What is the 120% rule and why do you care Well simply put it refers to the amount of power that can be supplied to your main service panels bus bars. super clear right ? well how about we take a look at this and I try to break it down a little for you.
Lets start with what the National Electrical Code Book has to say about this NEC705.12(B)(2)(3)(b) states that where there are two power sources, the grid and the solar inverter, the sum of their breakers cannot equal more than 120% of the busbar rating of the breaker box this is what the code book has to say. What does that mean ? Well the way I understand this is as follows if you have a 200A main breaker box with a 200 A main Circuit breaker in it then you CAN NOT add another breaker for the inverter larger than 40 Amps. This is because (Here is our 120% Rule) 200A x 120% = 240Amps, which equals the main 200A breaker plus the 40A inverter breaker. Also keep in mind that the 200 A and the 40 A circuit breakers need to be on opposite ends of the bus bars from each other to prevent concentration of current at one end of busbar. If this is not possible and both breakers are near each other the 120% rule no longer applies and can only equal 100% of the panel box rating.
to put this into practice you need to determine the current rating for your main breaker box busbar rating generally there is a label in the enclosure with model number and other important information about the electrical panels rating I suggest you take a picture of it as well as the main breaker then use this info to determine the size of system you can apply to your existing electrical system. I will follow up this blog with a couple of others that will go into Inverter breaker sizing, and a few different options of how you connect to the electrical panel.
I suggest you read and understand NEC705.12 for yourself it would be a good read prior to my next blogs on Breaker sizing and connection options also I know the code book can be confusing and totally boring to read however if you don’t understand the rules how can you confidently design a system?