Size your service the NEC way.

How the Standard Dwelling Calculation Works

The standard method in NEC Article 220, Part III, builds the service load one piece at a time. It starts with the general lighting and receptacle load at 3 volt-amps per square foot, adds the kitchen small-appliance circuits and the laundry circuit at 1500 volt-amps each, then applies the demand factors in Table 220.42 to that whole group. Larger fixed loads such as the range, dryer, fixed appliances, and heating or cooling are added on top with their own rules.

This calculator runs every one of those steps and shows the volt-amps at each stage, so you can see exactly where the final service size comes from.

The Table 220.42 Demand Factors

The combined lighting, small-appliance, and laundry load is not counted at full value. Table 220.42 allows the first 3000 volt-amps at 100 percent, the portion from 3001 to 120000 volt-amps at 35 percent, and anything above that at 25 percent. That reflects the reality that not every light and outlet runs at once.

Range, Dryer, and Noncoincident Loads

An electric range up to 12 kW is counted as 8000 volt-amps under Table 220.55, Column C, with 5 percent added for each kilowatt over 12. The dryer is counted at 5000 volt-amps or its nameplate, whichever is greater, under 220.54. Heating and air conditioning are noncoincident, so NEC 220.60 lets you count only the larger of the two because they never run at the same time.

Standard Method vs Optional Method

The standard method is the detailed approach and is required when a dwelling does not qualify for the simpler optional calculation. For most ordinary one-family homes you can also run the optional method in NEC 220.82, which usually gives a smaller service size. Try both with the same numbers on the Service Load Calculator and compare the results.