How to calculate the max breaker size for a solar system
Recently I have been getting a lot of questions on what size breaker can be installed for a solar system in your main service panel. I wanted to answer that question in this blog today. I have also made a video and shared it on all my social media platforms. I will embed the YouTube Short below.
How to calculate the max breaker size for a solar system
How to calculate the max breaker size for your solar system. It is very simple math. Just take the max bus rating of your panel. Using the 120% rule set by the NEC on a solar breaker. Example: 225 max bus rating would look like this. 225 x 120 = 270. 270 – main breaker size (ex: 200 amps) = 70amps. That is how I found that the 45amp breaker was allowed in my main panel. I will follow up on how I determined the 45amp breaker in another blog very soon.
The Code?
In the 2011 National Electrical Code (NEC), the language in 705.12(D)(2) is straightforward:
In the 2014 code, this one sentence has been revised to be several paragraphs long with different scenarios. However the philosophy holds true, and once you understand the philosophy of the simpler 2011 version of 705.12(D)(2) you will be able to understand NEC 2014’s more sophisticated version.
The Conclusion
Real-life examples are not always as cut and dry as 1-2-3. You can have cold sequence metering, tapered buses, interconnecting at subpanels, etc, things can get very tricky. As long as you apply the philosophy above to the new language in NEC 2014, you can work through the problem and ensure a load-side interconnection cant overload the bus.
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I cannot thank you enough for the article.Really thank you! Cool.
You are very welcome!
Thanks for your great information about DIY solar. I am just getting started with my project. In one of your videos you mentioned lyrasolar.com for their awesome planning packages. Unfortunately, it seems that they are now ‘waitlisting’ the regular DIY guy and instead focusing on larger commercial customers (> 5 plans per month). Bummer. I guess I am going to look for an alternate vendor for my plan package.