Mike Holt, National Electrical Code (NEC) expert discusses how to calculate branch-circuit loads in his latest article:

Article 220 of the 2020 National Electrical Code (NEC) 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., 120V, 120/240V, 120/208V, 240V, 277/480V, 480V) unless other voltages are specified [Sec. 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 dropped [Sec. 220.5(B)].

Calculate the floor area from the outside dimensions of the building, dwelling unit, or other area involved [Sec. 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.

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