Branch Circuits, Feeders, and Service Load Calculations
Core concepts
Article 210 governs branch circuits, Article 215 feeders, Article 220 calculations, and Article 230 services. The single most tested rule is the continuous-load adder: branch-circuit and feeder conductors and overcurrent devices must be sized at 125 percent of the continuous load plus 100 percent of the noncontinuous load. A continuous load is one expected to operate at maximum current for three hours or more.
For dwelling units, use the standard method in 220.42: general lighting at 3 VA per square foot, 1500 VA for each 20 A small-appliance branch circuit, 1500 VA for the laundry circuit, and nameplate ratings for fixed appliances. The general lighting and general-use receptacle load gets a demand factor - first 3000 VA at 100 percent, the remainder at 35 percent. Ranges use Table 220.55 demand factors rather than simple addition.
Non-dwelling receptacle outlets are computed at 180 VA per outlet, and 220.44 demand factors apply to the portion above 10 kVA.
Traps
- Adding 25 percent to a load after applying demand factors instead of to the actual load.
- Counting a receptacle outlet per device instead of per outlet (a duplex counts as one outlet).
- Forgetting the 25 percent adder on the largest motor for service and feeder calculations.
- Ignoring 310.12, which permits a dwelling service or feeder conductor sized at 83 percent of the service rating.
Worked example
A 2000 sq. ft. dwelling: 6000 VA lighting + 3000 VA small appliance + 1500 VA laundry = 10,500 VA. First 3000 at 100 percent = 3000; remaining 7500 at 35 percent = 2625. General load = 5625 VA before appliance and range loads are added.