Article 220 Branch-Circuit, Feeder, and Service Calculations

I. General

Figure 220.1 indicates the location in this section of the various calculation requirements.

Figure 220.1. Branch-circuit, feeder, and service calculation methods.

Table 220.3 lists additional load calculation references for specilized areas.

The nominal voltages to be used for branch-circuit and feeder load calculations are 120, 120/240, 208Y/120, 240, 347, 480Y.277, and 600 unless there are others specified. If there is a fraction of an ampere left when making the calculations, then if the fraction is less than 0.5 it can be dropped.

II. Branch-Circuit Load Calculations

Sections 220.12, 220.14, and 220.16 are to be used for branch circuit calculations.

Table 220.12 lists the minimum load required based on floor area, which is calculated using outside dimensions. Where an area cannot be used in dwelling units, garages, and open porches, it does not have to be included. It should be noted that these values are minimum load conditions and 100% power factor and may not provide sufficient capacity for the proposed installation.

[b] See Section 220.14(K).

[a] See Section 220.14(J).

There is a list of requirements for all other loads. An outlet for an appliance or other specific load not covered below is to be calculated on the ampere rating of the appliance or load. The load for electric dryers is based on Section 220.54 and electric ranges and other cooking devices on Section 220.55. Motor loads are to be calculated in accordance with Sections 430.22, 430.24, and 440.6. Luminaires (lighting fixtures) are to be calculated based on the maximum volt-ampere rating of the equipment and lamps. Heavy-duty lampholders are calculated at a minimum of 600 VA. Sign and outdoor lighting is to be calculated at a minimum of 1200 VA for each branch circuit noted in Section 600.5(A). Show windows are calculated at 200 VA per linear foot of window or the unit load as noted elsewhere in this section. Multioutlet assemblies need only be counted as one outlet (180 VA) for each 1.5 m (5-ft.) length unless many appliances are used simultaneously. If appliances are used simultaneously, then each 1.5 m (1-ft.) length is calculated at 180 VA. The portion containing the receptacle outlets can be used for calculations. Receptacle outlets on one strap other than those covered by the calculations in Section 220.14(J) are calculated at 180 VA each. If a single piece of equipment has multiple receptacles, then each receptacle is to be calculated at not less than 90 VA. Receptacles noted in Sections 210.11(C)(1) and (2) are not bound by the previous two sentences. In dwelling occupancies all receptacle outlets as noted in Sections 210.11(C)(3), 210.52(E), 210.52(G), and 210.70(A) and (B) are included in the general lighting load calculations in Section 220.12. In a bank or office building use the larger of either the calculated load of 220.14(I) or 1 volt-ampere/ft2. All other outlets are calculated at 180 VA.

Loads for additions or unwired portions greater than 46.5 square meters (500 square feet) to dwelling units are to be calculated in accordance with Sections 220.12 and 220.14. Additions to circuits should be calculated as noted in Sections 220.12 and 220.14. Loads for other than dwelling units are to be calculated as noted in Section 220.12 and 220.14.

The load should never exceed the rating of the branch circuit. Article 430 applies when only motor-operated loads are used. Article 440 is used for air-conditioning and refrigeration equipment. When more than one motor fastened in place is used, then you use 125% of the largest motor and the sum of the rest of the motor loads. Inductive lighting is calculated on the total ampere rating of the unit including ballasts, transformers, lamps, etc. Range load demand factors can be used in accordance with Table 220.55 including note 4.

III. Feeders and Services

Feeders must be able to supply the required load. They must be sized to supply the connected load with applied allowable demand factors.

To calculate the general lighting load, use the demand factors in Table 220.42.

[1] 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.

Use at least 660 VA per linear meter or 200 VA per linear foot for show windows and 150 VA for every 600 mm (2 ft.) for track lighting in other than dwelling units or hotels and motels. When multi-circuit track is used the load is to be considered divided equally between circuits.

If a receptacle load is calculated in accordance with Section 220.14(H) Fixed Multioutlet Assemblies and 220.14(I) Receptacle Outlets, then the demand factors in Table 220.42 or Table 220.44 can be used.

Calculate motor loads as directed by Sections 430.24, 430.25, and 430.26 and 440.6 for hermetic refrigerant motor compressors.

Generally, the load is to be calculated at 100% of the connected load. In no case can the service or feeder current rating be less than that of the largest branch circuit supplied. If, however, the total load is less as a result of intermittent operation or diversity, the authorities can reduce the calculated load provided the conductors have a rating for the determined load.

(A) Small-Appliance Circuit Load. Use 1500VA for each two-wire small-appliance branch circuit that is required in 210.11(C)(1). When the load is divided through two or more feeders, use 1500VA for each two-wire small-appliance branch circuit. This load can be included in the general lighting load and demand factors of Table 220.42 can be applied.

(B) Laundry Circuit Load. Use not less than 1500VA for each two-wire laundry branch circuit that is required in 210.11(C)(2). This load can be included in the general lighting load and demand factors of Table 220.42.

A demand factor of 75% can be applied to nameplate ratings of four or more appliances in a dwelling supplied by the same feeder or service. This does not apply to electric ranges, clothes dryers, and space heating or air-conditioning equipment.

Use a load of 5000 watts (volt-amperes) or the nameplate rating, whichever is larger. Demand factors listed in Table 220.54 can be used. If two or more single-phase dryers are served by a three-phase four-wire circuit, the total load is calculated at twice the maximum number connected between two phases.

The load for household electric ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking units larger than 1¾ kW are shown in Table 220.55. kVA and kW are considered equivalent for this section.

For single-phase ranges on a three-phase four-wire feeder or service, use twice the number of units between two phases.

This section applies to commercial cooking equipment, dishwasher booster heaters, water heaters, and other kitchen equipment. Demand factors shown in Table 220.56 can be used. They apply to thermostatic controlled or intermittently used kitchen equipment, not air conditioning, space heating or ventilating equipment. The feeder or service must always be larger than the sum of the two largest kitchen equipment loads.

If two loads on the same feeder or service will not be operating simultaneously, the smaller of the two can be omitted for calculation purposes.

The feeder or service neutral load is by definition the maximum unbalanced load between the neutral and one ungrounded conductor. In three-wire two-phase or five-wire two-phase systems the load is then multiplied by 140%. A factor of 70% is used in the case of household electric ranges, wall-mounted ovens, electric dryers, and counter-mounted cooking units, when the maximum unbalanced load for ranges is in accordance with Table 220.55, and for dryers, Table 220.54. A factor of 70% may also be used for load more than 200 A in systems with three-wire dc or single-phase ac; three-phase, four-wire; two-phase, three-wire; and two-phase, five-wire. Reduction of the neutral or the grounded conductor capacity of a 3 wire circuit with two phase wires and neutral of a four-wire wye-connected three-phase system is not permitted. Reduction of the neutral or grounded conductor capacity is also not permitted in a portion consisting of non-linear loads on a 3-phase, 4-wire, wye-connected system.

IV. Optional Feeder and Service Load Calculations

An optional calculation for feeder and service loads is permitted in accordance with this part (Part IV).

(A) Feeder and Service Load. This section can be used to calculate the load for a single three-wire, 120/240-V or 208Y/120-V feeder or service entrance with an ampacity of 100 A or larger. The neutral load can then be determined by Section 220.61. The load is the sum of (B) and (C) as follows:

(B) General Loads. These shall not be less than 100 percent of the first 10 kVA plus 40% of the remainder of the load listed as follows: each laundry and 20-A small appliance circuit at 1500 VA, for general lighting and receptacles use 33 VA per sq. m or 3 VA per sq. ft., the nameplate rating of all fastened in place appliances, all motors and low power factor loads use the nameplate ratings.

(C) Heating and Air Conditioning Load. Use the largest of the following.

1. 100% of the nameplate rating(s) of the air conditioning and cooling.

2. 100% of the nameplate ratings of the heating when a heat pump is used without supplemental heating.

3. 100% of the nameplate ratings of electric thermal storage and other heating systems where the usual load is expected to be continuous at the full nameplate value. Systems qualify under this selection shall not be figured under any other selection in (c).

4. 100% of the nameplate rating of the heat pump compressor and 65% of the supplemental electric for central electric space heating systems. If the heat pump compressor cannot operate at the same time as the supplemental heat, then it does not have to be added to supplemental heat for the total central space heating load.

5. 65% of the nameplate rating(s) of electric space heating if less than four separately controlled units.

6. 40% of the nameplate rating(s) of electric space heating of four or more separately controlled units.

This section applies only to existing 120/240-V and 120/208Y-V three-wire services. It gives an alternate means of calculating loads. Refer to the section in the Code.

This section permits an alternate means of calculation of feeder and service loads when each dwelling is supplied by a maximum of one feeder, has electric cooking, and has electric space heating or air conditioning. The alternate calculation is based on the demand factors shown in Table 220.84. These demand factors apply to the small-appliance branch-circuit load, 33 VA per sq. m or 3 VA per square foot general load, the nameplate rating of all appliances and motors, and the larger of the air-conditioning or space heating loads. House loads are calculated in accordance with Part III of this article and are in addition to the dwelling unit loads calculated by Table 220.84. There is a listing of the connected loads which apply to the table.

Refer to Table 220.86.

Actual demand data can be used if the maximum readings are known for at least 1 year, the existing demand multiplied by 125% and the new loads are not larger than the wire capacity, and proper feeder and service overcurrent protection are provided, in accordance with Sections 240.4 and 230.90, respectively. There is an exception if the maximum demand is not available for 1 year.

Table 220.88 can be used to calculate a service load or a feeder for a new restaurant if the feeder serves the total load. The service-entrance overload protection must be in accordance with Sections 230.90 and 240.4. The feeder conductors do not have to be larger than the service-entrance conductors. If this optional calculation is used, the neutral load of service or feeder conductors is determined by Section 220.61.

V. Farm Load Calculations

Note: Examples for the calculations referred to in Article 220 are shown in Annex D of the NEC®. They are shown in the same location in this book for correlation.