When it comes to sizing a solar PV system to install on your home, there are a number of factors that come into play -- how much energy you consume, how much of that usage you want to offset with solar electricity, and how much viable space you have on your roof, among others.
After weighing all these factors and determining the right system size for you, there is another important factor to bring into the equation, which could prove costly if not caught and addressed early, and that is whether your existing electrical panel (the main electrical box with all the breakers protecting the circuits in your home) can handle the solar inverter. In most scenarios, inverters get connected to an electrical panel with a breaker, which acts as an overcurrent protection device (OCDP).
The National Electric Code (NEC) has a very specific rule that dictates the maximum size breaker you can use to connect your solar PV system, commonly known as the 120% Rule.
What is the 120% Rule?
The 120% rule refers to NEC705.12(B)(2)(3)(b), a rule that determines the calculation of a maximum size breaker we can interconnect to the electrical panel and therefore the maximum size of the inverter we can use.
The code states the following:
“When two sources, one a primary power source and the other another power source, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the power source(s) output circuit current and the rating of the overcurrent device protecting the busbar, shall not exceed 120 percent of the ampacity of the busbar.”
Putting this code into an easy to read equation:
Maximum inverter breaker size = ((1) Busbar ampacity * 1.2) - rating of the main breaker.
The table below has a typical main electrical panel (busbar rating) and main breaker combinations, as well as the size of the inverter these combinations would allow for using the 120% rule:
If you’re curious as to how we arrived at these calculations, please see How do I calculate my inverter’s OCPD (breaker)?
As the code implies, this rule applies only if you are able to install the inverter breaker so that it is located at the opposite end from the main breaker on the busbar, that is, only if the electrical panel has the main breaker at the very top or bottom of the busbar, not in the middle. If the panel is center-fed (it has the breaker in the middle of the busbar) you may be subject to the 100% rule, meaning you won’t be able to apply 120% or the 1.2 multiplier to the busbar ampacity. This would significantly reduce the maximum size of the inverter you can connect to your electrical panel or even disqualify it.
Center Fed Panels:
Maximum inverter breaker size = (Busbar ampacity) - the rating of the main breaker.
If your main is center-fed you may be able to relocate the breaker to the top or bottom of the busbar. To find out if this is possible, first check with the Authority Having Jurisdiction (AHJ) and also with the electrical panel manufacturer to make sure this will not void the panel’s warranty.
What if the inverter requires a bigger breaker than is allowed by NEC code?
If you find out your main panel/main breaker combination is not suitable for your inverter, there are several workarounds.
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