Overcurrent Device in a Panel with a Backfed Breaker
A residential rooftop solar array feeds a 7.6 kW inverter into a 200 A main panel. The installer lands the inverter output on a 40 A two-pole breaker in the panel and installs the required dedicated disconnect at the meter. The panel's busbar rating is 225 A, and the main breaker is 200 A. The installer argues that because the busbar is 225 A, the sum of the main plus the backfed breaker can exceed the busbar rating by 25 A and still be fine. During inspection you also notice the backfed breaker is not marked as a supply-side or load-side connection, and the panel label has not been updated.
1. Apply the 120% busbar rule and determine whether the backfed breaker is acceptable.
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The relevant check is that the sum of the main overcurrent device rating plus the backfed source breaker rating should not exceed 120% of the busbar rating for a load-side connection. Here 200 A + 40 A = 240 A. The busbar is 225 A, and 120% of 225 A is 270 A. Since 240 A is below 270 A, the connection is acceptable under that rule. The installer's reasoning happens to reach the right result, but the inspector should verify the arithmetic and confirm the panel is listed for backfed connections. The inspector should also confirm the breaker is a listed backfeed device or is otherwise approved for the purpose, since not every breaker position is suitable.
2. What documentation and labeling deficiencies should the inspector cite, and why do they matter?
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The panel directory and label must identify the photovoltaic source and the backfed breaker clearly so that a future electrician or first responder knows the panel has a second source of power. The absence of that identification is a real defect, not paperwork. The inspector should require a durable label at the panel and at the disconnect stating the presence of the PV system, the source breaker location, and the disconnect location. The inspector should also verify the disconnect is labeled and accessible and that the permit documents match the installed equipment. These items matter because an unlabeled backfeed can lead to a worker assuming the main breaker de-energizes everything, which is a serious shock hazard.