Introduction
The calculator gives you a number — this guide shows you where every digit comes from. An electrical load calculation is not one formula but a sequence of seven formulas drawn from NEC Article 220: area-based lighting loads, fixed circuit allowances, demand factors that discount for diversity, appliance-specific rules, a continuous-load multiplier, and a final conversion to service amps. Work through them once by hand and the calculator’s output stops being a black box — you’ll know exactly which input drives your result, and which Code section backs it up.
The Electrical Load Calculator automates this whole sequence: enter floor area, circuits, and appliances, and it applies each formula in Code order. Below is the same sequence, derived step by step, so you can verify any result line by line.
Why the Code Works in Volt-Amperes
Before the formulas, the unit. Volt-amperes (VA) = volts × amperes — apparent power. For purely resistive loads (electric heaters, traditional lighting math) VA equals watts, which is why the two get casually mixed. For motors, compressors, and electronics they differ by the power factor, and the Code sizes conductors and services for the current that actually flows — hence VA throughout Article 220.
Practically: when a nameplate gives watts for a resistive appliance (water heater, range, dryer heating element), you may treat watts as VA. When it gives amps, multiply by the voltage yourself: a 30A, 240V dryer is 30 × 240 = 7,200 VA. The Watts Calculator converts between watts, amps, and volts for individual loads.
Formula 1: General Lighting Load
General lighting VA = floor area × 3 VA/ft² (NEC Table 220.12)
Every dwelling gets a minimum lighting-and-receptacle load of 3 volt-amperes per square foot of floor area, measured from the outside dimensions of the building, floor by floor. Exclude open porches, garages, and unfinished spaces not adaptable for future use — an unfinished attic with no flooring or stair access doesn’t count; a finished basement does.
Metric form: area (m²) × 32.3 VA/m² (3 ÷ 0.092903 = 32.29). A 200 m² home (≈ 2,153 ft²) gives 200 × 32.3 = 6,460 VA, matching the imperial result within rounding.
Formula 2: Small-Appliance and Laundry Circuits
Circuit VA = number of circuits × 1,500 VA (NEC 220.52)
On top of the lighting load, the Code requires fixed allowances: 1,500 VA per 20A small-appliance circuit (minimum two) and 1,500 VA per laundry circuit (minimum one) — Code minimums added whether or not the circuits run at full load.
Standard dwelling addition: 2 × 1,500 + 1 × 1,500 = 4,500 VA. Added to Formula 1’s result, this forms the “general load” subtotal that the demand factors act on.
Formula 3: General Demand Factors
Demand VA = (first 3,000 VA × 100%) + (remainder × 35%) (NEC Table 220.42)
This is the diversity discount. A home’s lights, receptacles, and small appliances never all run at full tilt simultaneously, so the Code lets dwellings count the first 3,000 VA of the general subtotal in full and everything above that at 35%. Watch what it does to the canonical example:
2,000 ft² home: 6,000 (lighting) + 4,500 (circuits) = 10,500 VA → 3,000 × 100% + 7,500 × 35% = 3,000 + 2,625 = 5,625 VA. Nearly half the connected load disappears — legitimately — because diversity is real. Skipping this step is the most expensive mistake in load estimating; the common mistakes guide shows what it costs.
Formula 4: Fixed Appliances, Dryers, and Ranges
Major appliances follow their own sections, added after the general demand calculation:
- Fixed-in-place appliances (NEC 220.53): dishwasher, disposal, compactor, microwave, and similar — if there are four or more, apply a 75% demand factor to their nameplate sum. Fewer than four: count at 100%.
- Electric dryer (NEC 220.54): 5,000 VA or the nameplate rating, whichever is greater, at 100%.
- Household range (NEC 220.55, Table 220.55): demand from the table, not the nameplate. A single 12 kW range → 8,000 VA demand; two 12 kW ranges → 11,000 VA. The table reflects that range elements cycle and rarely all run at once.
- Water heater and similar: nameplate at 100% — and remember the 125% continuous-load sizing when you get to conductors and breakers.
Formula 5: Heating vs. Cooling — the Larger Wins
Count only the larger of the heating and cooling loads (NEC 220.60)
Heating and air conditioning are noncoincident — they don’t run at the same time — so including both double-counts. Compare the two nameplates and carry only the larger. In most all-electric homes, heating wins; with a gas furnace and central AC, the AC wins. Note which one won; a future heat-pump conversion may flip the answer — the estimating guide covers where to find nameplates.
Formula 6: Continuous Loads at 125%
Sizing VA = continuous load VA × 125% (NEC 210.19, 230.42)
A 40A Level 2 charger draws 40 × 240 = 9,600 VA continuously, so it needs a 50A circuit and consumes 12,000 VA of planning capacity. Apply this after the demand calculation, when converting loads to equipment sizes.
Formula 7: VA to Service Amps
Service amps = total demand VA ÷ service voltage
The final conversion. For a typical North American single-phase dwelling: amps = VA ÷ 240. Then round up to the next standard service size:
| Service size | Capacity at 240V | Typical use |
|---|---|---|
| 100A | 24,000 VA | Code minimum for a modern dwelling; small all-gas homes |
| 150A | 36,000 VA | Mid-size homes, modest electric appliances |
| 200A | 48,000 VA | Common new-construction standard; EV-ready headroom |
| 400A | 96,000 VA | Large homes, all-electric, workshops, multiple EVs |
Never round down — the service must carry the full calculated demand. The Amperage Calculator performs this conversion for any voltage and phase configuration.
Unit Conversion Reference
| Conversion | Factor |
|---|---|
| ft² → m² | × 0.092903 |
| m² → ft² | × 10.7639 |
| VA/ft² → VA/m² | × 10.7639 (3 → 32.3) |
| VA → amps (240V single-phase) | ÷ 240 |
| kW → VA (resistive) | × 1,000 |
Full Worked Example
2,000 ft² home (186 m²), two small-appliance circuits, one laundry circuit, 5 kW dryer, 12 kW range, 4.5 kW water heater, 5 kW AC:
F1 — Lighting: 2,000 × 3 = 6,000 VA (metric check: 186 × 32.3 = 6,008 VA ✓)
F2 — Circuits: 3 × 1,500 = 4,500 VA → general subtotal 10,500 VA
F3 — Demand: 3,000 + (7,500 × 0.35) = 5,625 VA
F4 — Appliances: 5,000 (dryer) + 8,000 (range) + 4,500 (water heater) = 17,500 VA
F5 — HVAC: AC 5,000 VA carried (larger than heating in this example)
F6 — Continuous: water heater already at nameplate; no EV in this example
F7 — Service: (5,625 + 17,500 + 5,000) = 28,125 VA ÷ 240 = 117A → 200A service recommended
Related Guides
- Complete Guide — the full walkthrough: terms, framework, and worked example in one place.
- Estimating Guide — how to gather accurate nameplate and floor-area inputs.
- Common Mistakes — the formula misapplications that distort results.
- Pro Tips — headroom, future-proofing, and planning judgment beyond the formulas.
- Cost Guide — what the service size your calculation produces typically costs to install.
- Load Calculation Checklist — the formulas in checklist order, from measurement to inspection.
Frequently Asked Questions
Why does the Code use VA instead of watts?
Because conductors and breakers respond to current, not just real power. VA (volts × amps) captures the total current a load draws, including the reactive component in motors and electronics.
Where does the 35% demand factor come from?
NEC Table 220.42. It reflects measured diversity: in a dwelling, the general lighting and receptacle load above the first 3,000 VA is statistically unlikely to coincide, so the Code permits counting it at 35%.
Why is a 12 kW range only 8,000 VA in the calculation?
Table 220.55 recognizes that range elements cycle on thermostats and are rarely all at full power simultaneously. The demand value is the Code’s researched estimate of the realistic maximum.
Do I apply the 125% rule inside the load calculation?
The 125% continuous-load rule sizes conductors and overcurrent devices rather than changing the Article 220 demand total — apply it when selecting breaker and wire sizes for continuous loads like EV chargers.
Can I use the optional method instead?
NEC 220.82 and 220.83 offer simplified calculations for qualifying dwellings. They often give similar answers with less work, but they have eligibility conditions — confirm with your AHJ before relying on them.
Get Your Number
Seven formulas, one sequence: area to lighting load, circuits added, demand applied, appliances added per their sections, the larger of heat or cool, continuous loads respected, and VA converted to amps. Run your own numbers through the Electrical Load Calculator and check each line against the derivations above — when you can trace the result, you can defend it to an inspector, an electrician, or your own budget.
Disclaimer: planning estimates only. These formulas summarize NEC Article 220 for planning purposes — verify your final numbers with a licensed electrician and your local authority having jurisdiction (AHJ) before buying equipment or scheduling work. This guide is not engineering advice.
