Why do we derate a main breaker for solar?

[Amjad Kazmi]

December 20, 2023 at 2:04 pm

Can someone explain the reasoning behind creating the main panel breaker when installing a PV system?

[Asad Marri]

December 22, 2023 at 12:45 pm

In the USA the electrical code includes a thing we call “the 120% rules”.

A breaker panel has a current limit. The main bus bars can only take so much power. If you have a 100 amp panel, and you add a PV Solar power system that can push 50 amps into the panel, you could theoretically load up the panel to 150 amps without tripping a breaker. That could be a problem. So after testing, they came up with the 120% rule. The total of the main breaker from the grid, and the breaker coming back from the PV solar added together can be no higher than 120% of the breaker panel rating. Now at first glance, this seems like you can still pull 20% more power than the panel is rated for, but they work around that by placing the solar PV back feed breaker at the opposite end of the panel. If you have 120 amps of load in the middle of the breaker panel, the solar might be feeding 20 amps up from the bottom, and the grid is supplying 100 amps down from the top. No part of the bus bar will ever see more than the 100 amp rating. The loads can pull the extra 20% from the other end of the bus bar.

So derating the main breaker down to say 80 amps on a 100 amp panel would allow up to a 40 amp solar PV back feed breaker. The maximum you could pull from the middle of the panel is still the same 120 amps, but now 40 amps are coming up from the PV breaker and 80 amps are coming down from the grid main breaker. The maximum current in any one length of the bus bar is now only 80 amps, so it is completely safe in the panel. In theory, you could push more power, since it is fed from the opposite ends, but there is still heat buildup. It was decided that increasing the heat load in the breaker panel by 20% was still safe, but they didn’t want to push it any further. It seems like a reasonable compromise.