How much solar can you backfeed on a 125 A busbar with a 100 A main?

A 125 A busbar with a 100 A main breaker allows a maximum 50 A of backfed PV breaker under the 120% rule: 125 × 1.2 − 100 = 50 A (NEC 705.12(B)(3)(2)) — valid only when the PV breaker sits at the opposite end of the busbar from the main. Without the opposite-end placement, the sum-of-supplies rule allows 25 A in any position (705.12(B)(3)(1)).

In inverter terms: a 50 A backfeed breaker supports up to 40 A of continuous inverter output (breaker ≥ 125% of continuous output, 705.28(A)) — about 9.6 kW at 240 V. Conductors for that output must be sized for at least 50 A.

If your inverter needs more than this panel allows: downsizing the main to the next standard rung (100 A → 90 A, 240.6(A)) raises the 120%-rule limit to 60 A — provided the calculated load still fits the smaller main; a supply-side tap (705.11) removes the busbar constraint but involves the utility side of the service; and 705.12(B)(3)(3) permits sum-of-branch-OCPDs sizing when an engineered study supports it. Rejections cluster on the opposite-end placement — inspectors check the physical breaker position, not just the math.