When planning a solar system, you may come across the “20% rule.” This rule is an important safety and design guideline that helps ensure your solar installation works efficiently and follows local electrical codes. Let’s break it down in simple terms.
Understanding the 20% Rule
The 20% rule comes from the National Electrical Code (NEC), which sets standards for safe electrical installations.
It says:
- When solar panels are connected to a breaker in your electrical panel, the breaker size must be no more than 120% of the busbar rating.
- This means that your solar breaker should not exceed 20% of the panel’s main busbar capacity.
In short: your solar system can add only 20% more load to your electrical panel than its rated capacity.
Example Calculation
Suppose you have a 200-amp electrical panel at home.
- 20% of 200 amps = 40 amps.
- That means the maximum solar breaker you can add is 40 amps.
So, if your system needs a breaker larger than that, you may have to upgrade your electrical panel.
Why the 20% Rule Matters
- Safety – Prevents overheating and overloading your electrical panel.
- Compliance – Ensures your solar system passes inspection and follows NEC code.
System Longevity – Protects your home wiring and solar equipment from electrical stress.
Workarounds if You Hit the Limit
- Main breaker derating (reducing the main breaker size to free up capacity).
- Electrical panel upgrade (installing a larger busbar panel).
- Load-side vs. supply-side connections (depending on local code and installer approach).
Conclusion
The 20% rule for solar panels is all about making sure your electrical panel can safely handle the extra power from your solar system. Before installing solar, always have a qualified installer check your panel rating and design the system according to NEC standards. It keeps your home safe and your solar investment protected.
FAQ: The 20% Rule for Solar Panels
What is the 20% rule?
Your solar system should not exceed 20% of your electrical panel’s rated capacity for safety and compliance.Why is it important?
It prevents panel overload, ensures safety, and keeps your solar system working efficiently.How do I calculate it?
Multiply your main breaker amps by 20%. Example: 200A × 0.2 = 40A maximum solar input.Does it apply everywhere?
Mostly yes, especially under NEC codes, but local rules may vary.What if I exceed 20%?
You may need a panel upgrade or special connection; exceeding it can cause safety and compliance issues.
