Box Fill Calculator for Idaho
NEC 2020 box fill math for EV charger installers working in Idaho.
Every EVSE installation in Idaho eventually hits a junction or device box — disconnects, splice points, pull boxes — all of which must satisfy NEC 2020 Article 314.16 fill rules.
Worked example for Idaho
A 4-11/16" square × 2-1/8" deep box has a 42.0 in³ volume. Each #6 Cu conductor counts as 5.0 in³. With a 2-conductor + EGC EVSE branch landing in the box plus a device, you consume roughly 15-20 in³, leaving plenty of headroom — but a 60 A multi-port pull box can fill quickly with #4 or #2 AWG conductors.
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
Always run box-fill math when the EVSE disconnect lives more than a few inches from the unit itself — that intermediate junction is where Idaho inspectors most often catch fill violations.