Load Planning

The most vital, but often the last to be acquired, pieces of information for power system design are the load details. 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 demand upon the power source is always less than the total connected load (see IEEE Standard Terminal Markings and Connections for Distribution and Power Transformers IEEE Std. C57.12.70-2000.*). This concept is known as load diversity. The following standard definitions are given in IEEE Standard Terminal Markings and Connections for Distribution and Power Transformers IEEE Std. C57.12.70-2000.* and Electric Power Distribution System Design, New York Turan Gonen, : McGraw-Hill, 1986, p. 137. *and are tools to quantify it:

Demand: The electric load at the receiving terminals averaged over a specified demand interval. of time, usually 15 min., 30 min., or one hour based upon the particular utility’s demand interval. Demand may be expressed in amperes, kiloamperes, kilowatts, kilovars, or kilovolt amperes.

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

Peak Load: The maximum load consumed or produced by a group of units in a stated period of time. It may be the 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 as one-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 total connected load of the system, or part of the system, under consideration, that is:

Diversity Factor: The ratio of the sum of the individual maximum demands of the various subdivisions of a system to the maximum demand of the whole system, that is:

(3–2)

Where:

• D_i = maximum demand of load i, regardless of time of occurrence.

• D_g = 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:

• TCL_i = total connected load of load group i.

• DF_i = the demand factor of load group i.

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

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

These quantities must be used with each type of load to develop a realistic picture of the actual load requirements if 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 Electric Power Distribution System Design, New York Turan Gonen, : McGraw-Hill, 1986, p. 137. *, the following must be considered 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.

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

• Forecasted load growth over time.

Reference IEEE Recommended Practice for Electric Power Systems in Commercial Buildings IEEE Standard 241-1990, December 1990. *: and individual engineering experience on previous projects are both useful in determining demand factors for different types of loads. In addition, the National Electrical Code® NFPA 70, The National Fire Protection Association, Inc., 2020 Edition.* gives minimum requirements for the computation of branch circuit, feeder, and service loads.

NEC National Electrical Code® NFPA 70, The National Fire Protection Association, Inc., 2020 Edition.* Article 220 gives the basic requirements for load calculations for branch circuits, feeders, and services. To understand these requirements, the basic NEC definitions of branch circuit, feeder, and service must be understood, along with several other key terms:

Branch Circuit: The means to carry electric power from the final overcurrent device protecting the circuit to the outlets and equipment in residential, commercial, and industrial locations.

Feeder: All circuit conductors between the service equipment, the source of a separately derived system, or other power 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 system of the premises served.

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

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 receptacle is 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. For example, a single-phase 120 V circuit feeding an open-office lighting load (continuous) of 1,000 VA and a small cooling unit's condensate pump load (non-continuous) of 100 VA.

The NEC definition of Demand Factor is essentially the same as given above, so the points below must be considered when calculating the Demand Factor.

Minimum lighting load (Article 220.12): Minimum lighting load must not be less than as specified in Table 1 (NEC Table 220.12). Motors rated less than 1/8 HP and connected to a lighting circuit shall be considered general lighting load.

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 rating of the highest rated motor per a.) in Table 1 plus the sum of the full-load current ratings of all the other motors in the group, plus the ampacity required for the other loads (Article 430.24).

• For hermetic refrigerant motor compressors or multi-motor equipment employed as part of air conditioning or refrigerating equipment, use the equipment nameplate rated load current instead of the motor horsepower rating (Article 440.6). 

Luminaires (lighting fixtures) (Article 220.14(D)): Calculate an outlet supplying luminaire(s) based on the maximum VA rating of the equipment and lamps for which the luminaire(s) is rated.

Heavy-duty Lampholders (Article 220.14(E)): Calculate loads for heavy-duty lampholders at a minimum of 600 VA.

Sign and Outline Lighting (Article 220.14(F)): Calculate sign and outline lighting loads at a minimum of 1200 VA for each required branch circuit specified in article 600.5(A).

Show Windows (Article 220.14(G)): Calculate show windows in accordance with either:

• The unit load per outlet as required in other provisions of article 220.14.

• 200 VA per 300 millimeters (one foot) of show window.

Loads for Fixed Multi-outlet Assemblies: Calculate these in other than dwelling units or the guest rooms and guest suites of hotels or motels as follows (Article 220.14(H)):

• Where simultaneous use of appliances is unlikely, each 1.5 meters (five feet.) or fraction thereof of each separate and continuous length, must be considered as one outlet of 180 VA.

• Where simultaneous use of appliances is unlikely, each 300 millimeters (1 foot) or fraction thereof must be considered as an outlet of 180 VA. 

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, the minimum unit load shall be not less than 33 VA/m² (3 VA/ft²). The lighting and receptacle outlets specified in 220.14(J)(1), (J)(2), and (J)(3) are included in the minimum unit load. No additional load calculations are required for such outlets. The minimum lighting load is determined using the minimum unit load and the floor area as determined in 220.11 for dwelling occupancies. Motors rated less than 1⁄8 hp and connected to a lighting circuit are considered part of the minimum lighting load.

  (1) All general-use receptacle outlets of 20 A rating or less, including receptacles connected to the circuits in 210.11(C)(3) and 210.11(C)(4).

  (2)  The receptacle outlets specified in 210.52(E) and (G).

  (3)  The lighting outlets specified in 210.70.

• Office buildings (Article 220.14(K)): Calculate receptacle outlets to be the larger of either the calculated value per c.) below or 11 VA/m² (one VA/per square foot).

• All other receptacle outlets (Article 220.14(I)): Calculate each receptacle on one yoke as 180 VA. Calculate a multiple receptacle consisting of four or more receptacles at 90 VA per receptacle. 

Sufficient Branch Circuits: Incorporate sufficient branch circuits 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. Determine the total number of branch circuits from the calculated load and the size or rating of the branch circuits used. Evenly proportion the load 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 20 A small-appliance branch circuits (Article 210.11(C)(1)). 

• One or more 20 A 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 branch circuit overcurrent protection 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 overcurrent protection rating 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 load calculations, continuous loads should therefore be multiplied by 1.25 unless the circuit overcurrent device is 100% rated. Motor loads are not included in this calculation as the 125% factor is already included in the applicable sizing per above. 

Once the branch circuit loads are calculated, the feeder circuit loads may be calculated by applying demand factors to the branch circuit loads, so the points below must be considered when calculating the Demand Factor.

General Lighting Loads (Article 220.42): Calculate the feeder general lighting load by multiplying the branch circuit general lighting load calculated per the first bullet of Motor Loads (Article 220.14(C)) in NEC Basic Branch Circuit Requirements, for those branch circuits supplied by the feeder, by a demand factor per Table 2 (NEC Table 220.42). The demand factors specified in Table 220.42 apply to that portion of the total branch-circuit load calculated for general illumination. They do not be applied in determining the number of branch circuits for general illumination.

• Show Window or Track Lighting (Article 220.43): Show windows must use a calculated value of 660 VA per linear meter (200 VA per linear foot), measured horizontally along its base. Track lighting in other than dwelling units must be calculated at an 150 VA per 660 millimeters (two feet) of lighting track or fraction thereof. Where multi-circuit track is installed, the load is divided equally between the track circuits.

• Receptacles in Other than Dwelling Units (Article 220.44): Demand factors for non-dwelling receptacle loads are given in Table 3 (NEC Table 220.44) and Table 2 (NEC Table 220.42).

Motors (Article 220.50): Motor loads shall be calculated in accordance with 430.24, 430.25, and 430.26. 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 the second bullet of Motor Loads (Article 220.14(C)) in NEC Basic Branch Circuit Requirements plus the sum of the full-load current ratings of all the other motors in the group, plus the ampacity required for the other loads (Article 430.24). 

• Base the load calculation for factory-wired multimotor and combination-load equipment on the minimum 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 on the duty cycles of the motors. No demand factors are given in the NEC for this situation. 

Fixed Electric Space Heating (Article 220.51): Calculate the feeder loads for these at 100% of the connected load. However, in no case shall a feeder or service load current rating be less than the rating of the largest branch circuit supplied. 

Noncoincident Loads (Article 220.60): While unlikely that two or more non-coincident loads will be in use simultaneously, it is permissible to use only the largest loads for use at one time in calculating the feeder demand. 

Feeder neutral load (Article 220.61): The feeder neutral load is defined as the maximum load imbalance on the feeder. The maximum load imbalance for three- phase four-wire systems is the maximum net calculated load between the neutral and any one ungrounded conductor. A service or feeder supplying the following loads such as a feeder or service supplying household electric ranges, wall-mounted ovens, counter-mounted cooking units, and electric dryers and the unbalanced load in excess of 200 A shall be permitted to have an additional demand factor of 70%. 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 be necessary to oversize the neutral due to the current flow from non-linear load triple 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), size the branch circuit for the non-continuous load + 125% of the continuous load. In the final feeder circuit load calculation, the continuous portion of the load should therefore be multiplied by 1.25, unless the overcurrent device for the circuit is 100%-rated. Motor loads are not included in this calculation as the 125% 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. This data is not repeated here.

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