How to Do a Dwelling Load Calculation

Why we do the calc

A load calculation tells you how big a service and panel a house needs, in amps. Undersize it and the main trips or the equipment overheats. Oversize it and the customer pays for copper they will never use. The code gives us a standard method so the number is defensible to the inspector.

The whole trick is that not everything runs at once, so the code lets us apply demand factors instead of adding every nameplate at full value.

General lighting and receptacle load

Start with the general lighting load, calculated from the square footage at a set volt-amps per square foot. Add the required small-appliance circuits for the kitchen and the laundry circuit, each at a standard value.

• Take living area square footage times the code's per-square-foot value.
• Add the two or more kitchen small-appliance circuits.
• Add the laundry circuit.

This subtotal is the base you will apply the first demand factor to.

Apply demand factors

The general load gets a demand factor: the first chunk counts at one hundred percent, and everything above it counts at a reduced percentage, because you never light every room at once. Then add the appliances. Fixed appliances like a water heater, dishwasher, and disposal can get their own demand factor when there are enough of them.

The range gets its own demand table, and dryers have a standard demand value. Add heating or air conditioning using the larger of the two, since you never run both at full at the same time.

Convert to amps and size the service

Add up your demand-adjusted volt-amps for the total load, then divide by the service voltage, 240 volts for a typical home, to get amps. Compare that number to standard service sizes and pick the next one up: 100, 150, 200 amps, and so on.

Most new homes land at 200 amps once you count electric ranges, HVAC, and increasingly EV chargers. Do the math, keep your worksheet, and hand it to the inspector. A clean load calc on paper prevents the argument on site.