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Section 3: Load PlanningBill Brown, P.E., Square D Engineering Services

Basic PrinciplesThe most vital, but often the last to be acquired, pieces of information for power system design are the loaddetails. An important concept in load planning is that due to non-coincident timing, some equipment operating at less than rated load, and some equipment operating intermittently rather than continuously, the total demandupon the power source is always less than the total connected load [1]. This concept is known as load diversity. The following standard definitions are given in [1] and [2] and are tools to quantify it:

Demand: The electric load at the receiving terminals averaged over a specified demand interval. of time, usually15 min., 30 min., or 1 hour based upon the particular utility’s demand interval. Demand may be expressed inamperes, kiloamperes, kilowatts, kilovars, or kilovoltamperes.

Demand Interval: The period over which the load is averaged, usually 15 min., 30 min., or 1 hour.

Peak Load: The maximum load consumed or produced by a group of units in a stated period of time. It may bethe maximum instantaneous load or the maximum average load over a designated period of time.

Maximum Demand: The greatest of all demands that have occurred during a specified period of time such asone-quarter, one-half, or one hour. For utility billing purposes the period of time is generally one month.

Coincident Demand: Any demand that occurs simultaneously with any other demand.

Demand Factor: The ratio of the maximum coincident demand of a system, or part of a system, to the totalconnected load of the system, or part of the system, under consideration, i.e.,

(3-1)

Diversity Factor: The ratio of the sum of the individual maximum demands of the various subdivisions of asystem to the maximum demand of the whole system, i.e.,

(3-2)

where

Di = maximum demand of load i, regardless of time of occurrence.

DG = coincident maximum demand of the group of n loads.

Using (1), the relationship between the diversity factor and the demand factor is

(3-3)

where

TCLi = total connected load of load group i

DFi = the demand factor of load group i

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Load Factor: The ratio of the average load over a designated period of time to the peak load occurring in that period, i.e.,

(3-4)

If T is the designated period of time, an alternate formula for the load factor may be obtained by manipulating (3-4) as follows:

(3-5)

These quantities must be used with each type of load to develop a realistic picture of the actual load requirementsif the economical sizing of equipment is to be achieved. Further, they are important to the utility rate structure(and thus the utility bill).

As stated in [2], the following must be taken into account in this process:

� Load Development/Build-Up Schedule – Peak load requirements, temporary/construction power requirements, and timing

� Load Profile – Load magnitude and power factor variations expected during low-load, average load, and peak load conditions

� Expected Daily and Annual Load Factor

� Large motor starting requirements

� Special or unusual loads such as resistance welding, arc welding, induction melting, induction heating, etc.

� Harmonic-generating loads such as variable-frequency drives, arc discharge lighting, etc.

� Forecasted load growth over time

Reference [4] and individual engineering experience on previous projects are both useful in determining demandfactors for different types of loads. In addition, the National Electrical Code® [3] gives minimum requirements forthe computation of branch circuit, feeder, and service loads.

NEC Basic branch circuit requirementsNEC [3] Article 220 gives the basic requirements for load calculations for branch circuits, feeders, and services.In order to understand these requirements, the basic NEC definitions of branch circuit, feeder, and service mustbe understood, along with several other key terms:

Branch Circuit: The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).

Feeder: All circuit conductors between the service equipment, the source of a separately derived system, or otherpower supply source and the final branch-circuit overcurrent device.

Service: The conductors and equipment for delivering electric energy from the serving utility to the wiring systemof the premises served.

Outlet: The point on the wiring system at which current is taken to supply utilization equipment.

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Receptacle: A receptacle is a contact device installed at the outlet for the connection of an attachment plug. A single receptacle is a single contact device with no other contact device on the same yoke. A multiple receptacleis two or more contact devices on the same yoke.

Continuous Load: A load where the maximum current is expected to continue for three hours or more.

The NEC definition of Demand Factor is essentially the same as given above.

� Minimum lighting load (Article 220.12): Minimum lighting load must not be less than as specified in table 3-1(NEC Table 220.12):

Table 3-1: General lighting loads by occupancy (NEC [3] table 220.12)

a See NEC Article 220.14(J)b See NEC Article 220.14(K)

� Motor Loads (Article 220.14(C)): Motor loads must be calculated in accordance with Articles 430.22, 430.24, and 440.6, summarized as follows:):� The full load current rating for a single motor used in a continuous duty application is 125% of the motor’s

full-load current rating as determined by Article 430.6, which refers to horsepower/ampacity tables 430.247,430.248, 430.249, or 430.250 as appropriate (Article 430.22).

� The load calculation for several motors, or a motor(s) and other loads, is 125% of the full load current ratingof the highest rated motor per a.) above plus the sum of the full-load current ratings of all the other motors inthe group, plus the ampacity required for the other loads (Article 430.24).

Type of Occupancy Unit Load Volt-Amperes Per quare Meter

Unit Load Volt-Amperes per Square Foot

Armories and auditoriums 11 1

Banks 39b 3.5b

Barber shops and beauty parlors 33 3

Churches 11 1

Clubs 22 2

Court Rooms 22 2

Dwelling Unitsa 33 3

Garages – commercial (storage) 6 0.5

Hospitals 22 2

Hotels and motels, including apartment houses without provision for cooking by tenantsa

22 2

Industrial commercial (loft) buildings 22 2

Lodge rooms 17 1.5

Office buildings 39b 3.5b

Restaurants 22 2

Schools 33 3

Stores 33 3

Warehouses (storage) 3 0.25

In any of the preceding occupancies except one- family dwellings andindividual dwelling units of two-family and multi-family dwellings:

Assembly halls and auditoriums 11 1

Halls, corridors, closets, stairways 6 0.5

Storage Spaces 3 0.25

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� For hermetic refrigerant motor compressors or multi-motor equipment employed as part of air conditioning orrefrigerating equipment, the equipment nameplate rated load current should be used instead of the motorhorsepower rating (Article 440.6).

� Luminaires (lighting fixtures) (Article 220.14(D)): An outlet supplying luminaire(s) shall be calculated based uponthe maximum volt-ampere rating of the equipment and lamps for which the luminaire(s) is rated.

� Heavy-Duty Lampholders (Article 220.14(E)): Loads f for heavy-duty lampholders must be calculated at aminimum of 600 volt-amperes.

� Sign and outline lighting (Article 220.14(F)): Sign and outline lighting loads shall be calculated at a minimum of1200 volt-amperes for each required branch circuit specified in article 600.5(A).

� Show windows (Article 220.14(G)): Show windows can be calculated in accordance with either:� The unit load per outlet as required in other provisions of article 220.14.

� 200 volt-amperes per 300mm (1ft.) of show window.

� Loads for fixed multioutlet assemblies in other than dwelling units or the guest rooms and guest suites of hotelsor motels must be calculated as follows (Article 220.14(H)):� Where appliances are unlikely to be used simultaneously, each 1.5m (5 ft.) or fraction thereof of each

separate and continuous length must be considered as one outlet of 180 volt-amperes.

� Where appliances are likely to be used simultaneously, each 300mm (1 ft.) or fraction thereof must beconsidered as an outlet of 180 volt-amperes.

� Receptacle outlets (Articles 220.14(I), 220.14(J), 220.14(K), 220.14(L)): Loads for these are calculated as follows:� Dwelling occupancies (Article 220.14(J)): In one-family, two-family, and multifamily dwellings and in guest

rooms or guest suites of hotels and motels, general-use receptacle outlets of 20A rating or less are includedin the general lighting load per above. No additional load calculations are required for these.

� Banks and office buildings (Article 220.14(K)): Receptacle outlets must be calculated to be the larger ofeither the calculated value per c.) below or 11 volt-amperes/square meter (1 volt-ampere per square ft.).

� All other receptacle outlets (Article 220.14(I)): Each receptacle on one yoke must be calculated as 180 volt-amperes. A multiple receptacle consisting of four or more receptacles must be calculated at 90 volt-amperes per receptacle.

� Sufficient branch circuits must be incorporated into the system design to serve the loads per Article 220.10(summarized 1.) – 8.) above), along with branch circuits for any specific loads not covered in Article 220.10. The total number of branch circuits must be determined from the calculated load and the size or rating of thebranch circuits used. The load must be evenly proportioned among the branch circuits (Article 210.11(C)). In addition, Article 210.11(C) requires several dedicated branch circuits as follows for dwelling units:� Two or more 20A small-appliance branch circuits (Article 210.11(C)(1)).

� One or more 20A laundry branch circuits (Article 210.11(C)(2)).

� One or more bathroom branch circuits (Article 210.11(C)(3)).

� Continuous Loads (Article 210.20): The rating of the overcurrent protection for a branch circuit must be at leastthe sum of the non-continuous load +125% of the continuous load unless the overcurrent device is 100%-rated.Because the rating of the overcurrent protection determines the rating of the branch circuit (Article 210.3), thebranch circuit must be sized for the non-continuous load +125% of the continuous load. In load calculations,continuous loads should therefore be multiplied by 1.25 unless the circuit overcurrent device is 100% rated.Note that motor loads are not included in this calculation as the 125% factor is already included in the applicablesizing per above.

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NEC Basic Feeder Circuit Sizing RequirementsOnce the branch circuit loads are calculated, the feeder circuit loads may be calculated by applying demandfactors to the branch circuit loads.

� General Lighting Loads (Article 220.42): The feeder general lighting load can be calculated by multiplying thebranch circuit general lighting load calculated per B.) 1.) above, for those branch circuits supplied by the feeder,by a demand factor per table 3-2 (NEC table 220.42).

Table 3-2: Lighting load feeder demand factors (NEC [3] table 220.42)

� Show window or track lighting (Article 220.43): Show windows must use a calculated value of 660 volt-amperes per linear meter (200 volt-amperes per linear foot), measured horizontally along its base. Tracklighting in other than dwelling units must be calculated at an 150 volt-amperes per 660mm (2 ft.) of lightingtrack or fraction thereof.

� Receptacles in other than dwelling units (Article 220.44): Demand factors for non-dwelling receptacle loadsare given in table 3-3 (NEC table 220.44).

Table 3-3: Demand factors for non-dwelling receptacle loads (NEC [3] table 220.44)

Type of Occupancy Portion of Lighting Load to Which Demand Factor Applies(Volt-Amperes)

Demand Factor(Percent)

Dwelling units First 3,000 or less atFrom 3,001 to 120,000 atRemainder over 120,000 at

1003525

Hospitals* First 50,000 or less atRemainder over 50,000 at

4020

Hotels and motels, including apartment houses withoutprovision for cooking by tenants*

First 20,000 or less atFrom 20,001 to 100,000 atRemainder over 100,000 at

504030

Warehouses (storage) First 12,500 or less atRemainder over 12,500 at

10050

All others Total volt-amperes 100

* The demand factors of this table shall not apply to the calculated load of feeders or services supplying areas in hospitals, hotels,and motels where the entire lighting is likely to be used at one time, as in operating rooms, ballrooms, or dining rooms.

Portion of Receptacle Load to Which Demand Factor Applies (Volt-Amperes) Demand Factor (Percent)

First 10 kVA or less at 100

Remainder over 10 kVA 50

� Motors (Article 220.50): The feeder demands for these are calculated as follows:� The load calculation for several motors, or a motor(s) and other loads, is 125% of the full load current rating

of the highest rated motor per II.) B.) ii.) above plus the sum of the full-load current ratings of all the othermotors in the group, plus the ampacity required for the other loads (Article 430.24).

� The load calculation for factory-wired multimotor and combination-load equipment should be based upon theminimum circuit ampacity marked on the equipment (Article 430.25) instead of the motor horsepower rating.

� Where allowed by the Authority Having Jurisdiction, feeder demand factors may be applied based upon theduty cycles of the motors. No demand factors are given in the NEC for this situation.

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� Fixed Electric Space Heating (Article 220.51): The feeder loads for these must be calculated at 100% of the connected load.

� Noncoincident Loads (Article 220.60): Where it is unlikely that two or more noncoincident loads will be in usesimultaneously, it is permissible to use only the largest loads that will be used at one time to be used incalculating the feeder demand.

� Feeder neutral load (Article 220.61): The feeder neutral load is defined as the maximum load imbalance on thefeeder. The maximum load imbalance for three-phase four-wire systems is the maximum net calculated loadbetween the neutral and any one ungrounded conductor. A demand factor of 70% may be applied to thiscalculated load imbalance. Refer to NEC article 220.61 for neutral reductions in systems other than three-phase, four-wire systems. This demand factor does not apply to non-linear loads; in fact, it may benecessary to oversize the neutral due to current flow from non-linear load triplen harmonics.

� Continuous Loads (Article 215.3): The rating of the overcurrent protection for a feeder circuit must be at least the sum of the non-continuous load +125% of the continuous load, unless the overcurrent device is 100%-rated.Because the rating of the overcurrent protection determines the rating of the branch circuit (Article 210.3), the branch circuit must be sized for the non-continuous load +125% of the continuous load. In the final feedercircuit load calculation, the continuous portion of the load should therefore be multiplied by 1.25 unless theovercurrent device for the circuit is 100%-rated. Note that motor loads are not included in this calculation as the125% factor is already included in the applicable sizing per above.

Additional calculation data is given in NEC Article 220 for dwelling units, restaurants, schools, and farms. Thisdata is not repeated here. Refer to NEC Article 220 for details.

As this guide only presents the basic NEC requirements for load calculations, it is imperative to refer to the NECitself when in doubt about a specific load sizing application. Computer programs are commercially available toautomate the calculation of feeder and branch circuit loads per the NEC methodology described above.

ReferencesBecause the subject matter for this section is basic and general to the subject of electrical engineering, it isincluded in most undergraduate textbooks on basic circuit analysis and electric machines. Where material isconsidered so basic as to be axiomatic no attempt has been made to cite a particular source for it.

For material not covered per the above, references specifically cited in this section are:

[1] IEEE Recommended Practice for Electric Power Distribution for Industrial Plants, IEEE Standard 141-1993, December 1993.

[2] Turan Gonen, Electric Power Distribution System Design, New York: McGraw-Hill, 1986, pp. 37-51.

[3] The National Electrical Code, NFPA 70, The National Fire Protection Association, Inc., 2005 Edition.

[4] IEEE Recommended Practice for Electric Power Systems in Commercial Buildings, IEEE Standard 241-1990, December 1990.