Transformer Sizing Calculator for Colorado
NEC 2023 transformer sizing math for EV charger installers working in Colorado.
DCFC and large workplace EV deployments in Colorado 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 2023 continuous-load multiplier.
Worked example for Colorado
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. Colorado's 90°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 Colorado is the NEC 2023, which the state adopted in 2023. 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.
Colorado's primary EV-relevant utilities are Xcel Energy Colorado, Colorado Springs Utilities, Tri-State Generation. Each has its own service-upgrade timeline, EV rebate availability, and metering rules — confirm them before quoting commercial work.
Climate & Ampacity
In Colorado, the 90°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.
Colorado takeaway
Coordinate primary-side voltage, impedance, and fault-current specs with Xcel Energy Colorado early — interconnection lead times for new pad-mounts in Colorado can run 6-12 months on commercial DCFC sites.