Wire Size Calculator for Washington
NEC 2020 wire size math for EV charger installers working in Washington.
Wire sizing in Washington is governed by NEC 2020 Table 310.16, with the state's 87°F summer ambient driving a 0.88× correction factor at 75°C terminations per Table 310.15(B)(1).
Worked example for Washington
A 100 A continuous EV branch needs a conductor whose corrected ampacity meets or exceeds 100 A. In Washington's 0.88× correction, that means picking a conductor whose 30°C-rated ampacity is at least 114 A. For copper THWN-2 in EMT, that typically lands at #2 AWG; aluminum requires one to two sizes larger.
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
Don't forget conduit-fill derating per NEC 310.15(C)(1) when more than three current-carrying conductors share a raceway — a common condition on multifamily and workplace EVSE home-run racks in Washington.