The NEC requirements for branch-circuit load calculations are in Art. 220, Part II. These cover three classifications of loads:
- Lighting loads for specified occupancies (220.12)
- Other loads for all occupancies (220.14)
- Loads for additions to existing occupancies (220.16)
Let’s take a closer look at the lighting load requirement now.
The lighting loads must be of a minimum size, depending upon the occupancy. To find the minimum size, look in Table 220.12. If your occupancy isn’t listed, this subsection does not apply (see 220.14 or 220.16, as appropriate).
Using this table, the minimum load for a school (for example) is 3VA per square foot. Obviously, you must know the square footage (calculated from the outside dimensions of the building) to meet this requirement. Just multiply that value by 3, and you have the total VA for lighting. You then allocate that across the number of (typically 20A) branch circuits needed to carry that load.
This table provides only the minimum, and you may need to exceed that for a proper installation. Its values are based on 100% power factor and, according to the Informational Note, may not provide sufficient capacity.
New with the 2015 NEC is an Exception to this minimum load requirement (it was the subject of hot debate for some time). The Exception allows for compliance with energy codes, and it’s permitted if you meet the three requirements, which are shown at the end of 220.12.
