Residential Load Calculator

This calculator implements the optional method of NEC 220.82, which is what gets used for the large majority of ordinary single-family services. It applies to a dwelling unit fed by one 120/240 V or 208Y/120 V three-wire service or feeder rated 100 A or more — effectively every detached house built in the last seventy years.

The calculation has two halves that behave completely differently, and keeping them apart is most of what there is to understand.

The first half is the general load, and everything in it goes in at full value before being discounted once at the end. Floor area contributes 3 VA per square foot, which covers all the general lighting and general-use receptacles — you do not count them individually. Each 20 A small-appliance branch circuit adds 1,500 VA and the laundry circuit adds another 1,500 VA. Then every fixed appliance is added at raw nameplate: the range, the dryer, the water heater, the dishwasher, the disposal. No per-appliance table, no 75% reduction for four or more appliances. That absence is the difference between this method and the standard one.

The single demand factor comes next. The first 10,000 VA of that subtotal counts at 100%; everything above it counts at 40%. The reasoning is diversity — nothing about a house makes the range, the dryer and the water heater run at nameplate simultaneously, so sizing a service as though they might would buy copper that never carries current. The first 10,000 VA is not discounted because something in a house is always drawing.

The second half is heating and cooling, and it sits outside the factor entirely. Take the larger of the two loads, never the sum, and take it at 100%. Both parts of that rule matter: a house does not heat and cool at once, so adding them sizes for a condition that cannot occur — but whichever one is running on a design day runs flat out, so it gets no discount either.

Add the demand load to the HVAC load, divide by the service voltage, and round up to a service size that can actually be bought. A one-family dwelling never gets less than 100 A whatever the arithmetic says.

A 2,200 ft² house: two small-appliance circuits, a laundry circuit, and five fixed appliances — a 5,000 VA dryer, an 8,000 VA range, a 4,500 VA water heater, a 1,200 VA dishwasher and a 900 VA disposal. Cooling is 4,500 VA, heating 1,000 VA (the blower on a gas furnace). Service voltage 240 V.

General load. Area first: 3 × 2,200 = 6,600 VA. Small-appliance circuits: 2 × 1,500 = 3,000 VA. Laundry:1,500 VA. The five appliances sum to 5,000 + 8,000 + 4,500 + 1,200 + 900 = 19,600 VA — note how completely the appliances dominate; they are nearly three times the floor-area contribution. That gives a subtotal of 6,600 + 3,000 + 1,500 + 19,600 = 30,700 VA.

The demand factor. Split the subtotal at 10,000. The first 10,000 VA passes through untouched. The remainder is 30,700 − 10,000 = 20,700 VA, taken at 40%: 20,700 × 0.40 = 8,280 VA. The demand load is therefore 10,000 + 8,280 = 18,280 VA. The factor has just removed 12,420 VA — 40% of the connected load has vanished, and that single step is what makes a 100 A service legal here.

HVAC. Cooling at 4,500 VA is larger than heating at 1,000 VA, so 4,500 VA is added at 100% and the 1,000 VA heater is not counted at all. Total: 18,280 + 4,500 = 22,780 VA.

The answer. 22,780 ÷ 240 = 94.92 A, which rounds up to a 100 A service.

And now the part worth pausing on: 94.92 A is 94.9% of a 100 A service. Five amps of headroom. This house complies today and has almost nothing left — swap the gas furnace for a heat pump, or add an EV charger, and it needs a new service before it needs anything else. If the owner has any plan at all for the next decade, the conversation to have here is about 125 A or 150 A, and it costs far less to have it before the meter base goes on the wall than after.

Service equipment is not sold in every rating the NEC recognises. 240.6(A) lists 110 A, 175 A and 225 A as standard overcurrent ratings, but no supply house stocks a residential load centre at any of them, so a dwelling calculation gets rounded to the five sizes below rather than to the full series.

Use this table instead of the calculator when you already know the amperage and only want to sanity-check it against the kind of house it belongs to — or when you are deciding whether to take the next rung up. That second use is the more valuable one. The calculator returns a legal minimum; this table describes what each size actually lives with, which is the information you need to judge whether the minimum is a false economy.