Quiz in Progress

1)Homework 1 was?A. Easy

B. Hard

C. Easy except for problem 1

D. Easy except for problem 2

E. Easy except for problem 3

Selecting Electrical Systems

• MLO/MCB

• Aluminum wiring

2) Why do we need a transformer for panel D?

A. Lower voltage power is more efficient

B. Lower voltage power reduces peak demand

C. Lower voltage power is required for duplexes

D. Lower voltage power is safer

E. Transformer manufacturers lobbied to make transformers required under the NEC.

• Duplexes (receptacles)

• Main Lugs Only (remote protection)

• Phase total 60 kVA• 14 kVA (neutral)

• 30 poles

Panel D

• Duplexes (receptacles)• Main lugs only (remote protection)• 3 Phase total 60 kVA, 14kVA (neutral)• 30 poles• 60 kVA / (208V √3 ) = 167A• 200A panel (standard size)• Neutral 14000/120 = 117A• Text Table 12.2, NEC, Tao and Janis (2001) Table 11-2

• Hot wires should be 250 MCM, Neutral #2/0 aluminum

• 2½ inch conduit

Panel D

3) How do you calculate current flow through a neutral conductor in a 3 system?

A. I =√3 E P

B. I =P/ (√3 E )

C. I =P/ E

D. I= E P

4) Why is neutral load not 1/3 of hot load?

A. Because not all power goes through the neutral conductor

B. Because there can be different amounts of current on each hot conductor

C. Because the neutral is oversized for safety reasons

D. A. and B.

E. A., B., and C.

Selecting Electrical Systems

• MLO/MCB

• Aluminum wiring

5) What is the current in the hot conductors upstream of the transformer (I1)?

• E1 = V1 = 480V, E2 = V2 = 208V, I2 = 200A

A. 86 A

B. 100 A

C. 462 A

D. Can’t be determined

E. 149 A

Transformer capacity

• 200∙208∙√3 = 72 kVA ~ 75 kVA

• 200∙208 / 480 = 86 A• 100 A switch

• Neutral 117*120/277 = 50 A

• Three #2 and one #6 aluminum (could use #4)

• 1¼ inch conduit (probably actually 1½ inch)

• 100A fuse - could use 90, but likely same price

Selecting Electrical Systems

• MLO/MCB

• Aluminum wiring

6) What is lighting panel hot conductor current?

A. 65kVA/(480V∙√3) = 78 A

B. 65kVA/(480V) = 135 A

C. 65kVA/(277V∙√3) = 135 A

D. 65kVA/(277V) = 235 A

7) Does the neutral conductor carry more or less current than the hot conductor?

A. More current in neutral

B. Less current in neutral

C. No current in neutral

Lighting Panel

• 100A MLO 20 pole spaces

• Hot: 65kVA/(480V∙√3) = 78 A - #2 Aluminum

• Neutral: 22 kVA /277V = 79 A - #2 Aluminum

• 100 A three pole fused switch

• 1½ inch conduit

Selecting Electrical Systems

• MLO/MCB

• Aluminum wiring

8) The power panel has no neutral and therefore has how many wires in the

conduit at point 4

A. 3 hot + 1 neutral from panel = 4

B. 3 hot + 1 ground = 4

C. 3 hot = 3

D. 3 hot + 1 ground + 1 neutral = 5

E. 1 hot = 1

Power Panel

• No neutral? – all three pole circuits• 33 poles required – chose 42 standard size and

allows for expansion

• 165kVA/(480 √3) = 198 A

• Choose 225A switch (could use 200 A)

• 3 300 MCM aluminum wires – 2 inch conduit

Wireway

• 198A + 86 A + 79 A = 363A

• 400A would allow for some expansion, some would probably go to 600A

• 400A three-pole switch

• Six 250 MCM (unlikely to find conductors big enough to just have 3) and one #3/0 aluminum

• Neutral current = 130A

Summary Panel Sizing

• Find panel power usage and neutral usage (we will learn how to calculate in a minute)

• Size panel conductors and associated conduit and switches

NEC summary (Handout)

1. Minimum capacity2. Grounding necessary3. Only approved wires (voltage/temperature/location)

4. 200, 210, 215, 220 demand calculations5. Conduits, raceways required in commercial buildings

(usually rigid)6. 310 - Number of conductors in raceway7. Restrictions (to residential) on use of NM and NMC

conductors8. Connections and splices must be approved and inspectable9. Need overcurrent/overvoltage/and overload protection10. Need disconnect switches (also 11.)12. Not too many wires in a conduit

Branch Circuits (Handout)

• From panel to loads

• Balance loads on each phase of three phase panel

• NEC 430

Design Guidelines

1. No wire smaller than 14 AWG in residential• 12 AWG for commercial (2)

3. Continuous load per 120 V circuit limited to• 1.2kW for 15 A and 1.5kW for 20 A breaker

4. Continuous load for heavy duty circuits

Demand (kW) Conductor Size (AWG)

2 10

2.5 8

3 6

5. Do not exceed branch circuit rating by:

6. Panelboard to first lighting outlet• Use next highest conductor size if >75 ft• Only allowed >100 ft if <2% voltage drop (7)

8. Panelboard to first receptacle• If >100ft, use bigger than 10 AWG conductors• Can’t use branch circuit for convenience outlets

and display lighting

80 % Portable appliances

70 % Inductive lighting

50 % Fixed appliances

Ref: Tao and Janis (2001)

Ref: Tao and Janis (2001)