Code reference · NEC 705.12(B)(3)(1)-(3); 705.11
The NEC 705.12 120% rule, actually explained
When a PV inverter backfeeds a load-side busbar, 705.12(B)(3)(2) allows the main breaker plus the inverter breaker to total up to 120% of the busbar rating — provided the backfeed breaker sits at the opposite end of the bus from the main.
The canonical case: 200 A busbar with a 200 A main → 200×1.2 − 200 = 40 A maximum backfeed breaker (≈7.6 kW of inverter at 240 V). A 225 A busbar with a 200 A main allows 70 A. A 100 A subpanel with a 100 A main allows 20 A.
If you can't meet it: downsize the main, use the sum-of-supplies rule (705.12(B)(3)(1): main + PV ≤ 100% of busbar, any position), do an engineered load calculation, or connect supply-side under 705.11.
Run the numbers yourself
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