Ampacity Derating Calculator for Washington
NEC 2020 ampacity derating math for EV charger installers working in Washington.
Washington's 87°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 Washington
A conductor with a 30°C-rated ampacity of 55 A drops to roughly 48.4 A in Washington ambient conditions. Stack a 0.8× conduit-fill adjustment (NEC 310.15(C)(1)) on top and that same conductor is only good for 38.7 A.
Code & Utilities
The applicable code in Washington is the NEC 2020, 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.
Washington's primary EV-relevant utilities are Puget Sound Energy, Seattle City Light, Avista Utilities, Snohomish PUD. Always verify the applicable tariff and any utility-specific requirements (CT cabinets, metering enclosures, demand limiters) at design time.
Climate & Ampacity
Plan EV feeders against a 87°F ambient in Washington — the resulting NEC 310.15(B) correction of 0.88× is what trims a #6 THWN-2 down to its true continuous rating. 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.
Washington takeaway
Never size off the 30°C column in NEC Table 310.16 for Washington work — always start with the temperature-corrected number, then apply any conduit-fill adjustment.