Installing a residential solar panel system involves more than just placing panels on your roof. A critical part of a code-compliant solar installation is ensuring your system connects safely to your main electrical panel. One key regulation to be aware of is the 120% rule, which defines how much solar energy can be backfed into your Main Service Panel (MSP) without creating safety risks.
What Is the 120% Rule?
The 120% rule, outlined in NEC 705.12 of the National Electrical Code (NEC), governs how a solar panel system can be connected to your home’s electrical infrastructure. Specifically, it states:
The combined amperage of your main breaker and the solar backfeed breaker must not exceed 120% of your panel’s busbar rating.
This rule ensures the busbar—the conductive metal strip in your electrical panel—won’t overheat, which could otherwise lead to system failure or even fire hazards.
Why This Rule Matters for Solar Installations
Most residential solar systems use a load-side connection, where the solar inverter delivers AC power to your main panel via a circuit breaker. This is the most common and straightforward way to connect solar to your home.
However, failing to comply with the NEC’s solar interconnection guidelines—especially the 120% rule—can lead to failed inspections or unsafe operation. That’s why electrical code compliance for solar installations is crucial.
How to Calculate Maximum Inverter Size
If you’re wondering how much solar you can install on your panel, the formula below will help determine your system’s maximum allowable inverter output under the 120% rule:
U.S. Example (NEC 120% Rule)
- Busbar rating: 225A
- Main breaker: 200A
- Formula:
((225A × 1.20) − 200A) × 80% = 56A maximum inverter output
Canada Example (125% Rule)
- In Canada, the Canadian Electrical Code allows 125% of the busbar rating:
((225A × 1.25) − 200A) × 80% = 64A maximum inverter output
This formula takes into account the continuous output rating of the inverter, using a safety derating factor of 80%.
Optional: Derating Your Main Breaker
If your current main service panel doesn’t provide enough capacity for solar backfeed, one solution is derating the main breaker. This involves replacing the main breaker (e.g., from 200A to 175A), freeing up space under the 120% rule for additional solar inverter capacity.
Quick Reference Table
The following table provides a reference for the maximum allowable inverter output based on common main service panel and breaker ratings in 240V systems.
Table 1. Maximum System Size when Backfeeding into the Main Service Panel
| Bus Bar Rating (A) | Main Breaker (A) | Max Allowed PV Inverter Output (125%) (A) | Max Allowed PV Inverter Output (120%) (A) | Max PV Inverter AC Rating (125%) (kW) | Max PV Inverter AC Rating (120%) (kW) |
| 100 | 100 | 20 | 16 | 4.8 | 3.84 |
| 150 | 150 | 30 | 24 | 7.2 | 5.76 |
| 200 | 200 | 40 | 32 | 9.6 | 7.68 |
| 225 | 175 | 85 | 76 | 20.4 | 18.24 |
| 225 | 200 | 64 | 56 | 15.36 | 13.44 |
| 400 | 400 | 80 | 64 | 19.2 | 15.36 |
| 600 | 600 | 120 | 96 | 28.8 | 23.04 |
Final Thoughts
Grasping the key solar panel electrical requirements, especially the 120% rule defined by the National Electrical Code (NEC 705.12), is critical for a safe, efficient, and code-compliant residential solar installation. Whether you’re designing a grid-tied solar system or integrating battery backup, following the 120% guideline ensures your main service panel (MSP) operates within safe limits, helping you avoid failed inspections and reduce fire risks. Compliance with this rule is not just about passing code—it’s about maximizing the long-term safety, performance, and value of your home solar power system.