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Recently I got a bid from a solar company and they said I don't need to upgrade my electrical box since it is rated at 125 amps, and for that value, I can install a 7k system based on calculation here. I poked around the panel itself, and found two ratings, 100 amps and 125 amps, and I am not sure which rating that solar company should use, and blowing out my electrical box after solar panels are installed is definitely the last thing I want to see.

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Yes, the short of it is that what we’re concerned with on solar is the internal bus rating of the panel’s buses, not anything else.

Suppose you have a 100A main breaker because your wiring from pole through meter to main breaker is only 100A rated. Then you have 80A of solar back feeding your panel. There was a bungle in how your house was load factored, and as a result you are pulling 24A on each of a dryer, range, A/C and water heater, plus 80A off that Tesla fast charger that probably should not have been approved :)

So you have 180A of draw and 80A coming in from solar and 100A coming in from utility. That means you have 180A moving on the panel’s internal bus.

Now if that bus is 200A, no problem. UL and NFPA got together and determined even on a 150A bus that is unlikely to be a problem given the safety margins UL already requires in panels. Hence they give you a 20% “mulligan”.

But if you have a 100A bus, then no way, Jose.

On that particular panel, it was clear both versions of the panel had 125 A busing.

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