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Service Load Calculator

Size a one-family dwelling service the fast way using the NEC optional calculation method (220.82): 100 percent of the first 10 kVA of general load plus 40 percent of the rest, then the largest heating or cooling load.

Heated and habitable floor area in square feet.

Number of 20 amp kitchen small-appliance branch circuits, at least two.

Number of 20 amp laundry branch circuits, normally one.

Combined nameplate volt-amps of every appliance: range, oven, dryer, water heater, dishwasher, disposal, and the like. The optional method uses full nameplate, not the range table.

Nameplate volt-amps of fixed electric space heating. Counted at 65 percent under 220.82(C). Enter 0 for gas heat.

Nameplate volt-amps of air conditioning, counted at 100 percent. The calculator uses the larger of heating or cooling.

The optional method applies to a dwelling served by a single 120/240 volt set of service conductors.

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The Optional Method in NEC 220.82

NEC 220.82 gives a shorter way to size the service for a one-family dwelling, as long as it is served by a single 120/240 volt set of service conductors rated 100 amps or more. Instead of itemizing demand factors one load at a time, it lumps the general loads together and applies a single two-step demand, then adds the largest heating or cooling load separately.

This calculator follows that method and shows both the raw general load and the load after the demand factor, so the result is easy to check against an inspector.

What Counts as the General Load

Under 220.82(B) the general load is the sum of 3 volt-amps per square foot of living area, 1500 volt-amps for each small-appliance and laundry branch circuit, and the full nameplate rating of all appliances, including the range, oven, dryer, and water heater. Unlike the standard method, the optional method does not use the range demand table here. It uses the actual nameplate.

The 10 kVA Demand Step

Once the general load is totaled, 220.82(B) counts the first 10 kVA at 100 percent and everything above 10 kVA at 40 percent. That single demand step replaces the more detailed Table 220.42 factors of the standard method and is why the optional method is faster.

Heating and Cooling Under 220.82(C)

The heating and cooling load is added separately as the largest of several listed values. Air conditioning is taken at 100 percent and central electric space heating at 65 percent, and this calculator uses the larger of the two. Because heating and cooling do not run together, only one of them is counted.

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