How to calculate the NEC general lighting load

The first line of every service calc

Look up Tables 220.42(A) and 220.45 in the free NFPA viewer. NFPA offers read-only access to current NEC editions at no cost. This page explains how the tables work; it does not reproduce them. Written to the 2023 NEC — verify against the edition your AHJ has adopted.

It starts with square feet. The general lighting load is a minimum volt-amperes-per-square-foot figure set by occupancy. For dwellings it lives in 220.41 and also covers the general-use receptacles; for everything else, Table 220.42(A) lists unit loads by occupancy type. Multiply the area by the unit load and you have the connected general lighting load — unless the actual lighting is higher, in which case the actual load governs.

Then the demand factors. Table 220.45 recognizes that not everything is on at once. Dwellings, hotels, hospitals, and warehouses step down in blocks — the first block at full value and later blocks at reduced percentages. Most other occupancies, including offices and stores, stay at 100%. In a dwelling, the small-appliance and laundry circuits are added to the lighting total before those steps are applied.

2023 renumbering. Article 220 was reorganized: the old Table 220.12 unit loads are now Table 220.42(A), the dwelling minimum is 220.41, and the demand factors that used to be Table 220.42 are now Table 220.45. If a textbook or old calc sheet says “220.42 demand factors,” it is a 2020-or-earlier reference.

After the lighting load, the calc moves on to appliances, HVAC, and motors — see the cooking appliance demand guide and, for apartment buildings, the multifamily optional method. Size the conductors with the NEC 310.16 ampacity chart, pick the OCPD from the standard breaker sizes guide, lay out circuits on the panel schedule template, and check long runs with the voltage drop calculator.