NEC 705.12(B) — the 120% busbar rule

The most-cited residential solar interconnection rule. NEC 705.12(B) (the "120% rule") limits a load-side (back-fed breaker) connection so the busbar isn't overloaded: busbar rating × 1.2 ≥ main breaker + sum of back-fed solar breakers.

The 120% allowance by panel

Busbar	Main breaker	120% allowance	Max back-fed solar OCPD	Result
100 A	100 A	120 A	20 A	✓ within 120%
125 A	125 A	150 A	25 A	✓ within 120%
200 A	150 A	240 A	90 A	✓ within 120%
200 A	200 A	240 A	40 A	✓ within 120%
225 A	200 A	270 A	70 A	✓ within 120%

Example: a 200 A busbar allows 240 A total. With a 150 A main you may back-feed up to 90 A of solar; with a 200 A main only 40 A, so a main-breaker derate or a supply-side (line-side) tap under 705.11 is often needed.

When the 120% rule doesn't fit

If no legal main-breaker size fits the busbar, the code-compliant alternatives are a supply-side (line-side) tap per 705.11 (ahead of the service disconnect, so the busbar isn't back-fed), a main-breaker derate, or a line-side/feed-through connection. Solar Scripto flags the 120% result live and proposes the legal remedy.

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