Solar Backfeed Breaker & 120% Rule Calculator
Determine maximum allowable solar PV backfeed circuit breaker size and panelboard busbar ampacity limits under NEC 705.12(B) load-side interconnection rules.
Panelboard & Service Main
Solar Inverter AC Output
Solar Interconnection Compliance
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Frequently Asked Questions
What is the NEC 705.12 120% rule for solar interconnection?
The 120% rule permits the sum of the main service overcurrent device and all interconnected power sources (solar inverters, energy storage) to equal up to 120% of the panelboard busbar rating, provided the solar breaker is placed at the opposite end of the bus from the main breaker: (1.20 × Bus Rating) - Main Breaker = Max Solar Breaker.
Why must the solar breaker be placed at the opposite end of the busbar?
When the utility power enters from the top and solar power enters from the bottom, current flows toward branch circuit loads in the middle from opposite directions. Because load currents peel off along the bus length, the current at any single point on the busbar will never exceed the busbar rating.
What is a "main breaker de-rate" and when is it used?
If a homeowner or facility has a 200A busbar with a 200A main breaker, the 120% rule allows only a 40A solar breaker ((200 × 1.20) - 200 = 40A). If the solar system requires a 60A breaker, an electrician can replace the 200A main breaker with a 175A breaker. The allowed solar breaker becomes (240 - 175) = 65A, allowing the 60A solar system without replacing the entire electrical panel.