Ampacity Derating Calculator for Wisconsin
NEC 2017 ampacity derating math for EV charger installers working in Wisconsin.
Wisconsin's 88°F design ambient drives a 0.88× NEC 310.15(B)(1) correction at 75°C terminations — the single most-overlooked derate on hot-climate EV installs.
Worked example for Wisconsin
A conductor with a 30°C-rated ampacity of 75 A drops to roughly 66 A in Wisconsin ambient conditions. Stack a 0.8× conduit-fill adjustment (NEC 310.15(C)(1)) on top and that same conductor is only good for 52.8 A.
Code & Utilities
The applicable code in Wisconsin is the NEC 2017, which the state adopted in 2020. That includes Article 625 EVSE rules and the 125% continuous-load factor on charging branch circuits, though some 2020-cycle changes (like expanded EMS provisions) are not yet enforced statewide.
Wisconsin's primary EV-relevant utilities are We Energies, Madison Gas & Electric, Wisconsin Public Service, Xcel Energy Wisconsin. Each has its own service-upgrade timeline, EV rebate availability, and metering rules — confirm them before quoting commercial work.
Climate & Ampacity
In Wisconsin, the 88°F summer ambient drives a 0.88× 75°C ampacity correction. Bake this into every Level 2 and DCFC conductor pick before you commit to a wire size. Because the correction is below 0.9, conductors that "look fine" on a 30°C ampacity table will not carry their nameplate current here — always derate explicitly.
Wisconsin takeaway
Never size off the 30°C column in NEC Table 310.16 for Wisconsin work — always start with the temperature-corrected number, then apply any conduit-fill adjustment.