Electrical Service Load Calculator
What size service a house needs, worked both NEC ways side by side — because the standard and optional methods are both legal and disagree.
The house
Major appliances
Fixed appliances
Four or more fastened in place get a 75% factor, so the fourth one can lower the total.
Heating and cooling
Only the larger of the two counts — they cannot run at once.
Standard
Optional
Two legal methods, two different answers
That is not a flaw in the code, it is how Article 220 is written — and it is why an identical house down the street can legitimately have a smaller service than yours. The standard method applies a different demand factor to each category of load. The optional method (220.82) lumps almost everything together, takes the first 10 kVA at 100% and the rest at 40%, then adds the larger of heating or cooling. It is simpler and it almost always produces the smaller service, which is why most residential work uses it.
- A 12 kW range counts as 8 kW under the standard method, because nobody runs every element flat out for long enough to matter. The optional method uses the full nameplate instead — which is the main structural difference between them.
- The fourth fixed appliance can make your total go DOWN. NEC 220.53 gives a 75% factor once four or more are fastened in place, so adding a small one to three larger ones genuinely reduces the calculated load. It looks like a bug and is exactly what the code says.
- Heating and cooling never both count. 220.60 takes the larger and discards the other, since they cannot run at the same moment.
- Demand factors are not a fudge. Diversity is real — nobody runs the oven, the dryer, the water heater and the air conditioning simultaneously, and sizing as though they might would put a 400 A service on an ordinary house.
- EV charging is what is pushing houses past their service. It is continuous, so it goes in at 125%, and unlike a range or a dryer there is no demand factor to soften it. It is simply added.
A planning worksheet, not a design. Local amendments change the figures and the inspector has the final say. A service change is one of the few electrical jobs where getting it wrong is expensive rather than merely inconvenient — have a licensed electrician run the numbers before ordering anything.
How to use
- Enter the living area and the major appliances at their nameplate ratings.
- List fixed appliances — four or more fastened in place get a 75% factor.
- Enter heating and cooling separately; only the larger one counts.
- Compare the two methods, which are both legal and rarely agree.
Frequently asked questions
Why do the two methods give different answers?
Because Article 220 contains two of them and they work differently. The standard method applies a separate demand factor to each category of load. The optional method in 220.82 lumps almost everything together, takes the first 10 kVA at 100 percent and the remainder at 40, then adds the larger of heating or cooling. Both are legal, the optional one almost always comes out smaller, and that is why an identical house down the street can correctly have a smaller service than yours.
Which method should I use?
The optional method where you are eligible, because it is simpler and usually yields a smaller and cheaper service. It is restricted to a single dwelling served by one 120/240 volt feeder of 100 amps or more, so a subpanel calculation, a multi-family building or anything commercial has to use the standard method. If you are close to a service size boundary it is worth running both, since the difference can be a whole size.
Why is my 12 kW range counted as 8 kW?
Because NEC 220.55 says so, and the reason is that nobody runs every element flat out for long enough to matter. The table gives 8 kVA for any single range up to 12 kW, then adds 5 percent for each kilowatt above that. The optional method takes the opposite approach and uses the full nameplate, which is the main structural difference between the two calculations.
Can adding an appliance really lower my total?
Yes, and it is one of the genuinely counter-intuitive parts of the code. NEC 220.53 applies a 75 percent demand factor once four or more appliances are fastened in place. If you have exactly three large ones, adding a fourth small one triggers the factor and can reduce the calculated load. It looks like an arithmetic error and it is exactly what the article says.
Do heating and air conditioning both count?
No. NEC 220.60 takes the larger of the two and discards the other entirely, on the straightforward grounds that they cannot run at the same time. In a cold climate that usually means the heating load governs and the air conditioning never appears in the calculation at all. Heat pumps are the awkward case, since they are both, and their supplementary resistance heat needs handling carefully.
Why are the demand factors allowed at all?
Because diversity is real. Nobody runs the oven, the dryer, the water heater and the air conditioning at the same moment, and a code that assumed they might would require a 400 amp service on an ordinary three-bedroom house. The factors are derived from measured behaviour rather than invented, and they are the reason a 100 amp service has run American homes adequately for decades.
Will an EV charger mean I need a service upgrade?
Often, and it is the load doing most of the pushing at the moment. Charging runs for more than three hours so it counts as continuous and goes in at 125 percent, and unlike a range or a dryer there is no demand factor to soften it — it is simply added to the total. A 48 amp charger adds 14.4 kVA to the calculation, which is enough to move a typical house from a 150 amp service to a 200. Load management devices that pause charging when other loads run are the usual way around it.
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