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Lessons · Electrical · load calculation for a dwelling

Sizing a house service: count the load the standard way

A dwelling's load is added up in a fixed order: general lighting at 3 VA per square foot, plus 1500 VA for each small-appliance circuit and the laundry, with a demand factor applied because not everything is on at once; then the range and dryer from their tables and the fixed appliances; the total in VA divided by 240 V is the service amps.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

What it is for

A homeowner adds a hot tub, a car charger and a workshop and asks if the 100 A service will take it. The answer is a sheet of arithmetic, and the same sheet is what the utility and the inspector ask for on a new build. Guessing high costs a service upgrade; guessing low trips the main at dinner time.

How to think about it

Write the rows in order and keep the units in VA. Square feet times 3. Add 1500 per small-appliance circuit (at least two) and 1500 for laundry. Take the first 3000 VA at 100 percent and the rest at 35 percent. Then add the range (8000 VA for one range up to 12 kW), the dryer (5000 VA or the nameplate, whichever is more), and the rest of the fixed loads at full value. Divide by 240.

Worked example

1200 sq ft × 3 VA = 3600 VA
General lighting and receptacles.
+ 2 × 1500 + 1500 = 4500 VA, total 8100 VA
Two small-appliance circuits and the laundry circuit.
3000 × 1.00 + 5100 × 0.35 = 3000 + 1785 = 4785 VA
Demand: the first 3000 at 100 percent, the remainder at 35 percent.
+ range 8000 VA + dryer 5000 VA = 17785 VA
From their tables.
17785 / 240 = 74.1 A, so a 100 A service
The minimum for a dwelling is 100 A anyway; this one has room.

Your turn

A 2000 sq ft house. Write the general lighting load.

2000 × 3 =  VA

The trap

Taking 35 percent of everything. The demand factor is for the general load only, and only after the first 3000 VA. The range and dryer have their own tables, and a water heater or a car charger is counted in full.

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