Solar Backfeed Breaker

When setting up a generator to backfeed your house power can be pushed out through the main breaker to the transformer and cause unexpected high voltage.

Solar backfeed breaker.

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. The transfer switch or interlocked backfeed breaker acts to prevent the connection of the generator while the main breaker for the panel is closed. From there you d land your inverter s output into a 50a breaker and your done. This would make 45 amps solar into a 100 amp 125 amp rated panel.

The nec allows adding a back feed breaker so that the current supply is up to 120 of the rating of the bus bar. So you always want to be sure your sizing your wire to handle the output of each circuit in the system. 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 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.

For a 200 225 rated panel it would be 65 amps. 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. We sometimes see people adding a 60 or 80 a breaker instead. The main purpose of the fuses and breakers are to protect the wiring.

Then looking at that photo again you can see i have a 30a 2 pole breaker at the other end for my solar inverter. As noted in sec. The sizing of back feed breakers is another point of difficulty. If a 200 a busbar has a 200 a main breaker the code allows adding a up to a 40 a backfeed breaker.

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. Circuit requirements are outlined in part ii of this article.