Breaker Sizing Calculator for Missouri
NEC 2020 breaker sizing math for EV charger installers working in Missouri.
Every EVSE branch in Missouri is treated as a continuous load per NEC 2020 Article 625 — the OCPD must be sized at 125% of the EVSE's listed maximum draw.
Worked example for Missouri
A 60 A continuous EV load requires a breaker rated 75 A (60 × 1.25 = 75.0 A, rounded up to the next standard size). The conductor downstream must carry that 75 A after Missouri's 0.88× ampacity correction.
Code & Utilities
Missouri currently enforces the NEC 2020 edition, adopted in 2022. 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 Missouri include Ameren Missouri, Evergy Missouri, Empire District Electric. 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 95°F ambient in Missouri — 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.
Missouri takeaway
Use a 100%-rated breaker only when the panel and breaker are both listed for 100% continuous duty — otherwise the 125% rule applies. Missouri inspectors enforce this rigorously on Article 625 work.