Sorry in advance, this is all brand new to me. I am debating solar, but it is my understanding that if our panel bus rating is 100A or lower, we'll be limited to the size of the system we can have installed. But if it's 125A or higher, then we have a lot more options.

Since there is no main breaker switch, and the labels on the inside of the panel have worn off, I have no way to tell what our panel's bus rating is. Please help!

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  • 2
    I'd be willing to bet your service is 100 amps. This is just based on the age of your panel and the number of breaker spaces.
    – BillWeckel
    Nov 14, 2018 at 21:14
  • I'd seriously consider upgrading your service anyway, since you're doing all the other work. It's not terribly expensive and you can actually do it yourself with some good guidance.
    – isherwood
    Nov 14, 2018 at 21:51
  • 1
    That ol' dog could even b e a 60.
    – isherwood
    Nov 14, 2018 at 21:51
  • A lot of older homes had 60a services what is the size of the breaker feeding the panel?
    – Ed Beal
    Nov 14, 2018 at 22:44
  • 1
    Is this the only panel for your house, or is there a breaker somewhere else, such as at the meter? Nov 15, 2018 at 0:18

1 Answer 1


That's a Square D panel and in their world, a 12 space - 24 circuit panel is rated 125A. The largest 100A panel they sold was 8 space - 16 circuit. This is what they offer CURRENTLY, but in reality not much has changed in the last 30 odd years with them.

But that doesn't mean your SERVICE is rated for 125A, that would be determined by the main breaker that is feeding this panel. You will need that information as well. Here is the rule they are referring to:

(Busbar Rating (A) x 1.2) - Main Breaker Rating (A) = Max PV (A)

Most likely the solar contract already knows the size of your main, but he doesn't want to take on responsibility for determining the size of your panel bus. So if your panel bus is 125A and your main Service breaker is 100A, then your PV system limit is (125 x 1.2) - 100A = 50A of PV solar. But if your panel bus is only 100A and your breaker is 100A, your PV limit is only 20A.

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