Rules for Solar Breakers

When evaluating a residential solar system, verifying the safety of the main electrical panel is a top priority. The National Electrical Code (NEC) has strict guidelines—specifically NEC 705.12—to prevent electrical panels from overheating and creating severe fire hazards.

The metal bar inside the electrical panel that distributes electricity to the house is called the busbar, and it has a strict maximum amperage rating. Because a home with solar receives power from two sources simultaneously (the utility grid and the solar panels), we must ensure the busbar is not overloaded by that combined current.

To verify code compliance, our electrical inspections check the panel against two specific NEC rules:

1. The 100% Rule (Standard Compliance)If the combined amperage of the main utility breaker and the solar breaker is less than or equal to the busbar's total rating, the system passes standard compliance.

• The Math: Main Breaker + Solar Breaker <= Busbar Rating
• The Location: Because the combined power cannot physically exceed the busbar's limit, there is no risk of overloading. Therefore, the solar breaker can be safely located anywhere in the panel.

2. The 120% Rule (The Special Allowance)Often, older homes or larger solar systems result in the combined breakers exceeding the busbar's standard 100% limit. The NEC allows the combined amperage to reach up to 120% of the busbar’s rating, provided a strict safety condition is met.

• The Math: Main Breaker + Solar Breaker <= (Busbar Rating * 1.2)
• The Location Requirement: To safely utilize this extra 20% allowance, the solar breaker must be installed at the exact opposite end of the busbar from the main breaker. Placing the two power sources at opposite ends ensures the electrical current distributes evenly across the panel, preventing any single section of the metal busbar from overheating.

Why This Matters 

An improperly sized or incorrectly placed solar breaker is a serious safety hazard and a major compliance red flag during a real estate transaction.

Use our Code Compliance Validator below to instantly verify if a residential electrical panel meets NEC requirements.