Voltage Drop Calculator for Washington
NEC 2020 voltage drop math for EV charger installers working in Washington.
Washington EV installs frequently push past 100 ft of conductor — detached garages, parking-lot DCFC pedestals, and multifamily carport runs all add distance. NEC 2020 recommends a 3% branch / 5% total voltage-drop ceiling.
Worked example for Washington
A 40 A Level 2 charger at 240 ft on 240 V single-phase #6 Cu shows roughly 3.9% drop. That's above the 3% NEC recommendation, so you'd upsize to #4 or #2 AWG to land back under 3%. Washington's 0.88× ampacity correction is independent of voltage drop but applies on the same conductor pick.
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
Voltage drop is a recommendation, not a hard NEC rule — but EVSEs throttle aggressively below ~228 V on a 240 V circuit, so customers in Washington will notice any drop above 5%.