Size a dwelling service the fast way.
Enter the living area and the major loads to get the total calculated load in VA, the load in amps, and the minimum service size using the NEC 220.82 optional method.
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Load components
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How the optional method works
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.
Common questions
When can I use the optional method?
NEC 220.82 allows the optional method for a one-family dwelling, or an individual dwelling unit, served by a single 120/240 volt set of service conductors with an ampacity of 100 amps or greater.
Why is my optional method result smaller?
The optional method applies a 40 percent demand factor to all general load above the first 10 kVA, which is more generous than the standard method, so it usually produces a smaller calculated load and service size.
Do I use the range nameplate or the table?
In the optional method you use the full nameplate rating of the range and all other appliances. The Table 220.55 range demand is part of the standard method, not the optional one.
How is electric heat counted in the optional method?
Central electric space heating is counted at 65 percent under 220.82(C), and air conditioning at 100 percent. Only the larger of heating or cooling is included because they are noncoincident.
What is the smallest service allowed?
A one-family dwelling service must be at least 100 amps under NEC 230.79(C), even if the calculated load comes out lower.
Estimates only. The optional calculation per NEC 220.82 applies to a one-family dwelling served by a single 120/240 V set of service conductors. Confirm every service with your local code and inspector.