Load Calculations - Part 1, based on the 2020 NEC^®

Figure 01

For EC&M Magazine
By Mike Holt, NEC^® Consultant

Note: This article is based on the 2020 NEC.
Any underlined text denotes a change to the Code for the 2020 NEC.

Do you know how to calculate branch circuit loads?

Article 220 contains the requirements for calculating demand loads for branch circuits, feeders, and services. Table 220.3 lists references for branch-circuit calculations for specific equipment in Chapter 4.

Calculate your branch circuit, feeder, and service loads using nominal system voltages, (e.g., 120, 120/240V, 120/208V, 240V, 277/480V, 480V) unless other voltages are specified [220.5(A)].

What if you do a calculation and get a fraction of an amp? You can round the answer to the nearest whole number, with decimal fractions smaller than 0.50 are dropped [220.5(B)].

Calculate the floor area from the outside dimensions of the building, dwelling unit, or other area involved [220.11]. For dwelling units, the calculated floor area does not include open porches, garages, or unused or unfinished spaces not adaptable for future use.

Lighting load for non-dwelling occupancies
The general lighting load specified in Table 220.12 for non-dwelling occupancies, and the floor area determined per 220.11 must be used to calculate the minimum lighting load [220.12(A)]. When using the unit loads in Table 220.12 for calculating the minimum lighting load for a specified occupancy, never add the 125 percent multiplier for a continuous load [Note 1 to Table 220.12].

Where the building is designed and constructed to comply with an energy code adopted by the local authority, you can calculate the lighting load using the unit values specified in the energy code [220.12(B)]. But to do so, these conditions must be met:

(1) A power monitoring system provides continuous information about the total general lighting load of the building.

(2) The power monitoring system is set with alarm values to alert the building owner or manager if the lighting load exceeds the values set by the energy code. Automatic means are allowed to reduce the connected load.

(3) The demand factors specified in 220.42 are not applied to the general lighting load.

(4) The continuous load multiplier of 125 percent is applied.

What to do with some occupancies might be confusing. The Notes to Table 220.12 provides some clarity with this list:
a. Armories and auditoriums are considered gymnasium-type occupancies.
b. Lodge rooms are similar to hotels and motels.
c. Industrial commercial loft buildings are considered manufacturing-type occupancies.
d. Banks are office-type occupancies.
e. Garages—commercial (storage) are considered parking garage occupancies.
f. Clubs are considered restaurant occupancies.
g. Barber shops and beauty parlors are considered retail occupancies.
h. Stores are considered retail occupancies.

Other loads in occupancies
You'll find requirements for various types of loads in 220.14(A) through (M). Here are some highlights:
(D) Calculate the branch-circuit VA load for luminaires based on the maximum VA rating of the luminaires.

(F) Each commercial occupancy accessible to pedestrians must have at least one 20A sign outlet [600.5(A)] with a minimum branch-circuit load of 1,200 VA or the actual VA if larger.

(G) The branch-circuit VA load for show-window lighting must be calculated per (1) or (2):

(1) 180 VA for each show-window lighting outlet per 220.14(L).

(2) 200 VA for each linear foot of show window [220.43].

(H) Fixed multioutlet assemblies in commercial occupancies must be calculated per (1) or (2):

(1) If appliances are unlikely to be used simultaneously, each 5 ft or fraction of 5 ft, of multioutlet assembly is considered 180 VA.

(2) If appliances are likely to be used simultaneously, each 1 ft or fraction of a ft, of multioutlet assembly is considered 180 VA.

(I) In commercial occupancies, each 15A or 20A, 125V general-use receptacle outlet is considered 180 VA per mounting strap for each, except as covered in 210.14(J) and (K). 

(J) In one-family, two-family, and multifamily dwellings, the minimum unit load is 3 VA per sq ft. The lighting and receptacle outlets specified in 220.14(J)(1), (J)(2), and (J)(3) are included in the 3 VA per sq ft.

(1) General-use receptacle outlets of 20A rating or less, including receptacles connected to the bathroom or garage circuits in accordance with 210.11(C)(3) and 210.11(C)(4).

(2) Receptacle outlets specified in 210.52(E) and (G) for outdoor, garage, and basements.

(3) Lighting outlets specified in 210.70.

No additional load calculations are required for such outlets. Determine the minimum lighting load by using the 3 VA per sq ft unit load and the floor area as determined in 220.11.

There is no VA load for 15A and 20A, 125V general-use receptacle outlets because the loads for those devices are part of the 3 VA per sq foot for general lighting contained in Table 220.14(J) for dwelling units.

Although there is no limit on the number of receptacles on dwelling unit general-purpose branch circuits, the Code does require a minimum number of circuits for general-purpose receptacle and lighting outlets [210.11(A)]. Also, the receptacle and lighting loads must be evenly distributed among the appropriate number of circuits [210.11(B)].

(K) The calculated load for receptacle outlets in office buildings is based on the larger calculation of (1) or (2).

(1) The receptacle outlet load at 180 VA per receptacle yoke [220.14(I)], after all demand factors have been applied.

(2) The receptacle outlet load at 1 VA per sq ft.

Office general receptacle example problem: What is the calculated receptacle load for an 18,000 sq ft office space with one hundred sixty 15A, 125V receptacles? Figure 01
(a) 18,200 VA   (b) 19,400 VA   (c) 20,100 VA   (d) 22,200 VA
Answer: (b) 19,400 VA [220.14(K)(1) and 220.14(I)]
Solution:
160 Receptacles × 180 VA = 28,800 VA
First 10,000 VA at 100% = –10,000 VA × 100% = 10,000 VA
Remainder at 50% = 18,800 VA × 50% = + 9,400 VA
Receptacle Calculated Load = 19,400 VA
[220.14(K)(2)
18,000 × 1 VA per sq ft = 18,000 VA (omit)

(L) 180 VA for each receptacle and lighting outlet not covered in (A) through (K).

(M) In guest rooms or guest suites of hotels and motels, the lighting and receptacle outlets specified in 220.14(M)(1), (M)(2), and (M)(3) are included in the 1.70 VA per sq ft unit load in Table 220.12:

(1) General-use receptacle outlets of 20A rating or less, including receptacles connected to the bathroom or garage circuits per 210.11(C)(3) and 210.11(C)(4).

(2) Receptacle outlets specified in 210.52(E)(3) for balconies, decks, and porches.

(3) Lighting outlets specified in 210.70.

No additional load calculations are required for such outlets. The minimum lighting load is to be determined using the 1.70 VA per sq ft unit load and the floor area as determined in 220.11.

Maximum loads
Where a circuit supplies only a motor, the conductor and the overcurrent protection are sized per Article 430 [220.18(A)]. Where a circuit supplies only air-conditioning equipment, Article 440 applies.

Branch circuits that supply inductive and LED driver lighting loads must be sized to the ampere rating of the luminaire, not to the wattage of the lamps. [220.18(B)].

You can apply demand factors to range loads, per Table 220.55 [220.18(C)].

Eliminating errors
The rules for lighting load calculations vary by occupancy type. Remember that any given area served by a branch circuit can be a different kind of occupancy from the building it's in; for example, a gym within a police station.

Review your completed calculations to ensure you have included the applicable demand factors and continuous load multipliers. Omissions here are responsible for most branch circuit calculation errors.