How to use this calculator
- Enter the habitable area. Living space per the code rules — the 3 VA/ft² general lighting load comes from it.
- Count the required circuits. Two small-appliance circuits minimum plus the laundry circuit, at 1,500 VA each.
- Sum the appliance nameplates. Range, oven, dryer, water heater, dishwasher, disposal, EVSE — nameplate VA, no table factors.
- Describe heating and cooling. Enter what exists; the calculator evaluates every 220.82(C) option and takes the largest, naming which governed.
- Read amps and the standard size. Total VA ÷ 240 V, rounded up to the next standard rating (100 A minimum for this method).
How it works
Service sizing is a diversity argument: not everything runs at once, and the optional method encodes that in two moves. First, the general load — lighting by area, the required kitchen and laundry circuits, and every fastened appliance at nameplate — gets a steep demand curve:
demand = 10,000 VA at 100% + (remainder) × 40%
Second, heating and cooling are noncoincident, so only the largest of the listed options counts: A/C or heat-pump compressor at 100%; compressor plus 65% of supplemental strips; space heating at 65% (fewer than four units) or 40% (four or more); thermal storage at 100%. This calculator evaluates all of them from your inputs and reports which one governed — the step people miss when they hand-pick an option.
The companions: the feeder conductor calculator turns the amps into a conductor check, the voltage drop calculator handles long service laterals, and the transformer kVA calculator covers the utility side of the same number.
Worked example
Verified against the live calculator
A 2,500 ft² house with the two small-appliance circuits, one laundry circuit and 17,500 VA of fastened appliances:
general = 7,500 + 4,500 + 17,500 = 29,500 VA → 10,000 + 0.4 × 19,500 = 17,800 VA
Cooling is 6,000 VA and electric space heating 10,000 VA on one
thermostat: the heating option governs at 10,000 × 65% =
6,500 VA (beating A/C's 6,000). Total
24,300 VA ÷ 240 V = 101 A → next standard size
110 A, and the calculator flags how little headroom
that leaves — the 125 A or 200 A panel is the practical answer the
moment an EV charger enters the conversation. Add a 9.6 kW EVSE to the
appliances and the same house calculates to 117 A: still under a 125 A
service, which is exactly the case the optional method exists to make.
Frequently asked questions
How does the NEC optional method calculate a house service?
Per the 220.82 structure: add general lighting (3 VA/ft²), 1,500 VA per small-appliance and laundry circuit, and the nameplates of fastened appliances; take the first 10 kVA at 100% and the rest at 40%; then add the LARGEST heating/cooling option (A/C at 100%, heat pump + 65% supplemental, space heat at 65% or 40%, thermal storage at 100%). Divide by 240 V for amps.
When can I use the optional method instead of the standard method?
For a single-family dwelling served by a 120/240 V (or 120/208 V) single-phase service or feeder of at least 100 A. It usually computes lower than the standard method — which is why it is the go-to for justifying that an existing 200 A service can absorb an EV charger or heat pump.
Why is only the largest heating OR cooling load counted?
They are noncoincident — a house does not heat and cool at the same time. The code therefore takes the largest of the listed options rather than their sum, with the percentages reflecting diversity within each option (e.g., four or more separately controlled heaters rarely all run at once: 40%).
Do range and dryer demand factors apply in the optional method?
No — that is a standard-method feature (Table 220.55/220.54). In the optional method, ranges, dryers and other fastened appliances go in at NAMEPLATE, and the 40%-over-10-kVA demand factor does the diversity work instead.
Method & assumptions
- Implements the NEC 220.82 optional-method STRUCTURE (verified against published walkthroughs of the rule); the code text and adopted edition govern — percentages and applicability can be amended locally.
- Single-family dwelling on a ≥100 A, 120/240 V single-phase service; multifamily uses 220.84, existing-dwelling additions 220.83, and the standard method is always an alternative.
- Loads are your nameplate inputs; no code tables are embedded. Interlocked heat-pump supplemental heat may be treated as space heating (see the input tooltip).
- This is a design screen, not a permit document: the AHJ, the adopted edition and a licensed electrician have the final word.