Methodology & sources
We implement the arithmetic of each method in our own code and words and cite section numbers. We do not reproduce NFPA 70 text or tables. Read the code itself via NFPA's free access.
What the calculator does
| Method | Our implementation | Status |
|---|---|---|
| 220.83(A) 2020/2023 | 3 VA/sq ft, 1,500 VA per small-appliance and laundry circuit, nameplates; first 8 kVA at 100%, remainder 40% | Checked against the NEC Mastery worked example (94 A) |
| 220.83(B) 2020/2023 | When adding heat or A/C: larger of heating or cooling at 100%, the rest tiered | Secondary sources (NEC Mastery, Intrysys) |
| 120.83 2026 | 2 VA/sq ft; existing first 8 kVA 100%, remainder 40%; new EVSE 80%, new central resistance heat 80%, other new 50%; fill the 8 kVA tier with existing load first, then new loads lowest factor first | Assumption: from multiple secondary summaries, not checked against printed NFPA 70-2026; tier-fill order is our conservative reading |
| 220.82 / 120.82 | General load first 10 kVA (2026: 8 kVA) at 100%, rest 40%; plus largest of A/C & heat pump at 100%, heat pump + 65% supplemental, or resistance heat at 65% | 2020/2023 from secondary sources; 2026 changes are an assumption (The Power Estimator summary). Simplified: the 40% rule for four or more separately controlled heating units is not modeled |
| 220.87 / 120.87 | 12-month peak kW × 125% + new loads at full nameplate (conservative) | Secondary sources (Intrysys) |
| 220.57 EVSE | 2023: larger of 7,200 VA or nameplate; 2026: nameplate; 2020: nameplate | Secondary sources |
| Heating vs cooling | Only the larger side counts in every method (non-coincident loads, 220.60) | Assumption for 2020/2023 220.83(A), where some AHJs count both |
| Status | "Within rating" ≥10% headroom, "under 10% headroom", or "over rating" | Our presentation choice, not a code term |
Verified
- State NEC edition counts and examples as of August 3, 2026 from NFPA's enforcement map.
- 2023 220.83 arithmetic reproduces the NEC Mastery example (1,800 sq ft + 32 A EVSE ≈ 94 A); covered by our unit tests.
Not verified (labeled assumptions)
- Exact NFPA 70-2026 text of 120.83, 120.82 and 120.57.
- Local amendments, including AHJs that count EVSE at 100% (offered as a toggle).
- NFPA licensing terms for quoting code text; we avoid quoting it.
- Appliance defaults are typical values, not your nameplate.
Sources
- NEC Mastery: NEC 220.83 explained, with the 2026 changes
- NFPA: where the NEC is in effect (state enforcement map)
- Mike Holt: NEC adoption list by state
- Intrysys: NEC 2026 changes from NEC 2023
- Intrysys: residential load calculator (220.82 / 220.87)
- Electrical License Renewal: 120.83 Existing Dwelling Units (2026 summary)
- PNNL Building America Solution Center: electrical panel calculator for heat pump water heaters
- City of Sacramento: electrical load calculation worksheet (PDF)
- City of Campbell, CA: residential load calculation worksheet
- TradeTools: existing service load calculator (AHJ variants, 220.57)
- Fox Electric: can a 100 amp panel support an EV charger (625.42)
- NFPA 70 (National Electrical Code) free read-only access
- The Power Estimator: NEC 2023 vs 2026 load calculation methodology
- U.S. DOE Energy Saver: heat pump water heaters
- U.S. DOE Alternative Fuels Data Center: charging at home
Estimate only; a licensed electrician and your local authority having jurisdiction decide; not an engineering stamp.