Ampacity Derating Calculator for Idaho
NEC 2020 ampacity derating math for EV charger installers working in Idaho.
Idaho's 96°F design ambient drives a 0.82× NEC 310.15(B)(1) correction at 75°C terminations — the single most-overlooked derate on hot-climate EV installs.
Worked example for Idaho
A conductor with a 30°C-rated ampacity of 130 A drops to roughly 106.6 A in Idaho ambient conditions. Stack a 0.8× conduit-fill adjustment (NEC 310.15(C)(1)) on top and that same conductor is only good for 85.3 A.
Code & Utilities
Idaho currently enforces the NEC 2020 edition, adopted in 2021. 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.
Idaho's primary EV-relevant utilities are Idaho Power, Avista Utilities, Rocky Mountain Power. Their make-ready, time-of-use, and demand-charge structures vary widely; pull the specific tariff before sizing service equipment.
Climate & Ampacity
Plan EV feeders against a 96°F ambient in Idaho — the resulting NEC 310.15(B) correction of 0.82× 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.
Idaho takeaway
Never size off the 30°C column in NEC Table 310.16 for Idaho work — always start with the temperature-corrected number, then apply any conduit-fill adjustment.