marking for atesting agency listed series-rated systems.

On listed series-rated systems, the downstream equipment willbe marked by the manufacturer per applicable standards [240-83(c)]. The N.E.C. requires that the main or upstream protectivedevice be marked with a field installed label per N.E.C. Section110-22. This is the responsibility of the electrical contractor.

Short-circuit calculations must be performed at panellocations where series-rated systems are specified.

210-22(c) Covers Ratings of OvercurrentDevices on Branch Circuits ServingContinuous and Non-Continuous Loads

What is the importance of this Section?The overcurrent protective device provided for branch circuits,such as store lighting and restaurants, must not be less than thetotal non-continuous load, plus 125% of the continuous load

(defined as a load that continues for 3 hours or more).

Rating not less than = [(10A) x 1.0] + [(8A) x 1.25]= 20A

EXAMPLE

The branch circuit rating shall not be less than 20 amperes.

215-10 Covers Requirements for Ground-FaultProtection of Equipment on Feeders

What is the importance of this Section?

Equipment classified as a feeder disconnect, as shown in theseexamples, must have ground fault protection as specified inSection 230-95.

G.F.P. is not required on feeder equipment when it is provided onthe supply side of the feeder (except for certain Health CareFacilities requirements, Article 517).

See Section 230-95 for an in-depth discussion of Ground FaultProtection.

Ground fault protection without current-limitation may not protect systemcomponents. See Section 110-10.

20A Rating

Non-Continuous10A

Continuous Load8A

VIOLATION

High VoltageService 4160V

480Y/277V

Feeder W/OG.F.P.

1000Aor Greater

COMPLIANCE

High VoltageService 4160V

480Y/277V

FeederProvidedw/G.F.P.

1000Aor Greater

COMPLIANCE Feeder of any rating

no G.F.P. Required(Except Per Article 517)

480Y/277V

G.F.P.

1000AorGreater



10

230-65 Covers the Short-Circuit Rating of Service Entrance Equipment

What does the Section mean?Service equipment must be able to withstand available short-circuit currents. More specifically, the service switchboard,panelboard, etc., and the protective devices which theyincorporate must have a short-circuit rating equal to or greaterthan the short-circuit current available at the line side of the

equipment.

In this circuit, what must be the short-circuit rating of the switchboard?

At least 100,000 amperes.

What must be the interrupting rating of the fuses?100,000 amperes or greater. (Most current-limiting fuses have aninterrupting rating of 200,000 or 300,000 amperes.)

In this circuit, what must be the interrupting capacity of the main circuitbreaker, and the short-circuit rating of the switchboard?

At least 100,000 amperes.

As shown in the circuit, can fuses be used to protect circuit breakers witha low interrupting rating.

Yes. Properly selected fuses can protect circuit breakers as wellas branch circuit conductors by limiting short-circuit currents to alow level even though available short-circuit current is as high as100,000 amperes. (Buss LOW-PEAK YELLOW™ or T-TRON fusesgive optimum protection.)

Can cable limiters protect service entrance equipment from short-circuitcurrents?

Current-limiting cable limiters not only can be used to isolate a

“faulted” service cable, but also can help to protect utility meterswith low withstand ratings against high short-circuit currents. (SeeSection 230-82).

Application Note:Residential —100 ampere and 200 ampere fused main-branchcircuit breaker panels are commercially available. These loadcenters incorporate the small-sized T-TRON JJN fuses which makeit possible to obtain a 100,000 amperes short-circuit current rating.Mobile home meter pedestals are also available incorporating theT-TRON JJN fuses in a Fuse Pullout Unit.

Apartment Complexes —Have high densities of current and,therefore, high short-circuit currents for the typical meters.

Grouped meter stacks are commercially available using the T-TRON JJN fuses (up to 1200 amperes) to give the proper short-circuit protection. Meter stacks are also available with Class T fuse

pullouts on the load side of each meter.


Fuses must have100,000 amperesinterrupting ratingor greater

100,000Aavailablefault current MAIN

BREAKER

SWITCHBOARD

FEEDER CIRCUIT BREAKERS


200 ampere service entrance panelmust have a short circuit ratingequal to or greater than 100,000amperes

10,000A.I.C.breakers

CABLE LIMITER

UNDERGROUND CABLE

(Residential and lightcommercial buildings)

METER

METERS

METERS

JJN FUSE(up to 1200A)

JJN FUSE(up to 1200A)

CLASS T FUSES



11

RESIDENTIAL SERVICE ENTRANCE

(Single cable per phase)

What do exceptions 7 and 8 mean?The control circuit for power operable service disconnectingmeans and ground fault protection must have a means fordisconnection and adequate overcurrent protection–interruptingcapacity and component protection.

230-82 Covers Equipment Allowed to be Connected on the Line Side of theService Disconnect

What are the advantages of using cable limiters on the supply side of theservice disconnect.Typical cable installations are shown in the illustration below. Thebenefits of cable limiters are several:

1. The isolat ion of a faulted cable permits the convenientscheduling of repair service.2. Continuity of service is sustained even though one of morecables are faulted.3. The possibility of severe equipment damage or burn down as aresult of a fault is greatly reduced. (Typically, without cable limitersthe circuit from the transformer to the service equipment isafforded little or no protection.).4. Their current-limiting feature can be used to provide protectionagainst high short-circuit currents for utility meters and providecompliance with Section 110-10.

COMMERCIAL/INDUSTRIAL SERVICE ENTRANCE

(Multiple cables per phase)

ServiceDisconnect

(Open) (Open)

Faulted cable isolated; only the cablelimiters in faulted cable open; othersremain in operation

OpenFaulted cable isolated; the otherservices continue in operationwithout being disturbed

RESIDENCES

#4

#3

#2

#1

What is the importance of this section?This section means that 480Y/277 volt, solidly grounded “wye” only

connected service disconnects, 1000 amperes and larger, musthave ground fault protection in addition to conventional over-current protection. Ground fault protection, however, is notrequired on a fire pump or a service disconnect for a continuousprocess where its opening will increase hazards. All deltaconnected services are not required to have ground faultprotection. The maximum setting for the ground fault relay (orsensor) must be set to pick up ground-faults which are 1200amperes or more and actuate the main switch or circuit breaker todisconnect all phase conductors. A ground fault relay with adeliberate time delay characteristic of up to 3000 amperes for 1second can be used. (The use of such a relay greatly enhancessystem coordination and minimizes power outages).

Under short-circuit conditions, unlike current-limiting fuses,ground fault protection in itself will not limit the line-to-ground orphase-to-phase short-circuit current. When mechanical protectivedevices such as conventional circuit breakers are used with

G.F.P., all of the available short-circuit current will flow to the pointof fault limited only by circuit impedance. Therefore, it isrecommended that current-limiting overcurrent protective devicesbe used in conjunction with G.F.P. relays.

In this circuit, what protection does the fuse provide in addition to thatprovided by the ground fault equipment?

230-95 Covers Ground Fault Protection for Services

Current limitation under short-circuit conditions and high-levelground-faults.

In this circuit, is protection provided against high magnitude ground-faults as well as low level faults?

No, it is not. There is no current-limitation.

Is G.F.P. required on all services?No. The following do not require G.F.P.:

1. Continuous industrial process where non-orderly shutdownwould increase hazard.2. All services where disconnect is less than 1000 amperes.3. All 120/208 volts, 3Ø, 4W (wye) services.4. All single-phase services including 120/240 volt, 1Ø, 3W.5. High or medium voltage services. (See N.E.C. Sections 240-13and 215-10 for equipment and feeder requirements.)6. All services on delta systems (grounded or ungrounded) suchas: 240 volt, 3Ø, 3W Delta, 480 volt, 3Ø, 3W Delta, or 240 volt, 3Ø,4W Delta with midpoint tap.7. Service with 6 disconnects or less (Section 230-71) where eachdisconnect is less than 1000 amperes. A 4000 ampere servicecould be split into five 800 ampere switches.8. Resistance or impedance grounded systems.

1000 ampereswitch & fuseor larger

480Y/2773Ø, 4WService

Ground faultprotectionrequired

SWBD

1000 amperecircuit breakeror larger

480Y/2773Ø, 4WService

Ground faultprotectionrequired

SWBD



12

What is the importance of this Section?The basic purpose of overcurrent protection is to open a circuitbefore conductors or conductor insulation are damaged when anovercurrent condition exists. An overcurrent condition can be the

result of an overload or a short-circuit. It must be removed beforethe damage point of conductor insulation is reached. Conductorinsulation damage points can be established from availableengineering information, i.e., Publication P-32-382, “Short-CircuitCharacteristics of Cable”, ICEA, (Insulated Cable EngineersAssociation, Inc.)

When selecting an overcurrent protective device to protect a conductor,is it adequate to simply match the ampere rating of the device to theampacity of the conductor?No. Although conductors do have maximum allowable ampacityratings, they also have maximum allowable short-circuit currentwithstand rating. Damage ranging from slight degradation ofinsulation to violent vaporization of the conductor metal can resultif the short-circuit withstand is exceeded. (See Section 110-10.)

Why, in the circuit below, is the #10 wire protected even though theavailable short-circuit current exceeds the wire withstand? The #10conductor can withstand 4300 amperes for one cycle and 6020 amperesfor one-half cycle.**

**Footnote—From ICEA tables and formula.

Under short-circuits, the LOW-PEAK YELLOW Dual-Element fuse(30 ampere) is fast acting. It will clear and limit (cut off) short-circuitcurrent before it can build up to a level higher than the wirewithstand. The opening time of the fuse is less than one-half cycle(less than 0.008 seconds). In this particular example, theprospective current let-thru by the fuse is less than 1850 amperes.Thus, opening time and current let-thru of the fuse is far lower than

fuses in order to avoid outages. (Section 230-95 permits an inversetime-delay relay with a delay of up to 1 second at 3000 amperes.)

Conventional mechanical tripping overcurrent protectivedevices often do not permit a selectively coordinated system* andBLACKOUTS can occur. For ground faults (and short-circuitcurrent as well) of current magnitude above the instantaneous trip

setting on the main circuit breaker’s overcurrent element, the mainwill nuisance trip (open) causing a blackout even though the faultis on a feeder or branch circuit. Appropriate selection of current-limiting fuses with proper G.F.P. settings can provide the highestdegree of coordination and prevent blackouts.

A system wherein only the protective device nearest the fault operates andnone of the other protective devices in the system are disturbed.

What are some of the problems associated with G.F.P.?Incorrect settings, false tripping and, eventually, disconnection.(The knocking-out of the total building service or large feeders as aresult of minor faults or nuisance tripping cannot be tolerated inmany facilities). Unnecessary plant down time is often morecritical, or even more dangerous, than a minor ground fault.

Note: G.F.P. without current limitation may not protect systemcomponents. See Section 110-10 and 250-1 FPN.

How can ground faults be minimized?1. To prevent blackouts, make sure that all overcurrent protectivedevices throughout the overall system are selectively coordinated.When maximum continuity of electrical service is necessary,ground fault protective equipment should be incorporated infeeders and branch circuits. [Per Section 230-95 (FPN No. 2).]2. Insulating bus structures can greatly minimize the possibility offaults. The hazard of personnel exposure to energized electricalequipment is also reduced with insulated bus structures.3. Specify switchboards and other equipment with adequateclearance between phase conductors and ground. Ground faultsare rare on 120/208 volt systems because equipment manufactur-ers provide ample spacing for this voltage. Insist on greater

spacing for 277/480 volt equipment and the likelihood of groundfaults will be greatly reduced.4. Avoid unusually large services; split the service wheneverpossible.5. Adequately bond all metallic parts of the system to enhanceground fault current flow. Then, if a ground fault does occur, it ismore likely to be sensed by fuses or circuit breakers.

To respond properly to a line-to-ground type fault, what should be thesetting of a ground fault relay located on the main disconnect?The setting should allow the feeder circuit (or preferably thebranch) overcurrent protective devices to function withoutdisturbing the G.F.P. relay.

How is a G.F.P. setting determined?By making a coordination study. Such a study requires the plottingof the time-current curves of the protective devices.

A simple solution to the problem of coordinating ground faultrelays with overcurrent protective devices is shown in the systemrepresented in the graph at right. The G.F.P. relay coordinates withthe feeder fuses KTS-R 250. The G.F.P. relay with a degree ofinverse time characteristics provides coordination with feeder

230-95 Covers Ground Fault Protection for Services

6

43

2

1.8

.6

.4

.3

.2

.1.08.06

.04

.03

.02

.01

.05

.5

5

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

8 0 0

1 , 0

0 0

2 , 0

0 0

3 , 0

0 0

4 , 0

0 0

5 , 0

0 0

6 , 0

0 0

8 , 0

0 0

1 0

, 0 0 0

2 0

, 0 0 0

3 0

, 0 0 0

108

20

3040506080

100

200

300

KRP-C1600SP

KTS-R250

KTS-R125

KTS-R250

GFPset

at1200 AMPSPICK UP &0.5 SEC.

KRP-C

1600SP

KTS-R

125

CURRENT IN AMPERES

T I M E I N S E C O N D S

240-1 Covers the Scope of Article 240 on Overcurrent Protection

30ALow-Peak YellowClass RK1 Dual-ElementFuse

#10 THW COPPER WIRE40,000Aavailable

Short-Circuit

*



Copper—Thermoplastic Aluminum—ThermoplasticConductor Insulation Conductor Insulation

Where:I = Short-Circuit Current—AmperesA = Conductor Area—Circular Milst = Time of Short-Circuit—SecondsT1 = Maximum Operating Temperature—75°CT2 = Maximum Short-Circuit Temperature—150°C

Note: ICEA (Insulated Cable Engineers Association) is the most widely accepted authorityon conductor short-circuit withstand ratings. Their publication, P-32-382, is referenced inIEEE Buff, Red, Gray, and White Books and also by the Canadian Electrical Code.

13

the wire withstand. (Conductor protection is not a problem whenthe conductor is protected by current-limiting fuses which have anampere rating that is the same as the conductor. In the case ofshort-circuit protection only, fuses can often be sized many timeshigher than the wire current rating, depending upon the current-limiting characteristics of the fuse.)

Does the circuit below represent a misapplication? (#10 THW insulatedcopper wire can withstand 4300 amperes for one cycle and 6020amperes for one-half cycle).

Yes. The 40,000 ampere short-circuit current far exceeds the with-stand of the #10 THW wire. Note the table and chart which follow.

What can be done to correct the above misapplication?There are two possible solutions:1. Use a larger size conductor (i.e., 1/0), one with a withstandgreater than the short-circuit for one cycle (see chart below).2. Use an overcurrent protective device which is current-limitingsuch as that shown in the previous question.

The following table is based on Insulated Cable EngineersAssociation, Inc. (ICEA) insulated cable damage charts inPublication 32-382. This table assumes that the conductor ispreloaded to its ampacity before a short-circuit is incurred. Theformulas that are used to develop the ICEA Damage Charts aregiven following the table. These formulas can be used toextrapolate withstand data for wire sizes or time durations notfurnished in the ICEA Publication 32-382 charts. A sample chart isshown at right.

The mechanical overcurrent protective device opening time

and any impedance (choking) effect should be known along withthe available short-circuit current and cable withstand data todetermine the proper conductor that must be used.

Insulated Cable Damage Table (60Hz)†Wire Size Maximum Short-Circuit Withstand Current Amperes)

(THW Cu) at Various Withstand Times

1 Cycle 1/2 Cycle 1/4 Cycle 1/8 Cycle

#14 1,700* 2,400* 3,400* 4,800*

#12 2,700* 3,800* 5,400* 7,600*

#10 4,300 6,020* 8,500* 12,000*

#8 6,800 9,600* 13,500* 19,200*

#6 10,800 15,200* 21,500* 30,400*

#4 17,100 24,200* 34,200* 48,400*

† See Insulated Cable Engineers Association, Inc., “Short-Circuit Characteristics ofCable”, Pub. P-32-382, and circuit breaker manufactures’ published opening times forvarious types of circuit breakers.

240-1 Covers the Scope of Article 240 on Overcurrent Protection

1 C Y C

L E - 0 . 0 1 6 7

S E C

O N D

40,000 Amps - 1 Cycle

4,300Amps - 1Cycle

Conductor-Copper

Insulation-ThermoplasticCurves Based on Formula

I

A

2

t = .0297 logT2 + 234

T1 + 234

IAt

T1

T2

= Short-Circuit Current - Amperes= Conductor Area - Circular Mils= Time of Short-Circuit - Seconds= Maximum Operating Temperature -

75°C= Maximum Short-Circuit Temperature -

150°C

Where

S H O R T C I R C U I T C U R R E N T – T H O U S A N D S O F A M P E R E S

100

80

60

50

40

30

20

10

8

6

5

4

3

2

1

.8

.6

.5

.4

.3

.2

.1 1 0 8 6 4 2 1

1 / 0

2 / 0

3 / 0

4 / 0

A W G

2

5 0 M C M

5 0 0

1 0 0 0

CONDUCTOR SIZE

2 C Y C

L E - 0 . 0 3 3 3

S E C

O N D

4 C Y C

L E - 0 . 0 6 6 7

S E C

O N D

8 C Y C

L E - 0 . 1 3 3 3

S E C

O N D

1 6 C Y C

L E - 0 . 2 6 6 7

S E C

O N D

3 0 C Y C

L E - 0 . 5 0

0 0 S E C

O N D

6 0 C Y C

L E - 1 . 0 0 0 0

S E C

O N D

1 0 0 C Y C

L E - 1 . 6 6 6 7

S E C

O N D

Conductor mustbe protected forits entire length

30A MECHANICAL OVERCURRENTPROTECTIVE DEVICE(Clearing time 1 cycle;not current-limiting)

40,000Aavailable

Short-Circuit

#10 COPPER WIRE(THW insulated)

I 2

t = 0.0297 logT2 + 234

A T1 + 234 I 2

t = 0.0125 logT2 + 228

A T1 + 228

240-3 Covers the Protection of Conductors Other Than Flexible Cords andFixture Wires

What is the meaning of 240-3(b) and 240-3(c)?Where the ampacity of a conductor does not correspond with astandard rating (240-6) of a fuse, the next standard rating may beused as long as the fuse is not above 800 amps and theconductors are not part of a multi-outlet branch circuit supplyingreceptacles for cord and plug-connected portable loads.

What does 240-3(i) mean?

Conductors fed from single-phase, 2-wire secondary transformers

and three phase, delta-delta connected transformers with three-wire (single-voltage) secondaries can be considered protected bythe primary side fuses if the transformer is properly protected inaccordance with Section 450-3. The primary fuse must be lessthan or equal to the secondary conductor ampacity times thesecondary-to-primary transformer voltage ratio.

Also, Section 240-3 was completely rewritten in positivelanguage to improve comprehension and readability.



14

240-4 Covers Proper Protection ofFixture Wires and Flexible Cords

240-6 Covers Standard Ampere Rating

adjustable trip from 225 through 400 amperes, the rating of thebreaker would be 400 amperes, and 500 kcmil cable wouldtherefore be required, increasing costs significantly. However, ifthis adjusting means is not readily accessible, such as behind abolted door and reached only by a qualified person, then the ratingcan be considered to be equal to the adjusted setting.

Note: Standard ampere ratings for fuses and inverse time circuitbreakers are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110,125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700,800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000 and 6000amperes. In addition, standard fuse ratings are 1, 3, 6, 10 and 601.

240-8 & 380-7 Covers ProtectiveDevices Used in Parallel

What do these Sections mean?There are cases in which an original equipment manufacturer, forvarious reasons, must parallel fuses and receive an appropriateequipment listing. For example, this would be the case of some

solid-state power conversion equipment. However, for the standardsafety switch, conventional branch circuit applications, switch-boards, and panelboards, the use of parallel fuses is not allowed.Paralleling of circuit breakers is not allowed under any conditions.

240-9 Covers Thermal Devices

What does this Section mean?Thermal overload devices generally can neither withstand openinga circuit under short-circuit conditions nor even carry short-circuitcurrents of higher magnitudes. When using thermal overloadprotective devices, the use of a complementary current-limitingfuse will not only protect the circuit against short-circuit current, butalso the thermal overload device.

Note: Agency short-circuit tests on thermal devices, such asthermal cutouts, are run with a specified size and type of fuse.

240-10 Covers Requirements forSupplementary Overcurrent Protection

What is the importance of this Section?Supplementary fuses, often used to provide protection for lightingfixtures, cannot be used where branch circuit protection isrequired.

What are the advantages of supplementary protection?The use of supplementary protection for many types of appliances,fixtures, cords, decorator lighting (Christmas tree lights. . .)*, etc.,

is often well advised. There are several advantages:1. Provides superior protection of the individual equipment bypermitting close fuse sizing.2. With an occurrence of an overcurrent, the equipment protectedby the supplementary protected device is isolated; the branchcircuit overcurrent device is not disturbed. For instance, the in-line-fuse and holder combination, such as the Type HLR fuseholderwith Type GLR or GMF fuses, protects and isolates fluorescentlighting fixtures in the event of an overcurrent.3. It is easier to locate equipment in which a malfunction hasoccurred. Also, direct access to the fuse of the equipment ispossible.

Footnote–Supplementary protection for series connected decorator lighting sets andparallel sets (Christmas tree string lights) was required in 1982. Manufacturers haveimplemented this requirement.

What is the importance of this Section?Flexible cords and extension cords shall have overcurrent protec-tion rated at their ampacities. Supplementary fuse protection is anacceptable method of protection. For #18 fixture wire 50 feet or

over, a 6 ampere fuse would provide necessary protection, and for#16 100 feet or over, an 8 ampere fuse would provide thenecessary protection. #18 extension cords must be protected by a7 ampere fuse.

Also, Section 760-12, covering special non-power-limited fireprotective signaling circuits, requires 7 ampere protection for #18conductors and 10 ampere protection for #16 conductors.

240-6 Covers Standard Ampere Ratings

What is the importance of this Section?In addition to the standard ratings of fuses and circuit breakers,this section states that the rating of an adjustable trip circuitbreaker is considered to be the highest possible setting. Thisbecomes important when protecting conductors or motor circuits.For example, if a copper 75°C conductor is required to carry 200amperes continuously, a 250 kcmil cable might be chosen. If acircuit breaker were chosen to protect this cable with an external

Violation(EXTENSION CORD)

Receptacle

20A

Branch Circuits

#18 Extension Cord

Receptacle

20ABranch Circuits

Compliance(EXTENSION CORD)

#18 Extension Cord7 AmpFuse

#16 Fixture Wire100 ft. or over

Violation(FIXTURE WIRE)

20A FuseTo load

BRANCHCIRCUIT


Compliance(FIXTURE WIRE)

8A FuseTo load

BRANCHCIRCUIT


Violation(FIXTURE WIRE)

20A Fuse

To load

BRANCHCIRCUIT


Compliance(FIXTURE WIRE)

6A Fuse

To load

BRANCHCIRCUIT

*



15

What is the importance of this Section?

ACTION OF NON-CURRENT-LIMITING CIRCUIT BREAKER

ACTION OF CURRENT-LIMITING FUSE.

Simply stated, a current-limiting protective device is one whichcuts off a fault current in less than one-half cycle†. It thus preventsshort-circuit currents from building up to their full available values.

The greatest damage done to components by a fault currentoccurs in the first half-cycle (or more precisely, “the first majorloop” of the sinewave). Heating of components to very hightemperatures can cause deterioration of insulation, or evenexplosion. Tremendous magnetic forces between conductors cancrack insulators and loosen or rupture bracing structures.

The levels of both thermal energy and magnetic forces areproportionate to the square of current. Thermal energy isproportionate to the square of “RMS” current; maximum magneticfields to the square of “peak” current. If a fault current is 100 timeshigher than normal current, its increased heating effects equals(100)2 or 10,000 times higher than that of the normal current. Thus,to prevent circuit component damage, the use of current-limitingprotective devices is extremely important, particularly sincepresent-day distribution systems are capable of delivering highlevel fault currents.

Footnote: The more technical definition of a current-limiting protective device is expressedby 240-11.

240-11 Covers the Definition of Current-Limiting Overcurrent Protective Devices

To further appreciate current-limitation, assume for example,that the available prospective short-circuit current in a circuit is50,000 amperes. If a 200 ampere LOW-PEAK YELLOW fuse isused to protect the circuit, the current let-thru by the fuse will beonly 6500 amperes instead of 50,000 amperes. Peak current will beonly 15,000 amperes instead of a possible 115,000 amperes. Thus,

in this particular example, currents are limited to only 13% of theavailable short-circuit values.

As is true of fuse application in general, the application ofcurrent- l imit ing fuses in respect to current- l imitat ion andcomponent protection (110-10) is quite simple. Graphs or tablessuch as the one shown below permit easy determination of the “let-thru” currents that a fuse will pass for various levels of prospectiveshort-circuit currents. For example, the table below shows that the200 ampere LOW-PEAK YELLOW fuse will let-thru 6500 ampereswhen prospective short-circuit current is 50,000 amperes.

For the above circuit, the Size 1 Starter has a short-circuitwithstand rating of 5000 amperes.* The question is, with the 25,000ampere available short-circuit current, will a LOW-PEAK YELLOWfuse provide adequate protection of the starter? By referring to thetable below, it can easily be seen that for a prospective short-circuit current of 25,000 amperes, fuses with ratings less than 100amperes will limit fault currents to below the 5000 amperewithstand of the starter and, thus, provide adequate protection.

Current-Limiting Effects of RK1 LOW-PEAK YELLOW Fuses.Prospective Let-Thru Current (Apparent RMS Symmetrical)

Short-Circuit LPS-RK_SP (600V) Fuse Ratings

Current 30A 60A 100A 200A 400A 600A

5,000 980 1,600 2,100 3,200 5,000 5,000

10,000 1,200 2,000 2,550 4,000 6,750 9,150

15,000 1,400 2,300 2,900 4,800 7,850 10,200

20,000 1,500 2,500 3,150 5,200 8,250 11,300

25,000 1,600 2,650 3,400 5,450 9,150 12,20030,000 1,650 2,850 3,550 5,650 9,550 12,800

35,000 1,750 2,950 3,750 5,850 10,000 13,500

40,000 1,850 3,100 3,900 6,100 10,450 13,900

50,000 1,950 3,300 4,150 6,500 11,300 15,000

60,000 2,050 3,500 4,350 6,950 11,950 16,100

80,000 2,250 3,850 4,800 7,850 13,000 17,400

100,000 2,450 4,050 5,200 8,250 13,900 18,700

150,000 2,750 4,800 6,100 9,550 15,900 21,300

200,000 3,000 5,200 6,500 10,000 17,400 23,500

RMS Symmetrical Amperes

*Footnote: See discussion on Section 110-10 in this Bulletin.

The reader should note that much of the current-limitationclaimed by small ampere circuits breakers is actually the result ofthe significant impedance added to the circuit breaker test circuit

after the circuit has been calibrated. Refer to the circuit breakerprotection portion of Section 110-10 for further information oncircuit breaker test circuits.

Circuit breaker tripsand opens short-circuitin about 11 / 2 cyclesInitiation of

short-circuit current

Normalload current

Areas within waveformloops represent destructiveenergy impressed uponcircuit components

Fuse opens and clearsshort-circuit in lessthan 1 / 2 cycle


Size 1 Starter(Tested with 5000A available)

Short-Circuit

†



16

1. The 90 ampere breaker will unlatch (Point A) and free thebreaker mechanism to start the actual opening process.2. The 400 ampere breaker will unlatch (Point B) and it, too, wouldbegin the opening process. Once a breaker unlatches, it will open.The process at the unlatching point is irreversible.3. At Point C, the contacts of the 90 ampere breaker finally open

and interrupt the fault current.4. At Point D, the contacts of the 400 ampere breaker open. . .theentire feeder is “blacked out”!

Example of Non-Selective System.

Now, let’s take the case of fuse coordination. When selectivecoordination of current-limiting fuses is desired, the Selectivity Ratio Guide (next page) provides the sizing information necessary.In other words, it is not necessary to draw and compare curves.

Current- l imit ing fuses can be selectively coordinated bymaintaining at least a minimum ampere rating ratio between themain fuse and feeder fuses and between the feeder fuse andbranch circuit fuses.

These ratios are based on the fact that the smaller downstreamfuses will clear the overcurrent before the larger upstream fusesmelt. An example of ratios of fuse ampere ratings which provideselective coordination is shown in the one-line circuit diagram.

What is the importance of this Section?Whenever a partial or total building blackout could causehazard(s) to personnel or equipment, the fuses and/or circuitbreakers must be coordinated in the short-circuit range. It isacceptable for a monitoring system to be used to indicate anoverload condition, if the overcurrent protective devices cannot be

coordinated in the overload region. However, in the vast majority ofcases, both circuit breakers and fuses will be able to becoordinated in the overload range, so the monitoring systems willseldom be required. Typical installations where selectivecoordination would be required include hospitals, industrial plants,off ice buildings, schools, government buildings, mil i taryinstallations, high-rise buildings, or any installation where continuityof service is essential.

VIOLATION

Fault exceeding the instantaneous trip setting of all 3 circuit breakers in series willopen all 3. This will blackout the entire system.

COMPLIANCE

Fault opens the nearest upstream fuse, localizing the fault to the equipment

affected. Service to the rest of the system remains energized.

If the ampere rating of a feeder overcurrent device is larger than therating of the branch circuit device, are the two selectively coordinated?No. A difference in rating does not in itself assure coordination. Forexample, a feeder circuit breaker may have a rating of 400amperes and the branch breaker 90 amperes. Under overloadconditions in the branch circuit, the 90 ampere breaker will openbefore, and without, the 400 ampere breaker opening. However,under short-circuit conditions, not only will the 90 ampere deviceopen, the 400 ampere may also open. In order to determinewhether the two devices will coordinate, it is necessary to plot theirtime-current curves as shown at right. For a short-circuit of 4000amperes:

240-12 Covers System Coordination or Selectivity

1000AI.T.=10x

225AI.T.=8x

Opens

Opens

20AI.T.=8x

Opens

22,000 AmpShort-Circuit


Opens20A

225ANotOpen

1000ANotOpen

19-B

.001

.002

.003

.004

.006

.008

.08

.01

.02

.03

.04

.06

.1

.2

.3

.4

.6

.81

2

34

6

810

20

3040

80

60

100

200

300400

600800

1,000

1 0 0

2 0 0

3 0 0

6 0 0

4 0 0

8 0 0

1 , 0

0 0

2 , 0

0 0

3 , 0

0 0

4 , 0

0 0

6 , 0

0 0

8 , 0

0 0

1 0

, 0 0 0

2 0

, 0 0 0

3 0

, 0 0 0

3 0

, 0 0 0


CURRENT IN AMPERES

POINT D

POINT C

POINT B

POINT A

400 AMP

Circuit BreakerI.T. = 5X

90 AMP

CircuitBreaker

90A

Short-Circuit

400A

2:1 (or more)

LPS-RK400SPLPS-RK90SP

Short-Circuit



*Selectivity Ratio Guide (Line-Side to Load-Side) for Blackout Prevention.

Circuit Load-Side Fuse

Current Rating 601-6000A 601-4000A 0-600A 601-6000A 0-600A 0-1200A 0-600A 0-60A

Type Time- Time- Dual-Element Fast-Acting Fast-Acting Time-Delay Delay Time-Delay Delay

Trade Name & LOW-PEAK LIMITRON LOW-PEAK FUSETRON LIMITRON LIMITRON T-TRON LIMITRON SC

YELLOW YELLOWClass (L) (L) (RK1) (J)** (RK5) (L) (RK1) (T) (J) (G)

Buss KRP-C–SP KLU LPN-RK–SP LPJ–SP FRN-R KTU KTN-R JJN JKS SCSymbol LPS-RK–SP FRS-R KTS-R JJS

601 to Time- LOW-PEAK KRP-C–SP 2:1 2.5:1 2:1 2:1 4:1 2:1 2:1 2:1 2:1 N/A6000A Delay YELLOW (L)

601 to Time- LIMITRON KLU 2:1 2:1 2:1 2:1 4:1 2:1 2:1 2:1 2:1 N/A4000A Delay (L)

LOW-PEAK LPN-RK–SP – – 2:1 2:1 8:1 – 3:1 3:1 3:1 4:1YELLOW LPS-RK–SP

0 Dual (RK1)to Ele- (J) LPJ–SP** – – 2:1 2:1 8:1 – 3:1 3:1 3:1 4:1

600A ment FUSETRON FRN-R – – 1.5:1 1.5:1 2:1 – 1.5:1 1.5:1 1.5:1 1.5:1(RK5) FRS-R

601 to LIMITRON KTU 2:1 2.5:1 2:1 2:1 6:1 2:1 2:1 2:1 2:1 N/A6000A (L)

0 to Fast- LIMITRON KTN-R – – 3:1 3:1 8:1 – 3:1 3:1 3:1 4:1

600A Acting (RK1) KTS-R

0 to T-TRON JJN – – 3:1 3:1 8:1 – 3:1 3:1 3:1 4:1

1200A (T) JJS

0 to LIMITRON JKS – – 2 :1 2:1 8:1 – 3:1 3:1 3:1 4:1600A (J)

0 to Time- SC SC – – 3:1 3:1 4:1 – 2:1 2:1 2:1 2:160A Delay (G)

Note: At some values of fault current, specified ratios may be lowered to permit closer fuse sizing. Plot fuse curves or consult with Bussmann.

General Notes: Ratios given in this Table apply only to Buss fuses. When fuses are within the same case size, consult Bussmann.

Consult Bussmann for latest LPJ—SP ratios.

.

17

240-12 Covers System Coordination or Selectivity

What does this Section require?

Equipment ground fault protection of the type required in Section230-95 is now required for each disconnect rated 1000 amperes ormore in 480Y/277V systems that will serve as a main disconnectfor a separate building or structure. Refer to Sections 215-10 and230-95.

Note: G.F.P. that is not current-limiting may not protect systemcomponents. See Section 110-10 and 250-1 (FPN).

What are the requirements of 240-21(b)?The basic content of this section remains unchanged. However, ithas been rewritten to improve readability and comprehension.Typically, fuses must be installed at point where the conductorreceives its supply, i.e., at the beginning or line side of a branchcircuit or feeder. There are installations where this basic rule maynot have to be followed.

Fuses are not required at the conductor supply if a feeder tapconductor is not over ten feet long; is enclosed in raceway; doesnot extend beyond the switchboard, panelboard or control devicewhich it supplies; and has an ampacity not less than the combinedcomputed loads supplied, and not less than the rating of thedevice supplied, unless the tap conductors are terminated in afuse not exceeding the tap conductor's ampacity [240-21(b)]. Forfield installed taps, the ampacity of the tap conductor must be atleast 10% of the overcurrent device rating. See the following

example.

High VoltageService

Building A Service

800A480Y/277V

Building B Service

1000A or Greater480Y/277V

G.F.P. NotRequired

G.F.P. NotRequired

G.F.P.Required

240-13 Covers Ground Fault Protection of Equipment on Remote Structures

240-21 Covers Location Requirements for Overcurrent Devices and Tap Conductors

L i n e - S

i d e F u s e

*

**

M M M M M M

ComplianceViolation240V, 3Ø600A

EquipmentRoomWireway

1HP

5HP

7.5HP

15HP

20HP

25HP

10'TAP



18

length; the secondary conductors terminate in a set of fuses ratedat the ampacity of the tap conductors; and it the primary andsecondary conductors are suitably protected from physicaldamage.

What are the requirements of 240-21(e)?

Fuses are not required at the conductor supply if a feeder tap isnot over 25 feet long horizontally and not over 100 feet long, totallength, in high bay manufacturing buildings where only qualifiedpersons will service such a system. Also, the ampacity of the tapconductors is not less than 1 / 3 of the fuse rating from which theyare supplied, the size of the tap conductors must be at least No. 6AWG copper or No. 4 AWG aluminum. They may not penetratewalls, floors, or ceilings, and the taps are made no less than 30feet from the floor.

Note: Smaller conductors tapped to larger conductors can be aserious hazard. If not adequately protected against short-circuitconditions (as required in sections 110-10 and 240-1), theseunprotected conductors can vaporize or incur severe insulationdamage. Molten metal and ionized gas created by a vaporizedconductor can envelop other conductors (such as bare bus),causing equipment burndown. Adequate short-circuit protection isrecommended for all conductors. When a tap is made to aswitchboard bus for an adjacent panel, such as an emergencypanel, the use of BUSS Cable Limiters is recommended forprotection of the tapped conductor. These current-limiting cablelimiters are available in sizes designed for short-circuit protectionof conductors from #12 to 1000 kcmil. BUSS Cable Limiters areavailable in a variety of terminations to make adaption to busstructures or conductors relatively simple. For more information onBUSS Cable Limiters, see Buss Bulletin CL.

What are the requirements of 240-21(j)?Transformer secondary conductors of separately derived systemsdo not require fuses at the transformer terminals when all of thefollowing conditions are met.

1. Must be an industrial location.2. Secondary conductors must be less than 25 feet long.3. Secondary conductor ampacity must at least equal to the

secondary full load current of transformer and sum ofterminating, grouped, overcurrent devices.

4. Secondary conductors must be protected from physicaldamage.

Note: Switchboard and panelboard protection (384-16) andtransformer protection (450-3) must still be observed.

In the previous diagram, the feeder overcurrent devices aresized per the N.E.C for the load served.

All taps to the motors are 10 foot taps.Three of the motors are smaller motors:

one motor is a 1 HP motor,one motor is a 5 HP motor,

and one motor is a 71 / 2 HP motor.The 1, 5 and 71 / 2 HP motors will require a minimum of #14, #12

and #10 75°C conductors, respectively. For field wiring, these 10foot taps are not permitted since the line side overcurrent device is600 amperes. Section 240-21(b)(5) requires that the maximumovercurrent protection for field installations shall not exceed1000%, or 10 times the ampacity of the tap conductor, forexample:

#14 conductor, 20 amperes ampacity, maximum line sideovercurrent protection is 200 amperes.#12 conductor, 25 amperes ampacity, maximum line sideovercurrent protection is 250 amperes.#10 conductor, 35 amperes ampacity, maximum line sideovercurrent protection is 350 amperes.To tap the above conductors to a 600 amperes feeder

overcurrent device would be a violation of Section 240-21(b)(5) ofthe Code.

The solution is to feed the smaller motors from a branch circuitpanel or from a smaller feeder where the feeder overcurrentprotection does not exceed the 10 times rating of the tapconductor’s ampacity.

The smallest of the three larger motors is a 15 HP motor whichrequires a branch circuit conductor with a minimum ampacity of52.5 amperes and which could be tapped to the 600 amperefeeder since a No. 6 75° conductor has an ampacity of 65amperes and 10 x 65 = 650. In other words the No. 6 75°conductors could be tapped to an overcurrent device as high as650 amperes.

Motor tap conductors that have a 60 ampere ampacity orgreater could be tapped to a 600 ampere feeder overcurrentprotective device.

What are the requirements of 240-21(c)?

Fuses are not required at the conductor supply if a feeder tapconductor is not over 25 feet long, is suitably protected fromphysical damage; has an ampacity not less than 1 / 3 that of thefeeder conductors or fuses from which the tap conductors receivetheir supply; and terminate in a single set of fuses sized not morethan the tap conductor ampacity. See "Note".

What are the requirements of 240-21(d)?Fuses are not required at the conductor supply if a transformerfeeder tap has primary conductors at least 1 / 3 the ampacity, and/orsecondary conductors at least 1 / 3 the ampacity, when multiplied bythe approximate transformer turns ratio of the fuse or conductorsfrom which they are tapped; the total length of one primary plusone secondary conductor (excluding any portion of the primaryconductor that is protected at its ampacity) is not over 25 feet in

240-21 Covers Location Requirements for Overcurrent Devices and Tap Conductors

M M M

Wireway

200A

10'TAP

1HP

5HP

7.5HP

480V

150 AmpFeederFuse

TRANSFORMER2:1 RATIO 100 Amp

Fuse

100 Amp

RatedConductor

50 AmpRatedConductor

150 Amp

RatedConductor 240V

480V

300 AmpFeederFuse

TRANSFORMER1:1 RATIO 100 Amp

Fuse




480V

25 Feet or Less



240-40 Disconnecting Means for Fuses

19

The second exception allows one disconnect for multiple sets offuses as provided in 430-112 for group motor applications and424-22 for fixed electric space-heating equipment.

What does the Section require?A line side disconnecting means must be provided for all cartridgefuses where accessible to other than qualified persons and for anyfuse in circuits over 150 volts to ground. This section does notrequire a disconnecting means for the typical 120/240V singlephase residential plug fuse application.

The f irst exception removes the requirement for adisconnecting means ahead of a current-limiting cable limiter orother current-limiting fuse ahead of the service disconnectingmeans.

240-50 Covers Plug Fuses, Fuseholders, and Adapters

What does this Section mean?Normally, plug fuses are applied in 120 volt circuits for appliances,small motors, machines, etc. They may be used on 240/120 voltssingle-phase circuits, and 208/120 volt three-phase circuits, wherethe neutral is solidly grounded.

240-51 Covers Edison-Base Fuses

What are these fuse types?These are generally referred to as branch circuit listed fuses whichare NOT size rejecting. They can provide protection for appliancesand small motors in residential, commercial, and industrialapplications.

OR

PERMISSIBLE

M

Plug Fuses

208V120V

M

Plug Fuses

120V

240V

240-53 Covers Type S Fuses

What are these fuse types?These are branch circuit listed fuses that are size (ampere)rejecting. They become size rejecting when a special Type Sholder or Type S adapter is used. For example, when a 20 ampereadaptor is installed, it is very difficult to insert a 25 or 30 amperefuses.

Type S fuses are required for new installation where plug fusesare to be used as the branch circuit protection.

Edison-base fuses can be used for supplementary overcurrentprotection in new installations.



Note: Refer to Section 110-22 For marking requirements for the main or

upstream protective device.

What are the advantages of Type S Fuses?Type S fuses are size rejecting to prevent overfusing. They areused with special adapters that cannot easily be removed.

What does this Section mean?300 volt rated fuses can be used to protect single-phase line-neutral loads when supplied from three-phase solidly grounded480/277 volt circuits, where the single-phase line-to-neutral voltageis 277 volts.

240-54 Covers Type S Fuses, Adapters, and Fuseholders

What does this Section mean?All low voltage branch circuit fuses have an AC voltage ratingassociated with them. They can be properly applied at any systemvoltage up to that rating.

240-61 Covers Classification of Fuses and Fuseholders

240-60 Covers Cartridge Fuses and Fuseholders

20

480/277V

600 VoltFuses

277V 1Ø Loads

300V Fuses

Also, branch circuit listed fuses are designed so that it is verydifficult to replace an installed fuse with one of lesser capability.This can be based on a voltage or current rating or a current-limiting vs. non-current-limiting device.

240-83(c) Covers Marking–Interrupting Rating of Circuit Breakers and Series Ratings

240-100 Covers Feeder Overcurrent Protection Over 600 Volts, Nominal

What does the Section require?Special marking requirements now exist for series-rated systemswhich are tested and recognized. (One source is the U.L. YellowBooks.) This special marking on the equipment must state that thespecific circuit breakers in the equipment have been series testedwith special upstream devices. Any substitution of series-ratedcircuit breakers with a non-series-rated device will void therecognition and create a potentially dangerous situation. Thislabeling will be supplied by the manufacturer.

Note: Refer to Section 110-22 for marking requirements for themain or upstream protective device.

What does the Fine Print Note to this Section mean?The overcurrent device specified for the application must becapable of protecting the feeder conductors from short-circuitdamage.

(The reader should refer to N.E.C. Section 110-10 for a furtherunderstanding of this fine print note. Feeders must be protectedfrom short-circuit damage.)

30,000 Amps RMS Available

40,000 Amps RMS Available

200A Circuit Breaker65,000 A.I.R.

or200A Current-Limiting Fuse

20A

10,000 A.I.R.Circuit Breakers

If this is a testinglaboratory Recognizedseries rating, a special

label is required insidethe equipment, suppliedby the manufacturer.



What does the Fine Print Note #2 refer to in Section 110-10?This FPN stresses the importance of protecting the equipment and

materials that make up the ground path in order to facilitate thesafe opening of the overcurrent device.

What does this Section mean?The effective grounding path shall have a low enough impedanceto limit the voltage to ground and to facilitate the opening of theovercurrent protective device. It must be permanent and continuosand be able to safely conduct available fault current.

21

250-1 Covers the Requirements forProper Grounding and Bonding ofElectrical Installations

250-95 Covers Sizing of EquipmentGrounding Conductors

250-51 Covers the Requirements foran Effective Grounding Path

250-70 Covers Bonding Require-ments and Short-Circuit Withstand

250-75 Covers Bonding OtherEnclosures and Short-CircuitWithstand Requirements

250-95 Covers Sizing of EquipmentGrounding Conductors

What does this Section mean?All bonding provided must have the capacity to conduct safely anyfault current it is likely to see.

What do these Sections require?All materials used in the grounding and bonding of equipmentshall be capable of safely carrying the short-circuit current thatcould flow through the ground path. This will, in many cases,require the use of a current-limiting fuse to protect the equipmentfrom damage. See Section 110-10 for more on componentprotection.

What are the ramifications of 250-95 and especially the new note at thebottom of Table 250-95?The integrity of the grounding path is essential for safety; itfacilitates the operation of the overcurrent protective devices.Improper sizing of the grounding conductors can result in theirmelting or vaporizing before the protective device clears thecircuit. Generally, the grounding electrode conductor and theequipment grounding conductors are smaller than the circuitconductors and their ampere rating is less than that of theovercurrent protective device. The protective device may be tooslow to protect an undersized conductor against high fault currents(see Section 240-1 of this Bulletin). Consideration must be given tothe size of the grounding conductors, their withstand, themagnitude of ground fault currents, and the operat ingcharacteristics of circuit overcurrent devices. Where the protectivedevice is not fast enough to protect the undersized equipmentgrounding conductor, the conductor size may need to be

increased, or a different overcurrent device could be chosenwhich could provided adequate protection for the conductor. ThisSection of the N.E.C. now requires this analysis.

Since instantaneous only circuit breakers (MCP’s) can be set

as high as 1700% of motor full-load current, the equipmentgrounding conductor shall be sized based on the motor overloadrelay.

Note: Table 250-94 in the N.E.C. gives the sizes of the groundingelectrode conductors versus the sizes of the service entranceconductors. Caution, Table 250-95 in the N.E.C. gives the“Minimum Size Equipment Grounding Conductors for GroundingRaceway and Equipment.”

For example, Table 250-95 allows a circuit protected by a 400ampere overcurrent device to have a #3 copper equipmentgrounding conductor. If the 400 ampere overcurrent device takesone cycle to open in a circuit where 50,000 amperes are available,typical cable manufacturer’s withstand charts show that the #3conductor would be damaged. One solution would be to install a#2/0 copper equipment grounding conductor which would be ableto withstand the 50,000 amperes for one cycle. The otheralternative is to limit the 50,000 amperes to within the 22,000ampere for one cycle limit of the #3 conductor. This can beaccomplished easily with the use of current-limiting fuses.

#1/0 CopperGroundingElectrodeConductor

50,000A RMS

400A Non-Current-Limiting Device

Service EquipmentMetal Enclosure

Non-MetallicRaceway

GroundedServiceNeutral

#3 CopperEquipmentGroundingConductor

VIOLATION

Would need to increaseEquipment GroundingConductor to 2/0.

3ØLoad

Metal Enclosure

500 kcmil Copper

#1/0 CopperGroundingElectrodeConductor

50,000A RMS

400A Current-Limiting Device

Service EquipmentMetal Enclosure

Non-MetallicRaceway

Grounded

ServiceNeutral

#3 CopperEquipmentGroundingConductor

3ØLoad

Metal Enclosure

500 kcmil Copper

COMPLIANCE

Conforms to Section 110-10

and 250-95.



22

What is the purpose of the Fine Print Note in this Section?The fine print note is intended to point out the need for conductorderating at high ambient temperatures. It also directs the user tobe aware of other information, such as conductor size andnumber, to assure proper application.

310-10 Covers Temperature Limitation of Conductors

364-11 Covers Protection at a Busway Reduction

384-16 Covers Panelboard Overcurrent Protection

What does this Section mean?

Lighting and appliance branch circuit panelboards must beprotected by a main overcurrent device (up to two sets of fuses, aslong as their combined ratings do not exceed that of thepanelboard), unless the feeder has overcurrent protection notgreater than the rating of the panelboard.

What is the importance of this Section?This is a new Section of the N.E.C. It offers an overview ofprotection for motors, motor circuits, motor controllers, and motor

control centers.

430-1 Covers Scope of Motor Article

What is the importance of this Section?It states that conductors supplying motors shall be selected fromapplicable Tables in Article 310. The determination of conductorampacity, or ampere rating of switches, branch circuit protection,etc., should be taken from the motor F.L.A. tables in Article 430,Tables 430-147 through 430-150.

35°CEnvironment

3 #12 75°C Copper Conductors

in a Raceway

This fuse is sized at 25 (amperes) x .94 (temperature deratingfactor) = 23.5 amperes. The next standard size is 25 amperes,but the obelisk directs the reader to the bottom of Table 310-16,where the maximum overcurrent device is given as 20 amperes.

35°CEnvironment

9 #12 75°C Copper in a Raceway

This fuse is sized at 25 (amperes) x .94 (temperature deratingfactor) x .70 (9 conductors in a raceway derating factor fromNote #8 to ampacity tables) = 16.45 amperes. The nextstandard size is a 20 ampere Fuse.

General Comment—The service entrance split bus load center or

panelboard having up to 6 main disconnects is no longer permittedon new installations.

The tap rules found in Section 240-21 do not remove theserequirements for lighting and appliance branch circuit panelboardprotection, nor do they remove the requirements for transformerprotection found in Section 450-3.

430-6 Covers Ampacity of Conductors for Motor Branch Circuits and Feeders

The separate overload device should be based on thenameplate current rating.

What does this Section mean?Overcurrent protection is required whenever busway is reduced inampacity unless all of the following conditions are met:1. Industrial establishment only.2. Length of smaller bus does not exceed 50 feet.3. Ampacity of smaller bus must be at least 1/3 that of theupstream overcurrent device.4. Smaller bus must not contact combustible material.



430-32 Covers Motor Overload Protection

23

What does this Section require?This Section clarifies the need for overload protection in all threephases of a 3-phase, 3-wire system, where one phase also servesas the grounded conductor.

430-36 Covers Fuses Used to Provide Overload and Single-Phasing Protection

What are the typical ways of providing motor overload protection externalto the motor?Generally, motor starters with overload relays and/or dual-elementfuses are used to provide motor running protection.

Typically, how are the devices selected for protection of motors?With starters and overload relays, the proper heater element isselected from manufacturers’ tables based on the motor full-loadcurrent rating. The level of protection reached in this selectionprocess complies with Article 430.

When employing dual-element fuses for motor running overloadprotection, the rating of the fuse should be as follows:

M

M

LOW-PEAK YELLOWDual-ElementFuse

Do fuses sized as above also provide branch circuit protectionrequirements?Yes. Sizing FUSETRON® and LOW-PEAK YELLOW™ Dual-Elementfuses for motor running overload protection also provides thenecessary short-circuit protection per 430-52. The use of thesedual-element fuses permits close sizing. Thus, fuse case sizes

often can be smaller, thereby permitting the use of smallerswitches.

Can circuit breakers and fuses other than dual-element fuses be used togive motor overload protection?Not generally. The conventional circuit breakers usually must besized at 250% of the motor full-load amperes to avoid tripping onmotor start ing current, and thus cannot provide overloadprotection. Instantaneous only circuit breakers or motor short-circuit protectors are only equipped with a short-circuit trippingelement and, therefore, are incapable of providing overloadprotection. For motor applications, the non-time-delay fuses suchas the LIMITRON® KTS-R fuses normally have to be sized at 300%of a motor full-load current rating to avoid opening on motor start-up and, therefore, do not provide overload protection.

When single-phasing occurs on a 3-phase motor circuit,unbalanced currents flow through the motor, which can damage

the motor if not taken off-line. Dual-element, time-delay fuses, sizedfor motor overload protection, can provide single-phase damageprotection . See Section 430-36.

Footnote–Abnormal Motor Operation: The application of motors under certain abnormaloperating conditions often requires the use of larger size fuses than would normally berequired. The use of oversize fuses limits protection to short-circuit or branch circuitprotection only. The types of abnormal motor installations that may be encountered includethe following: (a) Fuses in high ambient temperature locations. (b) Motors having a highCode Letter (or possibly no Code Letter) with full-voltage start. (c) Motors driving highinertial loads or motors which must be frequently cycled off-and-on. Typical high inertialloads are machines such as punch presses having large mass flywheels, or machinessuch as centrifugal extractors and pulverizes, or large fans which cannot be brought up tospeed quickly. (d) High efficiency motors with high inrush currents.

M M

LOW-PEAKYELLOW orFUSETRONDual-elementFuse

Size at 115%or less of motorfull-load amps

S.F. less than 1.15ortemp. rise over 40°C.

S.F. 1.15 or higherortemp. rise 40°C.or less

Size at 125%or less of motorfull-load amps

LOW-PEAKYELLOW orFUSETRONDual-ElementFuse

480 Volts10HPF.L.A. = 14A

LPS-RK171/2SP

B

A

C

LPS-RK171/2SP

LPS-RK171/2SP

M



24

What is the basic content of this Section?This Section deals with the protection of motor branch circuitsagainst short-circuit damage. It establishes the maximumpermissible settings for overcurrent protective devices. (Branch

circuits include all the circuit components–wire, switches, motorstarters, etc.) As is apparent in Code Table 430-152, maximumsettings vary with different types of motors, each type havingunique starting characteristics. Motors to which the maximumpermissible settings or ratings apply (shown in the condensedTable following) include all types of single-phase, three-phasesquirrel cage (other than Design E) and three-phase synchronousmotors.

These maximum values do not preclude the application oflower sizes. Also, compliance with Sections 110-10 must beanalyzed. Motor starters have relatively low short-circuit currentwithstands. Refer to Buss Bullet in SPD for specif ic fuserecommendations.

Maximum Rating or Setting of Protective Devices†

Fuse Circuit Breaker*

Non-Time-Delay Dual-Element Instantaneous Inverse

All Class CC Time-Delay Type Only Time Type300% 175% 800% 250%

†See Article 430, Section 430-52.*For latest information, check manufacturer’s data and/or Underwriters’ Laboratories U.L.Standard #508 for damage and warning label requirements.

What about starter withstandability and Section 110-10 requirements forcomponent protection?

Under short-circuit conditions, the branch circuit protective devicemust protect the circuit components from extensive damage.Therefore, the following factors should be analyzed: availableshort-circuit current, let-thru characteristics of the overcurrentprotective device, and starter withstandability.

As an Example, a Size 1 Starter has been tested by U.L. with5000 ampere minimum available short-circuit current per U.L.Standard 508. Thus, in the example above, the available short-circuit currents should not exceed 5000 amperes since the circuitbreaker is not current-limiting.

Additionally an MCP, if used in a combination controller, mustbe listed for that specific combination. The MCP cannot be usedas a separate motor branch circuit short-circuit protective deviceto protect a motor controller. Applications of MCP’s on manymotors, i.e., high efficiency or high Code Letter, may cause theMCP to operate needlessly, even when sized at 1700% of motorcurrent. Some MCP’s require an additional component which

accommodates or overlooks the inrush to allow motor start-up.

In the circuit below using a Buss LOW-PEAK YELLOW™ dual-elementtime-delay fuse, can available short-circuit current exceed 5000amperes?

Yes. Because the LOW-PEAK YELLOW fuse is “current-limiting,”good short-circuit protection is provided, even though availableshort-circuit current greatly exceeds 5000 amperes. (Specifically,the LOW-PEAK YELLOW fuse would give protection against fault

currents through 200,000 amperes.) It is also significant to notethat because the LOW-PEAK YELLOW fuse is a time-delay fuse, itactually could be sized at 125% of full-load current or the nextlarger size (30 amperes) with the advantage of permitting the useof a smaller disconnect switch, and providing backup overloadprotection and even better short-circuit protection.

These maximum sizing allowances are all overridden if amanufacturer's label shows overcurrent protection values lowerthan what 430-52 allows.

The overload relay heater elements of a motor controller oftenhave relatively low short-circuit current withstand ratings. Themaximum ratings of protective devices given in Table 430-152,thus, do not necessarily apply since they are too large to provideadequate protection. Consequently, the starter manufacturerincludes an overload relay table within the starter enclosure. Thistable states the maximum fuse size ratings to be used which willadequately protect the overload relay heaters. The protective

device used, in such cases, must be a fuse.

TYPICAL EXAMPLE: The chart shown below is typical for startermanufacturers and may be found on the inside of the door of thestarter enclosure. (See starter manufacturer for specif icrecommendation.)

Heater Full Load Current Max.

Code of Motor (Amperes) Fuse

Marking (40°C Ambient)

XX03 .25- .27 1

XX04 .28- .31 3

XX05 .32- .34 3

XX06 .35- .38 3

XX14 .76- .83 6

XX15 .84- .91 6

XX16 .92-1.00 6XX17 1.01-1.11 6

XX18 1.12-1.22 6

Above Heaters for use on Size 0

Section 240-6 has an exception listing additional standard fuseampere ratings of 1, 3, 6 and 10 amperes. The lower ratings wereadded to provide more effective protection for circuits with smallmotors, in accordance with Sections 430-52 and 430-40 andrequirements for protecting the overload relays in controllers forvery small motors. Fuse manufacturers have available otherintermediate fuse ampere ratings to provide closer circuitprotection (such as sizing dual-element fuses at 125% of motorcurrent) or to comply with “Maximum Fuse” sizes specified incontroller manufacturer’s overload relay tables.

A paragraph of Section 430-52 allows other fuses to be used in place of

those allowed in Table 430-152. Why is this Code provision necessary?Some “solid-state” motor starters and drives require fusesspecifically designed to protect semiconductor components. TheCode provision was necessary in order to give branch circuit,short-circuit and ground fault “recognition” to these fuses.

430-52 Covers the Sizing of Various Overcurrent Devices for Motor BranchCircuit Protection

M

NON-CURRENT-LIMITINGCIRCUIT BREAKER

SIZE 1 STARTER LISTED FOR 5000AMPS WITH THE 50A BREAKER

Short-circuit currentshould not exceed5000 amperes

71/2 HP(22A)

SIZE 1 STARTER LISTED FOR 200,000AMPS WITH A 40A CLASS R FUSE

LOW-PEAK DUAL-ELEMENT FUSEMax. size: 175% x 22 = 40A

230V3Ø M

71/2 HP(22A)



Comparison By Largest HP Motor (460V) Circuit Where Branch Circuit Protective Device Is Considered To Protect The Control Conductors Per430-72(b) (2), Exc. 1 and 2.Protective Approx. Level Of Control Circuit Control Circuit

Device Size As Percent Protection Within Enclosure Extending Beyond Enclosure

Motor F.L.A. #18 #16 #14 #18 #16 #14

125% Overload 15HP 25HP 60HP 3HP 5HP 25HP

LOW-PEAK YELLOW and Branch

or FUSETRON Fuse Circuit

175% 10HP 15HP 40HP 2HP 3HP 15HP

Non-Time-Delay 300% 5HP 71 /2HP 20HP 1HP 11 /2HP 10HP

Fuse

Thermal Magnetic 250% Branch 5HP 10HP 30HP 11 /2HP 2HP 10HP

Circuit Breaker Circuit

Instantaneous 1000%* Only 1HP 2HP 5HP 1 /4HP 1 /2HP 2HPOnly Circuit

Breaker

*Instantaneous only circuit breakers cannot provide any overload protection. Typically to hold starting currents, instantaneous trip is set at 1000% to 1700% of motor full-load amperes.

26

Do the two circuits shown below require individual control circuit protec-tion?

No. The LPS-RK40SP ampere fuses are sized within the 40 ampererequirement for #16 conductor within an enclosure. (See Table430-72(b).)

Yes. Individual control circuit fuses are required since the 80ampere circuit breaker has a rating in excess of the 40 ampererequirement for #16 conductor within an enclosure. (See Table430-72(b).

Note: Sections 110-10 and 240-1 require that component withstand not be exceeded. Notall overcurrent devices set at 400% can protect small conductors.

What does Exception No. 2 mean?If the control conductors leave the enclosure, they can beconsidered to be protected by the branch circuit fuse, if that fusedoes not exceed the values of Table 430-72(b) Column C.

What does Exception No. 3 mean?Primary fusing of a control transformer can be considered toprotect the 2-wire, secondary conductors if the fuse rating does notexceed the value of multiplying the appropriate rating from Table430-72(b) with the secondary-to-primary voltage ratio.

From Table 430-72(b)

Wire Size Max. Protection#18 Copper 7 Ampere Fuse

Maximum primary fuse shall not exceed 1.75A as determined by—

120V

x 7A = 1.75A480V

430-72(b) Covers Motor Control-Circuit Conductor Protection

Control conductorsextending beyond enclosure

(Exception No. 2)

M

The motor branch circuit protective device is considered also toprotect the control conductors if it does not exceed the values ofColumn C.

No. 16 Wire Within Enclosure

LPS-RK40SP

M25 HP34A

M25 HP34A

No. 16 Wire Within Enclosure

80A

10A Required

M

#18 Copperwithinenclosure

.5 Amp120V2 WIRE

480V 100VA

BRANCH

CIRCUITFUSE

Even though a fuse or circuit breaker can be sized at 300% or 400% ofthe conductor ampacity, what level of control conductor protection can beexpected?The protective device would respond only to high level conductorovercurrents; the control conductors would not be protectedagainst lower overcurrent levels. This lack of protection couldresult in a prolonged 200% control circuit overcurrent and eventual

insulation breakdown and melting of the conductors. For example,if the control circuit run were of considerable length, the conductorimpedance might be sufficiently high to limit fault currents to 200%to 400% of the conductor ampacity. Thus, oversized overcurrentdevices would provide inadequate protection. In contrast, fusessized to the conductors ampacity would provide full-rangeovercurrent protection; their use is to be recommended.



27

What does this Section Mean?

Primary Fuse Protection Only.Transformer Primary Fuse

Primary Ampacity

Current Must Not Exceed

Less than 2 amperes 500%* (Exception No. 2)*2 to 9 amperes 167%

9 amperes or more 125%

*Primary protection using time-delay fuses sized not in excess of 250% is recommended.

Primary and Secondary Fuse Protection.Primary Fuse Secondary Secondary

Does Not Exceed Current Fuse

250% 9 amperes or more 125%

250% Less than 9 amperes 167%

Application Guideline to 430-72(c), Exception No. 1.

The conditions of 430-72(c), Exception No. 1, permits the use of acontrol transformer rated less than 50 VA* without the inclusion ofindividual protection on the primary side of the transformer in thecontrol circuit proper. Thus, protection of the transformer primary

against short-circuit currents is dependent upon the device usedfor branch circuit protection. However, consideration should begiven to protecting the control transformer on the primary side withindividual fuses specifically sized for control transformerprotection.

Take the case, for instance, in which a short occurs in a controltransformer (such as would result from insulation deterioration andbreakdown). (See diagram above in which a 60 ampere branchcircuit fuse is shown.) Now, if the overcurrent drawn by the controlcircuit as a result of the shorted control transformer is relatively low(actually could be less than 60 amperes) compared to theresponse time of the 60 ampere branch circuit fuse or circuitbreaker, the transformer could become so hot that extensivedamage could be done to the insulation of the control conductors. . . the transformer itself could burst into flames.

*Control Transformers rated less than 50 VA are usually impedance protected or haveother types of protection, such as inherent protection.

However, inclusion of fuse protection in the primary of thecontrol transformer would minimize this type of hazard. Buss typeFNQ or FNQ-R Time-Delay fuses could be sized as low as 125% ofthe transformer full-load amperes. Buss type KTK or KTK-R fast-acting fuses could typically be sized at 300% of the primary full-load amperes. When applying fuses, the t ime-current

characteristics should be checked to determine if the fuse can holdthe inrush magnetizing current of the transformer.

Fuses Commonly Used in Control Circuits.There are several fuse types which have small dimensions that areideally suited for control circuit protection. The KTK-R, FNQ-R andLP-CC fuses are listed as Class CC fuses, and JJN (JJS) fuses arelisted as Class T fuses. The other fuses shown are listed assupplementary protection. When used for control transformer, coil,or solenoid protection, the fuse should be selected to withstand theinrush current for the required time.

Symbol Voltage Ampere Interrupting Comment

Rating Rating Class Rating

Branch Circuit Rejection Fuses

FNQ-R 600V 15 / 100 thru 10 CC* 200KA

Time-delay inLP-CC 600V 1 / 2 thru 30 CC* 200KA

overload regionLPJ 600V 1 thru 600 J* 300KASC 480V 6 thru 60 G* 100KA

KTK-R 600V 1 / 10 thru 30 CC* 200KA No intentional

JJN 300V 1 thru 1200 T* 200KA time-delay

JJS 600V 1 thru 800 T* 200KA in the overload

SC 480V 1 / 2 thru 5 G* 100KA region

Supplementary Fuses

FNQ 500V 1 / 10 thru 30 SUP.* 10KA

FNW 250V 12 thru 30 SUP.* 10KA

FNM 250V 0 thru 1 SUP.* 35A

FNM 250V 1.1 thru 3.5 SUP.* 100A

FNM 250V 3.6 thru 10 SUP.* 200A Time-delay in the

FNM 125V 10.1 thru 15 SUP.* 10KA overload region

FNM 32V 15.1 thru 30 SUP. 1KA

FNA 250V 1 / 10 thru 8 / 10 SUP.* 35A

FNA 125V 1 thru 15 SUP.* 10KA

FNA 32V 15.1 thru 30 SUP. 1KA

KTK 600V 1 / 10 thru 30 SUP.* 100KABAF 250V 1 / 2 thru 1 SUP.* 35A

BAF 250V 1.1 thru 3.5 SUP.* 100A

BAF 250V 3.6 thru 10 SUP.* 200A No intentional

BAF 250V 10.1 thru 15 SUP.* 750A time-delay

BAF 125V 15.1 thru 30 SUP. 10KA in the overload

BAN 250V 2 / 10 thru 1 SUP. 35A region

BAN 250V 1.1 thru 3.5 SUP. 100A

BAN 250V 3.6 thru 10 SUP. 200A

BAN 250V 10.1 thru 15 SUP. 750A

BAN 250V 15.1 thru 30 SUP. 1500A

* U.L. Listed

430-72(c) Covers Motor Control-Circuit Transformer Protection

.05A normal F.L.C.(breakdown of transformer windings could causecurrent to increase many times over normal level butless than 60A) *Conductor protection still required perSection 430-72(b)

*

480V 120V

(25VA)

60A



COMPLIANCE COMPLIANCE

430-94 Covers Motor Control Center Protection

What are the requirements of this Section?Where motor control centers (MCC) are specified, properovercurrent protection shall be supplied in the MCC as an integralmain, or remote main. These devices should be rated based onthe common power bus rating.

LPS-RK600SP

600A Bus

600A MCC

LPS-RK600SP

600A Bus

600A MCC

LPS-RK600SP

28

What are the requirements of this Section?

If the nameplate on the equipment controller is marked with "MAXFUSE", that means a fuse must be used to protect the equipment.See Section 110-3(b) for proper installation and protection.

What are the requirements of this Section?The branch circuit protective device may be sized at the maximumvalue of 175% of the motor-compressor rated load current. If themotor cannot start due to high inrush currents, this value may beincreased to, but cannot exceed, 225% of motor rated current.

The required secondary protection may be satisfied with multipleovercurrent devices that protect feeders fed from the transformersecondary. The total ampere rating of these multiple devicescannot exceed the allowed value of a single secondaryovercurrent device. If this method is chosen, dual-element, time-delay fuse protection offers much greater flexibility.

Note the following examples:This design utilized a single secondary overcurrent device. It

provides the greatest degree of select ively coordinatedtransformer protection, secondary cable protection, and switch-board/panelboard/load center protection. The transformer cannotbe overloaded to a significant degree if future loads are added

(improperly) in the future.If the single secondary overcurrent device is eliminated, much

of the protection will be reduced.

440-5 Covers Marking Requirements on HVAC Controllers

440-22 Covers Application and Selection of the Branch Circuit Protection forHVAC Equipment

450-3 Covers Protection Requirements for Transformers

M M M M M

250%

150 KVA

208/120V IFLA = 417A

This fuse or circuit breaker maybe sized at 1.25 x 417A = 522A.The exception allows the nextstandard size of 600A to be used.

200ASwitch

LPN-RK110SP

LPN-RK110SP

LPN-RK110SP

LPN-RK110SP

LPN-RK110SP

83A 83A 83A 83A 83A

200ASwitch

200ASwitch

200ASwitch

200ASwitch

440-22(e) states that if the manufacturer's heater table shows amaximum protective device less than that allowed above, theprotective device rating shall not exceed the manufacturer's values(refer to Section 430-52 also).



29

Using the same logic, if the single secondary main is eliminatedand thermal magnetic circuit breakers are utilized as branch circuitprotection, only three of the motors can be connected because thethermal-magnetic breakers will have been sized at approximately250% of motor F.L.A. (83 x 250% = 207.5A)

Using a 200 ampere circuit breaker would allow three(600 ÷ 200) motors to be connected.

If the single secondary main is eliminated and MCP's areutilized as branch circuit protection, the transformer will beseriously underutilized because only one motor can be connected.For one motor, 1 x 700% of 83 = 581 amperes. For two motors, 2 x700% of 83 = 1162 amperes. Since the sum of the devices cannotexceed 600 amperes, only one motor can be connected when themotor circuit is protected by an MCP.

If the MCP will not hold at the 700% setting due to a high

starting current, it cannot be adjusted beyond 722% (600÷83) andtherefor it may not be able to be used.

If the single secondary main is eliminated, and dual-elementfuses are utilized as branch circuit protection, the transformer cancontinue to be loaded with the five 83 ampere motors because 5 x110 = 550 amperes (which is less than the maximum of 600amperes).

450-3 Covers Protection Requirements for Transformers

M M M

250%

150 KVA

208/120V IFLA = 417A

No Single Secondary Device

83A 83A

200AThermal-MagneticCircuitBreaker

83A



M M M M M

250%

150 KVA

208/120V IFLA = 417A

200ASwitch

LPN-RK110SP

LPN-RK110SP

LPN-RK110SP

LPN-RK110SP

LPN-RK110SP

83A 83A 83A 83A 83A

200ASwitch

200ASwitch

200ASwitch

200ASwitch


450-3(a) Covers Protection Requirements for Transformers Over 600 Volts

M

250%

150 KVA

208/120V IFLA = 417A

Only one motor can be connectedwhen the MCP is utilized.

83A

581A MCP


What is the general content of this Section? This part of the Section sets the overcurrent protection require-ments of transformers (over 600 volts): The primary should beprotected by an individual protective device with fuse rating not inexcess of 300% of the primary's rated current. Secondary sizing isdependent upon whether or not the location is "supervised".

Fuse at300% of F.L.A. of primary

UnsupervisedLocation

Fuse at125% ofF.L.A. of secondary

PRI.over600V

SEC.600Vor less

Z = 6% (or less)



A maximum fuse rating LPS-RK100SP will meet the 125%requirements.

What does this Section require?Current-limiting cable limiters shall be used on each end of the tieconductors, specified per the size of the conductors.

30

What is the general content of this Section?This section covers protection requirements of transformers, 600volts or less. Fusing requirements are shown in the illustratedexample below.

Protection of circuit conductors is required per Articles 240 and310; protection of panelboards per Article 384. Specific sectionswhich should be referenced are Sections 240-3, 240-21 andSection 384-16d.

Note: Transformer overload protection will not be provided by using

overcurrent protective devices sized much greater than the trans-former F.L.A. The limits of 167%, 250% and 300% will not adequatelyprotect transformers. It is suggested that for transformer overloadprotection, the fuse size should be within 125% of the transformer full-load amperes.

There is a wide range of fuse ampere ratings available toproperly protect transformers. FUSETRON® (Class RK5) and LOW-PEAK YELLOW™ (Class RK1) dual-element fuses can often be sizedon the transformer primary and/or secondary, rated as low as 125%of the transformer F.L.A. These dual-element fuses have sufficienttime delay to withstand the high magnetizing inrush currents oftransformers. There is a wide ampere rating selection in the 0 to 15ampere range for these dual-element fuses to provide protection foreven small control transformers.

450-3(b) Covers Protection Requirements for Transformers 600 Volts or Less

450-6(a)(3) Covers Tie Circuit Protection

455-7 Covers Overcurrent Protection Requirements for Phase Converters

460-8(b) Covers Overcurrent Protection of Capacitors

PRIMARY PROTECTION ONLY

Fuse must not belarger than 125%of F.L.A. of primary

No secondaryprotection

PRI. & SEC.600V or less

PRIMARY AND SECONDARY PROTECTION ONLY

Fuse no larger than250% of F.L.A.of primary whensecondary fusesare provided at 125%

125% of F.L.A.of secondary(except as noted)

PRI. & SEC.600V or less

What does this Section mean?

Phase converters can best be protected by dual-element time-delay, current-limiting fuses sized at not more than 125% of theP/C nameplate single-phase input full-load current.

For converters supplying specific leads, the protection shall notbe more than 250% of the load, times the ratio of output to inputvoltage.

Where the required rating does not correspond to a standardrating, sizes up to the next standard rating may be used.

LPS-RK100SP 3Ø Motor

M

P/C

Nameplate =80 Amperes

What are the requirements of this Section?Overcurrent protection must be provided in each ungrounded

conductor supplying a capacitor bank, except for a capacitorlocated on the load side of a motor overload protective device.

The rating of this overcurrent protective device shall be as lowas practical. Generally, dual-element time-delay fuses can besized at 150% to 175% of the capacitor rated current.



501-6(b) Covers Fuses for Class I, Division 2 Locations

What is the importance of the addition of “non-indicating, silver sand,current-limiting type” to Section 501-6(b)(3)?A reference to a “non-indicating, silver sand, current-limiting type”fuse has been added to Section 501-6(b)(3). The intent of thisreference is to suggest the use of non-indicating, filled, current-limiting fuses. The description “silver sand” is used as a generic

reference in describing filled, current-limiting fuses, not as adescription for a specific type of fuse link or filler material. Thefollowing is a partial list of non-indicating fuses which are current-limiting:

Class CC LP-CC 1/2 - 30, KTK-R 3 - 30, FNQ-R 2 - 10Class J LPJ_SP 15 - 600, JKS 0 - 600Class L KRP-C_SP 601 - 6000, KTU 601 - 6000,

KLU 601 - 4000Class RK1 KTN-R 1 - 600, KTS-R 35 - 600

What does this Section require?Compliance with Sections 110-9 and 110-10 is mandatory. Highavailable fault currents require the use of current-limiting fuses.

If the available short-circuit current exceeds the withstand rating of theswitchboard, what can be done to comply with this Section?Use current-limiting fuses on the line side of the switchboard toprovide short-circuit protection.

31

What does this Section mean?If ground fault protection is placed on the main service or feeder ofa health care facility, ground fault protection must also be placedon the next level of feeders. The separation between ground faultrelay time bands for any feeder and main ground fault relay mustbe at least 6 cycles in order to achieve coordination between thesetwo ground fault relays. In health care facilities where no groundfault protection is placed on the main or feeder, no ground faultprotection is necessary at the next level down. Therefore, if therequirements of Sections 230-95 and 215-10 do not require groundfault protection, then no ground fault protection is required on thedownstream feeders either.

If the ground fault protection of the feeder coordinates with the mainground fault protection, will complete coordination between the main andfeeder be assured for all ground faults?No, not necessarily! Merely providing coordinated ground faultrelays does not prevent a main service blackout caused by feederground faults. The overcurrent protective devices must also beselectively coordinated. The intent of Section 517-17 is to achieve“100 percent selectivity” for all magnitudes of ground fault currentand overcurrents. 100% selectivity requires that the overcurrentprotective devices be selectively coordinated for medium and highmagnitude ground fault currents because the conventionalovercurrent devices may operate at these levels. (See discussionof Section 240-12, System Coordination, for a more detailedexplanation of selective coordination).

What is one of the most important design parameters of the powerdistribution system of a health care facility?Selective coordination. To minimize the disruption of power andblackouts in a distribution system, it is absolutely mandatory thatthe overcurrent protective devices be selectively coordinated.

What is selective coordination?A selectively coordinated system is one in which the overcurrentprotective devices have been selected so that only the overcurrentdevice protecting that circuit in which a fault occurs opens; othercircuits in the system are not disturbed. The danger of a majorpower failure in a health care facility such as a hospital is self

evident. In any facility, a rampant power failure is at leastinconvenient, if not quite costly; in a hospital, it can easily give riseto panic and endanger lives. Continuity of electrical service byselective coordination of the protection devices is a must. (SeeSection 240-12, System Coordination, of this Bulletin for a moredetailed explanation of selective coordination).

517-17 Covers Requirements for Ground Fault Protection and Coordination inHealth Care Facilities

20' #12 WIRE2,000 Amperes

AvailableRow of Fluorescent

Fixtures

GLR FuseOpens

FixtureFaulted Ballast Ballasts

20ACIRCUIT

BREAKER

LightingPanel

520-53(f) Covers Protection of Portable Switchboards on Stage

CURRENT-LIMITINGFUSE

50,000Aavailable faultcurrent

Switchboard withstandrating 50,000A when

protected by a current-limiting fuse

What is the importance of 501-6(b)(5)?General Comment–These fuses are used to isolate a faulted fixtureballast and maintain continuity of service. Listed or recognizedbranch circuit or supplementary fuses may be used. Additionally,the GLR fuse is used on ballasts that have a 200 ampere short-circuit withstand rating such as Class P ballasts.



.001

.002

.003

.004

.006

.008

.08

.01

.02

.03

.04

.06

.1

.2

.3

.4

.6

.8

1

2

34

6

810

20

30

40

80

60

100

200

300400

600

8001,000

1

0 0

2

0 0

3

0 0

6

0 0

4

0 0

8

0 0

1 , 0

0 0

2 , 0

0 0

3 , 0

0 0

4 , 0

0 0

6 , 0

0 0

8 , 0

0 0

1 0 , 0

0 0

2 0 , 0

0 0

3 0 , 0

0 0

3 0 , 0

0 0


CURRENT IN AMPERES

400 AMPCircuit BreakerI.T. = 5X

90 AMPCircuitBreaker

90A

4000A

400A

ElevatorController

32

What does this Section mean?Branch circuit fuses installed in a mobile home should not exceedthe rating of the conductors they supply. These fuses should notbe more than 1.5 times the rating of an appliance rated 13.3

amperes or more on a single branch, and not more than the fusesize marked on the air conditioner or other motor operatedappliance.Do these branch circuit fuses conform to the requirements of 550-6(b)?Yes.

Note: If the nameplate on a device states “Maximum Fuse Size”, then fuses that size orsmaller must be used somewhere in the circuit.

What does this Section mean?#18 conductors can be used in control circuits of cranes andhoists if they are fused at not greater than 7 amperes.

What does this Section require?When a feeder supplies more than one elevator, the elevatorovercurrent protective devices must selectively coordinate with allupstream feeders and mains. This requirement was addedbecause the upstream device may be a long distance from thecontroller.

To be "selectively coordinated" means that should a fault (L-G,L-N, L-L, L-L-L) occur anywhere in a system, ONLY the firstovercurrent device upstream of the fault will open. Thus, power ismaintained on all other branches (feeders) in the system.

In the following diagram, a fault at X would trip both the 90ampere breaker and the 400 ampere breaker. This non-selectivitywould be present for all short-circuit current values higher than the

instant trip setting of the 400 ampere breaker. In the example. . .400 x 10 = 4,000 amperes. This results in total loss of power to theother elevators. The aftermath can be PANIC. This installation is aclear VIOLATION of Section 620-62.

If LOW-PEAK YELLOW™ fuses were used (see the SelectivityRatio Guide), a fault at X clears the 90 ampere fuses ONLY. The400 ampere fuses remain intact, thereby maintaining power to theother elevators. This installation is in COMPLIANCE with Section620-62.

550-6(b) Covers Overcurrent ProtectionRequirements for Mobile Homes and Parks

620-62 Covers Selective Coordination ofOvercurrent Protective Devices for Elevators

610-14(c) Covers Conductor Sizesand Protection for Cranes and Hoists

What does this Section mean?If a main overcurrent protective device is provided on a metalworking machine tool, the nameplate shall state, among otherthings, the interrupting capacity of the device. The machine shallalso be marked “overcurrent protection provided at machinesupply terminals”.

#14 Conductor

20A FUSE

Air Conditioning Unitmarked max. fuse 20 Amp.

M

15A FUSE

20A FUSE

60AMAIN

13.3 AmpsAPP

620-62 Covers Selective Coordination of

Overcurrent Protective Devices for Elevators

670-3 Covers Protection of IndustrialMachinery



33

What does this Section require?Emergency lighting systems cannot allow a blackout in any arearequiring emergency illumination due to the failure of any oneelement of the lighting system. Such failures could be caused bythe burning out a light bulb or the opening of a branch circuitprotective device due to a faulted ballast. The solution to the burntout light bulb is to have additional bulb(s) in the area. The solutionto the open branch circuit protective device is to install U.L. 198Glisted supplementary fuses on each ballast. In that way, a faultedballast would be taken off the line by the supplementary fuse, notby the branch circuit protective device, allowing the rest of the

emergency system to remain energized.

The fault in Fixture #3 causes the 20 ampere branch circuit overcurrentdevice to open, causing a blackout in the entire area.

The fault in Fixture #3 will open just the supplementary fuse. The 20ampere branch circuit device does not open and Fixtures 1, 2 and 4remain energized, preventing a blackout.

700-5 Covers Emergency Systems – Their Capacity and Rating

700-16 Covers Emergency Illumination

What does 700-5(a) require?Emergency systems and equipment must be able to handle theavailable short-circuit current at their line side. If the equipmentcannot, it may be damaged, causing additional hazards topersonnel. The use of current-limiting fuses can be a solution tothis high fault current problem.

Fixture No. 1

Fixture No. 2

Fixture No. 4

Fixture No. 3 Fault

VIOLATION

20A

Branch(Opens)

Fixture No. 3Fault

COMPLIANCE

20A

Branch(Remainsenergized)

Fixture No. 1

Fixture No. 2

Fixture No. 4

(Open)

BLACKOUT PREVENTION!Increased Reliability

Fault opens the nearest upstream fuse, allowing other circuits to remain

energized. Reliability of the emergency system is increased.

†Selective coordination of overcurrent protective devices is addressed in PublicationNFPA110 “Emergency and Standby Power Systems”. Section 4-5 (1988) states thatthese devices shall be coordinated to ensure selective tripping of the circuit overcurrentprotective devices when a short-circuit occurs. (Appendix A, Section A-4-5.1 alsoaddresses selective coordination).

What does this fine print note require?In order to maximize the reliability of emergency systems, theovercurrent devices must be selectively coordinated. †Time-current curves of both fuses and circuit breakers must beexamined to determine whether or not only the overcurrent deviceclosest to a fault opens. If additional upstream devices open, thesystem is not selectively coordinated, causing additional sectionsof the emergency system to black out and therefore, reducing thereliability of that system.

BLACKOUT!Reduced Reliability

Fault exceeding the instantaneous trip setting of all three circuitbreakers in series will open all three. This will blackout the entireemergency system.

700-25 Covers Emergency System Overcurrent Protection Requirements (FPN)

1000A

I.T.=10x

225AI.T.=8x.

Opens

Opens

20AI.T.=8A

Opens



Opens20A

225ANotOpen

1000ANotOpen

38-B

VIOLATION

COMPLIANCE



34

What do these three Sections mean?The N.E.C. requires that emergency and standby systems shallhave the capability of safely interrupting the available short-circuitcurrent available at the line terminals of the equipment. Refer toSections 110-9 and 110-10.

701-6 Covers Legally Required Standby Systems – Their Capacity and Rating

702-5 Covers Optional Standby Systems – Their Capacity and Rating

705-16 Covers Interconnected Electric Power Production Sources – TheirInterrupting and Withstand Rating

725-23 Covers Overcurrent Protection for Class 1 Circuits

What does this Section mean?Class 1 Control Circuit Conductors shall be protected by fuses attheir ampacities. In addition, #18 and #16 shall be protected at 7amperes and 10 amperes, respectively.

What must be added to this Control Circuit to comply with 725-23?

A 7 ampere fuse must be added to protect the #18 control wire.

#18 Control Wire

20 Amp

BRANCH

7 Amp Fuse

COMPLIANCE

#18 Control Wire

20 Amp

BRANCH

VIOLATION

760-23 Covers Requirements for Nonpower-Limited Fire Protective SignalingCircuits

What does this provision require?Fire protective signaling circuits with conductors #18 and largermust be protected at their ampacities as shown:

#18 …7 ampere fuse maximum#16 …10 ampere fuse maximum#14 (and larger). . .Max. fuse size as dictated in Section

310-15.

Fuses shall be located at the supply terminals of the conductor.



Buss Fuse Selection Chart (600 Volts or Less).

Circuit Load Ampere Fuse Symbol Voltage Class Interrupting Remarks

Rating Type Rating Rating

(AC) (KA)

Conventional Dimensions–Class RK1, RK5 (1/10-600A), L (601-6000A)

All type loads– 1 / 10 LOW-PEAK LPN-RK_SP 250V* RK1†† 300 All-purpose fuses.

resistive or to YELLOW™ LPS-RK_SP 600V* Unequaled for combined

inductive (optimum 600A (dual-element, short-circuit and

overcurrent time-delay) overload protection.

protection). 601 to LOW-PEAK KRP-C_SP 600V L 300 (Specification grade product)

6000A YELLOW™

(time-delay)

Motors, welders, 1 / 10 FUSETRON® FRN-R 250V* RK5†† 200 Moderate degree of

transformers, to (dual-element, FRS-R 600V* current limitation. Time-delay

capacitor banks 600A time-delay) passes surge currents.

(circuits with heavy 601 to LIMITRON® KLU 600V L 200 General purpose fuse.

Main, inrush currents). 4000A (time-delay) Time-delay passes

Feeder surge-currents.

and Non-motor loads KTN-R 250V RK1†† 200 Same short-circuit protectionBranch (circuits with no 1 KTS-R 600V as LOW-PEAK fuses, but

heavy inrush toLIMITRON®

must be sized larger for

currents). 600A(fast-acting)

circuits with surge currents,

LIMITRON fuses i.e., up to 300%.

suited for circuit 601 to KTU 600V L 200 A fast-acting, high-breaker protection. 6000A performance fuse.

Reduced Dimensions For Installation in Restricted Space–Class J(1-600A), T(1-1200A), CC(1/10-30A), G(1/2-60A)

All type loads LOW-PEAK LPJ_SP 600V* J 300 All-purpose fuses.

(optimum YELLOW™ Unequaled for combined

overcurrent (dual-element, short-circuit and overload

protection). 1 time-delay) protection. (Specification

to grade product).

Non-motor loads 600A LIMITRON® JKS 600V J 200 Very similar to KTS-R

(circuits with no (quick-acting) LIMITRON, but smaller.

heavy inrush 1 to T-TRON™ JJN 300V T 200 The space saver (1 /3 thecurrents). 1200A JJS 600V size of KTN-R/KTS-R).

Control transformer 1 / 10 to 30A LIMITRON® KTK-R 600V CC 200 Very compact (13 /32" x

circuits and lighting (fast-acting) 11 /2"); rejection feature.

ballasts, etc. 1 / 4 to 10A CC-TRON™ FNQ-R Excellent for control

Branch (time-delay) transformer protection .

All type loads - LOW-PEAK LP-CC

especially small 1 / 2 to 30A YELLOW™

HP motors (time-delay)General purpose, 1 / 2 SC SC 300V G 100 Current-limiting;

i.e., lighting to 13 /32" dia. x varying

panelboards. 60A lengths per ampere rating.

Miscellaneous. 1 / 8 ONE-TIME NON 250V H or K5† 10 Forerunners of

to NOS 600V the modern

600A SUPER-LAG® REN 250V H 10 cartridge fuse.

General RENEWABLE RES 600V

Purpose Plug fuses can 1 / 4 FUSTAT® S 125V S 10 Base threads of Type S(non- be used for to (dual-element, differ with ampere ratings.current- branch circuits 30A time-delay) T and W have Edison-base.limiting and small FUSETRON® T 125V ** 10 T & S fuses recommendedfuses) component (dual-element, for motor circuits. W not

protection. time-delay) recommended for circuits

Buss Type W W 125V ** 10with motor loads.

* LPN-RK_SP, 125VDC; LPS-RK_SP, 300VDC. FRN-R, 125VDC; FRS-R, 300VDC; LPJ_SP, 300VDC.

** Listed as Edison-Base Plug Fuse.† Some ampere ratings are available as Class K5 with a 50,000A interrupting rating.

†† RK1 and RK5 fuses fit standard switches, fuseblocks and holders; however, the rejection feature of Class R switches and fuseblocks designed specifically for rejection type fuses(RK1 and RK5) prevent the insertion of the non-rejection fuses (K1, K5 and H).

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