Solar Backfeed Breaker

The main purpose of the fuses and breakers are to protect the wiring.

Solar backfeed breaker.

The nec allows adding a back feed breaker so that the current supply is up to 120 of the rating of the bus bar. If a panel is rated at 125 amps and the breaker is 100 amp it seems to me that you could add 25 amps backfeed plus another 20 amps based on the 120 rule. So you always want to be sure your sizing your wire to handle the output of each circuit in the system. Then looking at that photo again you can see i have a 30a 2 pole breaker at the other end for my solar inverter.

The sizing of back feed breakers is another point of difficulty. However if we add our pv breakers into a sub panel that is dedicated to only the pv breakers we can land the sub panel into a 40a breaker in the main panel board 10 4a 21a 31 4a x 1 25 39. From there you d land your inverter s output into a 50a breaker and your done. We sometimes see people adding a 60 or 80 a breaker instead.

Circuit requirements are outlined in part ii of this article. The transfer switch or interlocked backfeed breaker acts to prevent the connection of the generator while the main breaker for the panel is closed. The sum of these breakers is 45a and we already know we cannot land them in the main panelboard because it will only allow us 40a using the 120 rule. This would make 45 amps solar into a 100 amp 125 amp rated panel.

As noted in sec. Nec705 12 b 2 3 b states that where there are two power sources the grid and the solar inverter the sum of their breakers cannot equal more than 120 of the busbar rating of the breaker box. The backfeed generator breaker can t be closed while there is main power on the panel and when there is a power failure it can only be closed after the main breaker is open. If a 200 a busbar has a 200 a main breaker the code allows adding a up to a 40 a backfeed breaker.

690 10 e plug in type back fed circuit breakers connected to a stand alone inverter output in either stand alone or utility interactive systems shall be secured in accordance with 408 36 d. For a 200 225 rated panel it would be 65 amps.