Service Load
Square footage plus what's plugged into the house, run through the optional dwelling calculation — and the service size that carries it.
Estimating only. A service upgrade is permitted, inspected and usually utility-coordinated work — have a licensed electrician run the calculation for the permit.
Service size
100
amp service · calculated load 0 A- General load
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- After demand factor
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- Heating or cooling
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- EV charging
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- Total
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- Headroom
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At 240 volts, the standard residential service.
How the optional dwelling calculation works
NEC 220.82 exists because the standard method — adding every load at full nameplate — sizes houses for a day that never happens. Nobody runs the range, the dryer, the water heater and the air conditioner in the same minute. So the optional method adds the general load honestly, then discounts it: the first 10,000 VA counts in full, and everything past that counts at 40 %.
The general load is three parts. Three volt-amps for every square foot of conditioned floor area covers lighting and general receptacles. Each 20 amp small-appliance circuit in the kitchen and each laundry circuit adds 1,500 VA. Then every fastened-in-place appliance goes in at its nameplate rating. Heating and cooling sit outside the discount — the larger of the two is added at 100 %, because whichever one is running will run hard.
Reading the answer
Divide the total by 240 volts and you have the calculated load in amps. Round up to the next service the utility sells — 100, 125, 150, 200 — and check the headroom line. A one-family dwelling may not be served by less than 100 amps in any case (NEC 230.79(C)), so a small house calculating out at 55 amps still gets a 100 amp service. Under about 20 % spare capacity, think hard before adding an EV charger or a heat pump: those are the two loads that turn a comfortable 150 amp service into a 200 amp upgrade.
On the counter
The assumptions this calculator ships with — every one is adjustable in the form.
- Method
- NEC 220.82 optional calculation for a dwelling unit
- General lighting
- 3 VA per square foot of conditioned floor area
- Branch circuits
- 1,500 VA per small-appliance circuit (two minimum) and per laundry circuit
- Demand factor
- First 10,000 VA at 100 %, everything above it at 40 %
- Heating vs cooling
- The larger of the two at 100 % — they do not run together
- EV charging
- Added at 125 % of nameplate with no demand factor — NEC 625.42 makes it continuous
- Service floor
- 100 amps is the code minimum for a one-family dwelling — NEC 230.79(C)
Questions from the aisle
Do I need a 200 amp service?
Less often than people assume. A 1,200 square foot house with gas heat, a gas range and a window unit computes out near 50 amps; 1,800 square feet with an electric range, dryer and central air lands close to 90. Even an all-electric 2,400 square foot house is around 100. What pushes a service past 150 is a Level 2 EV charger, a heat pump with resistance backup, or both — each of those adds 40 to 60 amps on its own.
Is this the same calculation an electrician does?
It is the optional method in NEC 220.82, the one used for most existing dwellings. The standard method in 220.42 through 220.55 applies different demand factors appliance by appliance and sometimes lands lower. An electrician doing a service upgrade will run one of the two on paper for the permit.
What counts as an appliance load?
Nameplate VA of everything fastened in place or on its own circuit: range, wall oven, cooktop, dryer, water heater, dishwasher, disposal, microwave, well pump, hot tub. Portable things you plug into a general receptacle are already covered by the square-footage allowance. EV charging is the exception that gets its own field above — NEC 625.42 treats it as a continuous load, so it counts at 125 % and never gets the 40 % discount.
Why is my panel full but my load fine?
Breaker spaces and amperage are unrelated. A 100 amp panel with twenty full slots can be well under its calculated load while having nowhere left to land a wire. Tandem breakers or a subpanel solve spaces; only a service upgrade solves amps.
The reasoning behind this one
Does my panel have room for it
Where the assumptions above come from, what changes them, and the mistakes that cost a second trip. Free to read, no account needed.