Residential Load Calculation: The Standard Method

The dwelling unit load calculation is a long problem with many small steps. Each step is simple, but the order matters and one skipped demand factor changes the final answer. This guide walks through the standard method for a single-family house, one line at a time, ending with the service size and service conductors.

The example house

We will use one house for the whole calculation. Treat each nameplate watt value as volt-amperes, as this example does throughout.

• 2,000 sq ft of living area
• Two small-appliance branch circuits and one laundry circuit (the minimum)
• Electric range, 12 kW
• Electric clothes dryer, 5.5 kW
• Water heater 4,500 W, dishwasher 1,200 W, disposal 900 W, built-in microwave 1,500 W
• Electric space heat, 10 kW
• Air conditioner, 5,000 VA
• 120/240 V single-phase service

Step 1: General lighting load

Dwelling units use 3 VA per square foot (Table 220.12 in NEC 2020). Use the outside dimensions of the living area. Open porches, garages and unused spaces not adaptable for future use are not included.

Step 2: Small-appliance and laundry circuits

Each small-appliance branch circuit is figured at 1,500 VA, with at least two required (220.52(A)). The laundry circuit adds another 1,500 VA (220.52(B)). These are added to the general lighting load before demand factors are applied.

Step 3: Apply the lighting demand factors

Table 220.42 (NEC 2020) applies to the subtotal from Step 2. For dwelling units: the first 3,000 VA at 100%, the portion from 3,001 to 120,000 VA at 35%, and anything over 120,000 VA at 25%.

The classic mistake here is applying 35% to the whole 10,500 VA. The first 3,000 VA always stays at 100%.

Step 4: Range

For one household range rated not over 12 kW, Table 220.55 Column C gives a demand of 8 kW. Ranges over 12 kW use the percentage increase described in the notes to that table.

Step 5: Clothes dryer

220.54 sets the dryer load at 5,000 W or the nameplate rating, whichever is larger.

Step 6: Fastened-in-place appliances

Under 220.53, when there are four or more fastened-in-place appliances, you can take 75% of their total. Ranges, dryers, space heating and air conditioning are handled separately and are not part of this group.

Step 7: Heating or air conditioning, not both

Heating and cooling do not run at the same time, so 220.60 lets you use only the larger of the two noncoincident loads.

Step 8: Add it up and convert to amperes

Divide by 240 V, not 120 V. The service is 120/240 V single-phase and the load calculation is balanced across both ungrounded conductors.

Step 9: Service rating and service conductors

The service must be rated at least the calculated load. 146.7 A fits under a 150 A service. For the service conductors of a 120/240 V single-phase dwelling service that carries the entire load, 310.12 (NEC 2020 and 2023) allows conductors sized at 83% of the service rating, using Table 310.16.

The 83% rule applies only to the conditions in 310.12. It does not apply to a feeder that serves only part of the dwelling, and it is not a general derating rule.

What about the neutral?

The neutral can be smaller than the ungrounded conductors because it only carries the unbalanced load. 220.61 covers this: 240 V loads like the water heater, heat and air conditioning put nothing on the neutral, and the range and dryer can be taken at 70% of their demand on the neutral. If an exam question asks for the neutral load, rebuild the list with only the loads that use the neutral and apply those rules.

Checklist for the standard method

1. Square footage × 3 VA.
2. Add 1,500 VA per small-appliance circuit (at least two) and 1,500 VA for laundry.
3. Apply the demand factors: 3,000 at 100%, next portion at 35%.
4. Range from Table 220.55.
5. Dryer at 5,000 W or nameplate, whichever is larger.
6. Fastened-in-place appliances: 75% if four or more.
7. Larger of heat or AC.
8. Total, divide by 240 V, pick the service, size the conductors.

Write every step on its own line in the margin of your scratch paper. If you lose your place, you can find it in seconds instead of starting over. For a related check on the service conductors, see ampacity and derating, and if the service is far from the panel, run the numbers in the voltage drop calculator.