Ampacity Derating Calculator for Maryland
NEC 2020 ampacity derating math for EV charger installers working in Maryland.
Maryland's 91°F design ambient drives a 0.88× NEC 310.15(B)(1) correction at 75°C terminations — the single most-overlooked derate on hot-climate EV installs.
Worked example for Maryland
A conductor with a 30°C-rated ampacity of 95 A drops to roughly 83.6 A in Maryland ambient conditions. Stack a 0.8× conduit-fill adjustment (NEC 310.15(C)(1)) on top and that same conductor is only good for 66.9 A.
Code & Utilities
EV installations in Maryland are governed by the 2020 National Electrical Code, in force since 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.
In Maryland, you'll most often interconnect with BGE, Pepco, Delmarva Power, Potomac Edison. Always verify the applicable tariff and any utility-specific requirements (CT cabinets, metering enclosures, demand limiters) at design time.
Climate & Ampacity
In Maryland, the 91°F summer ambient drives a 0.88× 75°C ampacity correction. Bake this into every Level 2 and DCFC conductor pick before you commit to a wire size. 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.
Maryland takeaway
Never size off the 30°C column in NEC Table 310.16 for Maryland work — always start with the temperature-corrected number, then apply any conduit-fill adjustment.