Solar Backfeed Breaker

The sizing of back feed breakers is another point of difficulty.

Solar backfeed breaker.

We sometimes see people adding a 60 or 80 a breaker instead. If a 200 a busbar has a 200 a main breaker the code allows adding a up to a 40 a backfeed breaker. So you always want to be sure your sizing your wire to handle the output of each circuit in the system. From there you d land your inverter s output into a 50a breaker and your done.

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. Then looking at that photo again you can see i have a 30a 2 pole breaker at the other end for my solar inverter. 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. Circuit requirements are outlined in part ii of this article.

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. The nec allows adding a back feed breaker so that the current supply is up to 120 of the rating of the bus bar. The main purpose of the fuses and breakers are to protect the wiring. As noted in sec.

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. 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. 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. 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 transfer switch or interlocked backfeed breaker acts to prevent the connection of the generator while the main breaker for the panel is closed. Article 690 requirements apply to solar pv systems.