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690.8(B) Breaker Sizing


Q:

Hello
regarding question 6 explanation. I understand how you arrived at the 20 A breaker for the 100 A busbar. I am not familiar why you then took the 20 A breaker divided by 1.2 to arrive at 16 A.

A:

A breaker has to be sized based on 125% of the current of the device (and then round up). This comes from 690.9(B) of the NEC, which states:

"Overcurrent device ratings shall be not less than 125 percent of the maximum currents calculated in 690.8(A)"

This is usual throughout the NEC for other equipment that is not solar.

This means that if we are starting with the maximum breaker size, we can divide the breaker by 1.25 to get the maximum inverter size for the breaker (simple algebra).

Another shortcut is instead of dividing by 1.25, we can multiply by the inverse of 1.25, which is 0.8

1/1.25 = 0.8

20A x 0.8 = 16A
(this you can do in your head, since 8 x 2 is 16)

The reason the NEC wants us to not put a 20A inverter in a 20A breaker is to prevent false trips of the breaker, because in theory a 20A breaker should trip at 20A.

If you have a 20A inverter and wanted to determine the breaker size required, then:

20A x 1.25 = 25A

If you have a 25A inverter and you wanted to size the breaker:

25A x 1.25 = 31.25A

They don't make a 31.25A breaker, so you round up to a 35A breaker.

If you are over 800A you do not round up, but under 800A you do.

Thanks,
Sean White


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