Because a home Level 2 session often lasts three hours or longer at a steady current, EV charging infrastructure is treated as a continuous load in the National Electrical Code framework. The U.S. DOE Alternative Fuels Data Center says so explicitly and points installers to Article 625. The familiar 125% arithmetic (32 A × 1.25 = 40 A) is how that continuous load is usually converted into a branch-circuit size. It is not a mysterious “safety fudge factor” you can skip.
Two different 125% factors get mixed up, and the calculator shows them separately on purpose:
- Branch-circuit sizing. The circuit that feeds the EVSE is commonly sized at 125% of the continuous charging current. That is why the presets on the calculator pair 32 A with a 40 A circuit and 48 A with a 60 A circuit.
- Measured-demand service check. The existing-loads method (NEC 2023 220.87 / 2026 120.87, as commonly described) already multiplies the recorded peak by 125%, then adds the new load. Commentaries disagree about whether the new EV load is also taken at 125% in that service check, or only at nameplate. The calculator’s primary line adds nameplate current; an optional conservative line applies 125% to the new EV load as well.
Neither line is a permit result. See the panel headroom calculator and the measured-demand question page.
This is not a permit calculation, a utility approval, or a substitute for a licensed electrician, HVAC contractor, or the authority having jurisdiction. Panel Headroom is an independent educational site. It is not a utility, a certification body, or a contractor. Electrical and generator work that is done incorrectly can cause fire, shock, or carbon monoxide poisoning. Use the adopted electrical code and manufacturer instructions; pull permits where required.