How the two methods differ — and why the answer changes
Article 220 gives you a choice, and the choice has money in it. The optional method (220.82) usually gives a smaller service for a modern all-electric home, because it applies a single 40% factor to everything above 10 kVA rather than demanding each category separately.
The three mistakes that fail an inspection
- Adding heat and A/C together. 220.60 calls them noncoincident — you take the larger and ignore the smaller. They do not run at the same time, so the service never sees both. Adding them oversizes the service and costs the customer real money.
- Using the range nameplate. A 12 kW range does not count as 12,000 VA. Table 220.55 Column C says one range up to 12 kW is 8,000 VA. Above 12 kW you add 5% per kW.
- Forgetting the dryer floor. 220.54 sets a 5,000 VA minimum regardless of nameplate. A 4,000 VA dryer still counts as 5,000.
Which method should you use?
Whichever gives the smaller service — both are legal. The optional method is available for any dwelling served by a single 120/240 V or 208Y/120 V three-wire service of 100 A or more, which covers essentially every house. This calculator runs both and tells you which one wins.
NEC Dwelling Load Calculator — frequently asked
Use NEC Article 220. Take 3 VA per square foot of living space for general lighting, add at least two 1,500 VA small-appliance circuits and a 1,500 VA laundry circuit, then apply the 220.42 demand factor: the first 3,000 VA at 100% and the remainder at 35%. Add the range demand from Table 220.55 Column C, the dryer (5,000 VA minimum), fastened appliances, and the larger of heating or air conditioning. Divide the total by the service voltage to get amps.
No. NEC 220.60 treats them as noncoincident loads — they cannot run at the same time, so you use the larger of the two and ignore the smaller. Adding them is one of the most common load-calculation errors and it oversizes the service.
8,000 VA, not 12,000. NEC Table 220.55 Column C gives the demand for household electric ranges: a single range rated 12 kW or less counts as 8,000 VA. Above 12 kW, Note 1 adds 5% to the Column C value for each kW above 12.
NEC 230.79(C) requires a minimum of 100 A for a one-family dwelling, regardless of what the calculation gives. If your calculated load is 54.6 A, you still install a 100 A service.
The standard method (220.42–220.60) applies a separate demand factor to each category of load. The optional method (220.82) sums all nameplate loads, takes the first 10 kVA at 100% and everything above that at 40%, then adds HVAC separately. Both are legal; the optional method usually gives a smaller service for a modern all-electric home.
Run the calculation. A typical 2,000–2,500 ft² all-electric home with a range, dryer, water heater and central heat usually lands between 150 A and 200 A by the optional method. Adding an EV charger, a hot tub or a heat pump can push it past 200 A — which is why the calculation exists rather than a rule of thumb.
NEC 220.12 measures the outside dimensions of the dwelling and includes areas adaptable to future use, but excludes open porches, garages, and unused or unfinished spaces not adaptable for future use. A finished basement counts; an unfinished crawl space does not.