Solar PV & EV Charging Calculator
The 120% rule for a load-side (busbar) PV interconnection, PV source-circuit conductor sizing at 125% of rated short-circuit current, and EV charging equipment feeder sizing.
What it calculates
- Maximum inverter/PV breaker allowed on a busbar without upsizing the busbar or main breaker (705.12(B)(3)(2))
- PV source-circuit conductor ampacity requirement at 125% of the module's rated Isc (690.8)
- EV charging equipment branch-circuit and feeder sizing per Article 625
Typical inputs
- Busbar and main breaker rating
- PV module rated short-circuit current (Isc)
- EV supply equipment nameplate rating
Known limits
- Every state page in the Engineering Hub's code lookup states which adopted NEC edition carries these articles for that jurisdiction — confirm the edition before relying on section numbers here.
Frequently asked
What is the 120% rule and why does it exist?
705.12(B)(3)(2) allows a PV system to connect at the opposite end of a busbar from the main breaker without upsizing that busbar, as long as the sum of the main breaker and the PV breaker doesn't exceed 120% of the busbar's rating — a code-recognized exception that avoids a full panel upgrade for many residential solar installs.
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