Electrical Service Load Calculator

A dwelling load calculation exists to answer one question: how large does the electrical service have to be so that the main breaker never trips under normal use, and the conductors never carry more current than they can safely dissipate as heat. It is not a measurement of how much electricity a house uses — that is energy, billed in kilowatt-hours. It is a measurement of the worst plausible instant, in volt-amperes.

The interesting part is the word "plausible". A house might contain a 12,000 VA range, a 5,000 VA dryer, a 4,500 VA water heater and a 4,800 VA air conditioner, which is over 26,000 VA before the lighting. If the code required a service that could run all of that simultaneously, every American home would need 200 A. In reality nobody bakes, dries laundry, showers and cools the house at full tilt in the same sixty seconds, and the National Electrical Code encodes that observation as demand factors.

Demand factors are the difference between connected load — the sum of every nameplate — and calculated load, which is what the service is actually sized against. They are conservative, empirically derived, and the reason a 100 A service ran a typical mid-century house perfectly well.

Article 220 of the NEC offers two routes to a dwelling service size, and they are not simplified versions of each other — they apply different factors to different categories and can produce different answers for the same house.

The standard method, in 220.42 through 220.55, handles each category separately: general lighting gets its own graduated demand factors, ranges get a table of their own, dryers another, and four or more fixed appliances get a flat 75%. It is more work and it occasionally produces a smaller number, particularly for a house with an unusually large cooking appliance.

The optional method in 220.82 — the one this calculator implements — throws almost everything into a single pot and applies one rule: the first 10,000 VA at 100%, everything above at 40%. It is far simpler, it is what most residential permits use, and for a typical modern house it is also the more favourable of the two. Where a project is close to a service-size boundary, running both is worth the half hour.

One condition attaches to the optional method: it applies to a dwelling unit served by a single 120/240 V set of service conductors with an ampacity of 100 A or greater. For very small services the standard method is the one available.

Electric vehicle charging is the load that has broken more residential services than anything since central air conditioning, and the code has been catching up with it in stages.

The 2023 NEC added 220.57, which fixes the charger load itself: it is the greater of 7,200 volt-amperes or the equipment nameplate. The 7,200 figure corresponds to a 30 A circuit at 240 V, and it exists so that installing a small charger today does not understate a service that will predictably see a larger one later.

What 220.57 did not settle cleanly is where that load sits within the optional method. If it joins the other general loads in 220.82(B), most of it is absorbed by the 40% demand factor — a 9,600 VA charger contributes only about 3,840 VA to the calculated load. If instead it is added at full value after the demand factors, it contributes the whole 9,600. On a house near a boundary, that is the difference between keeping a 100 A service and paying for a 200 A upgrade, and inspectors genuinely take both positions. This calculator reports both totals rather than pretending the question is resolved.

There is also a third path that avoids the argument. An energy management system — required to be listed for the purpose — monitors the service and reduces or interrupts charging when the rest of the house draws heavily. The code permits the service to be sized on the managed maximum rather than the charger nameplate, and such a device is usually a small fraction of the cost of a service upgrade.

When an electrician quotes a service upgrade to add a charger, the calculation behind it is almost always 220.82 — a from-scratch computation that assumes the house might run everything it contains. For an existing home there is a second, entirely legitimate route that frequently reaches the opposite conclusion.

Section 220.87 permits an existing feeder or service to be evaluated on measured reality. Take the maximum demand the utility recorded over the past twelve months, multiply by 125%, add the new load — continuous loads at 125%, non-continuous at 100% — and compare the total to the service rating. If a 12-month figure is not available, an electrician may connect a recording meter for a minimum of 30 days, taken while the house is occupied and including the larger of the heating or cooling load.

The gap between the two methods is routinely enormous, because real houses draw far less than their nameplates imply. A 2,400 square foot home might calculate at 122 A under 220.82 while its actual recorded peak over a year was 45 A — which at 125% is 56 A, leaving room for a 40 A charger on a 100 A service with amps to spare.

The catch is evidence. This is a documented-measurement route, not an assertion, and an inspector will ask where the number came from. Most utilities will supply interval or peak demand data on request, often through the same online portal that shows the bill.

Passing a load calculation is necessary and not sufficient. Several other limits sit between a number on a worksheet and a working installation.