What does a load calculation tell you?
It estimates the demand a dwelling puts on its service, so you can choose a service size, disconnect and conductors that are safe for the loads. NEC Article 220 covers branch-circuit, feeder and service calculations. This guide covers the optional dwelling method as described in trade references. Always check the text of the code edition your jurisdiction has adopted, and have the calculation reviewed where required.
What loads go into the optional method?
Add these general loads, which exclude heating and cooling:
- General lighting and receptacles: 3 VA per square foot, using outside dimensions. Open porches, garages and unused or unfinished spaces not adaptable for future use are excluded.
- Small-appliance circuits: two 20 amp circuits at 1,500 VA each.
- Laundry circuit: one 20 amp circuit at 1,500 VA.
- Fixed appliances at nameplate rating, such as a range, dryer, water heater and dishwasher.
- Motors not part of an appliance, at nameplate rating.
How are demand and heating or cooling applied?
Take 100 percent of the first 10,000 VA of the general load and 40 percent of the remainder. Then add heating or cooling. For air conditioning, add 100 percent of the nameplate. Electric space heating of three or fewer units is taken at 65 percent, and the largest applicable value is added. Divide the total VA by the nominal voltage, 240 for most homes, to get amps.
Section 240.6(A) lists the standard sizes for the service disconnect, and conductors are sized from the code tables.
What does a worked example look like?
1418 Ridge Rd is a 2,000 square foot home with a 3 ton A/C unit. Appliance values are examples, so use the nameplates on a real job.
General lighting and receptacles: 2,000 x 3 = 6,000 VA. Small-appliance circuits: 2 x 1,500 = 3,000 VA. Laundry: 1,500 VA. Range 12,000 VA, dryer 5,000 VA, water heater 4,500 VA, dishwasher 1,200 VA. Total general load: 6,000 + 3,000 + 1,500 + 12,000 + 5,000 + 4,500 + 1,200 = 33,200 VA.
Demand: first 10,000 VA at 100 percent = 10,000 VA. Remaining 23,200 VA x 0.40 = 9,280 VA. Subtotal: 19,280 VA.
Add the A/C at an example nameplate of 4,800 VA (240 volts x 20 amps): 19,280 + 4,800 = 24,080 VA. Amps: 24,080 / 240 = 100.3 amps. That is just over a 100 amp service.
What happens when you add an EV charger?
Add an 11,500 VA charger to the general load: 33,200 + 11,500 = 44,700 VA. Demand: 10,000 + (34,700 x 0.40 = 13,880) = 23,880 VA. Add the A/C 4,800 VA = 28,680 VA. Amps: 28,680 / 240 = 119.5 amps.
On paper that points to at least a 125 amp service. Most shops quote 200 amps for headroom. Check how your inspector treats EV charging in the calculation, since rules on continuous loads and load management differ. Existing dwellings have a separate method in 220.83.
Step by step
- Total the general load. Add lighting and receptacles at 3 VA per sq ft, small-appliance and laundry circuits, fixed appliances and motors.
- Apply demand. Take 100 percent of the first 10,000 VA and 40 percent of the remainder.
- Add heating or cooling. Add the larger applicable value, with A/C at 100 percent of nameplate and electric heat per 220.82(C).
- Convert to amps. Divide total VA by 240 volts, or 208 where applicable.
- Select the service. Choose a standard disconnect size at or above the result, and size conductors from the code tables.
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Sources: EC&M: Dwelling Load Calculations Per the National Electrical Code