How it works & the NEC rules behind it
When connecting solar photovoltaic (PV) inverters or battery energy storage systems (ESS) into an electrical panelboard, current can flow from both the utility grid AND the solar inverter simultaneously. To prevent the busbar from overheating, the National Electrical Code (NEC §705.12) restricts total power source input.
The Five Interconnection Methods (NEC §705.12):
- 1. The 120% Rule — Opposite End (§705.12(B)(3)(2)): Where the solar backfeed breaker is located at the opposite end of the busbar from the main breaker, the sum of the main breaker and 125% of the inverter output must not exceed 120% of the busbar rating:
(Busbar Rating × 1.20) − Main Breaker = Max Allowable Solar Breaker. - 2. Main Breaker Derating: If the required solar backfeed breaker exceeds the 120% limit, you can downsize (derate) the main breaker (e.g., swapping a 200A main breaker for a 175A or 150A main on a 200A busbar) to make room for more solar.
- 3. The 100% Rule (§705.12(B)(3)(1)): In center-fed panels or panels where opposite-end placement is impossible, the sum of all power source breakers cannot exceed 100% of the busbar rating:
Max Solar = Busbar Rating − Main Breaker. - 4. Sum of Breakers Rule (§705.12(B)(3)(3)): The sum of all branch-circuit breakers plus the solar breaker cannot exceed the panel busbar rating.
- 5. Supply-Side Tap (§705.11): Connected between the utility meter and the main service disconnect. Avoids busbar limitations entirely, limited only by service conductor ampacity.