Transformer Sizing Calculator for Wisconsin
NEC 2017 transformer sizing math for EV charger installers working in Wisconsin.
DCFC and large workplace EV deployments in Wisconsin typically need a dedicated 480 V three-phase service, which means sizing a pad-mount or dry-type transformer against the connected charger load plus the NEC 2017 continuous-load multiplier.
Worked example for Wisconsin
A 240 kW DC fast charger draws roughly 289 A at 480 V three-phase. Applying the 125% continuous-load factor (240 × 1.25 ≈ 300 kVA), then rounding up to the next standard transformer rating gives a 300 kVA minimum. Wisconsin's 88°F summer ambient does not directly derate the transformer, but it does push the secondary feeder ampacity down by 0.88× — so the secondary copper has to be sized accordingly.
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
Coordinate primary-side voltage, impedance, and fault-current specs with We Energies early — interconnection lead times for new pad-mounts in Wisconsin can run 6-12 months on commercial DCFC sites.