Box Fill Calculator for California
NEC 2023 box fill math for EV charger installers working in California.
Every EVSE installation in California eventually hits a junction or device box — disconnects, splice points, pull boxes — all of which must satisfy NEC 2023 Article 314.16 fill rules.
Worked example for California
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
California currently enforces the NEC 2023 edition, 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.
Major electric utilities serving California include Pacific Gas & Electric (PG&E), Southern California Edison, San Diego Gas & Electric, LADWP, SMUD. Each has its own service-upgrade timeline, EV rebate availability, and metering rules — confirm them before quoting commercial work.
Climate & Ampacity
Plan EV feeders against a 95°F ambient in California — 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.
California 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 California inspectors most often catch fill violations.