What is the 120 rule for solar panels?

The 120 percent rule, NEC 705.12, governs how much solar can land on a panel's busbar. The main breaker plus the solar backfeed breaker may total up to 120 percent of the busbar rating, provided the solar breaker sits at the opposite end from the main.

Concrete case: a 200 amp busbar with a 200 amp main leaves room for a 40 amp solar breaker, which caps the inverter around 7.6 kW.

The opposite-end placement matters because the two sources then feed the bus from both directions and no point of the bar carries their sum.

Bigger systems have outs: derate the main breaker, upgrade the panel, or use a supply-side connection ahead of the main.

The solar wire ampacity calculator covers the conductor side of the same interconnection math.