The 120% rule comes from Article 705. 12 (B) (2) in the National Electrical Code (NEC) in the United States. It says that the combined power from your utility company and your solar panels can't exceed 120% of your electrical panel's busbar rating. The NEC 120 % busbar rule that limits how big the back‑fed solar breaker can be inside your main service panel. Understanding both. . There are several possible options for connecting your solar system to the utility grid, but the most common—point of interconnection—that is used in residential solar is the 120% rule from the National Electric Code (NEC). Ignore it, and you're looking at potential overheating, failed inspections, and some very expensive headaches.
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