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Residential electrical load and panel headroom

Two educational methods: measured utility peak demand, or a simplified optional dwelling-unit estimate. This is not a permit calculation and does not replace a licensed electrician.

Methods described: NEC 2023 220.87 / 2026 120.87 (measured demand) and the structure of 220.82 / 120.82 (optional method). National Electrical Code is a copyrighted NFPA standard. Updated 2026-11-11.

Service
Most U.S. houses are 120/240 V single-phase.
Method
12-month peak demand

Use the highest demand from about 12 months of utility data when you can get it. The check shown here is 125% of that peak, plus new load, compared with the service rating — the structure commonly described for NEC 220.87 / 120.87. The 30-day recording exception has extra limits (including where solar or peak shaving is present).

New loads you want to add
Circuit sizes above follow the usual 125% continuous-load rule for branch-circuit sizing (16×1.25=20, 32×1.25=40, 40×1.25=50, 48×1.25=60). Confirm with the EVSE nameplate and Article 625 as adopted locally. The car’s onboard charger may be the real limit.

What this calculator is doing

The measured-demand path is usually the better fit for an existing house that already has a year of utility data: existing load is 125% of the recorded maximum demand, plus the new load, compared with the service or feeder rating. Public permit checklists, including the City of Orlando EV charging checklist, paraphrase those conditions from the existing-loads rule.

The optional-method path is a nameplate estimate for when demand data is missing. It is easy to misuse. It is not the same as the standard method, and we do not reproduce copyrighted NEC tables. If your jurisdiction has adopted the 2026 edition, the toggle reflects the publicly described changes (2 VA/ft², an 8 kVA first tier, and EVSE added at 100% outside that tier).

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