Transformer Sizing Calculator for Illinois
NEC 2020 transformer sizing math for EV charger installers working in Illinois.
DCFC and large workplace EV deployments in Illinois 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 2020 continuous-load multiplier.
Worked example for Illinois
A 150 kW DC fast charger draws roughly 180 A at 480 V three-phase. Applying the 125% continuous-load factor (150 × 1.25 ≈ 188 kVA), then rounding up to the next standard transformer rating gives a 200 kVA minimum. Illinois's 91°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
EV installations in Illinois are governed by the 2020 National Electrical Code, in force since 2022. That includes Article 625 (Electric Vehicle Power Transfer System) requirements: 125% continuous-load sizing on EVSE branch circuits, GFCI protection at outdoor receptacles, and provisions for energy management systems on shared circuits.
In Illinois, you'll most often interconnect with ComEd, Ameren Illinois, MidAmerican Energy. Each has its own service-upgrade timeline, EV rebate availability, and metering rules — confirm them before quoting commercial work.
Climate & Ampacity
Illinois's representative summer design ambient is around 91°F, which yields a 0.88× ampacity correction factor at 75°C terminations per NEC 310.15(B)(1). 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.
Illinois takeaway
Coordinate primary-side voltage, impedance, and fault-current specs with ComEd early — interconnection lead times for new pad-mounts in Illinois can run 6-12 months on commercial DCFC sites.