Load calculation

It is not a sum of everything

Add every nameplate rating in a modern house at full power and you would conclude that most homes need a service several times larger than they have. That number describes a moment that never happens.

So the calculation applies demand factors: the first portion of a load counts fully, the remainder counts at a reduced percentage, on the reasoning that you will not run the oven, the dryer, the air conditioning and the car charger simultaneously at maximum. The result is a realistic worst case rather than an impossible one.

What goes into it

Living area drives a base figure — a bigger house has more lighting and more general receptacles. On top of that come the loads big enough to be counted individually: electric range, water heater, air conditioning or electric heat, and any EV charger.

Heating and cooling are usually counted as one, taking whichever is larger, because a house does not heat and cool at the same time.

Two methods, take the larger

The code provides more than one way to arrive at the answer, and the requirement is to size the service to the greater result. This is why a calculator can show you two numbers and then tell you which one governs.

It is a floor, not a target. Meeting it means the service is legal for the loads declared, not that there is room for whatever you add next.

Why an on-site check still happens

The calculation runs on what you tell it. It cannot see that the panel is a discontinued model with no breakers available, that half the circuits are ungrounded, or that a previous owner added a subpanel nobody mentioned. The number is sound; the property is the variable.