Derating a Subpanel

[Solar Noob]

I'm working on a job right now that has a 400A split buss MSP, with each split buss pushing 200A. The subpanel is connected to the house, where all of the loads are at. I'm not sure if I am correctly understanding interconnections when subpanels are involved. Can you derate a subpanel? The backfeed from the inverters is 45.375A, which exceeds the 120% rule for the subpanel (which is 200A main 200A buss). In this case, if it were the MSP we could just derate and be okay after checking the load calcs. But because it is a subpanel I'm not sure if this is allowed. Also, if we were to "line side tap" the subpanel doesn't that just constitute as a protected load side tap since there are breakers in the subpanel and the MSP? If anyone can clear this up that would be awesome.

 

[ggunn]

You can derate the main breaker in a subpanel to make electrical space for a backfed breaker, however, you have to be compliant with 705.12 in every panel between the PV breaker and the service.

 

[jaggedben]

The rules for subpanel busbars are no different than main panel busbars, as long as your not on the line side of the service disconnecting means.

If you tap the subpanel feeder you need to protect the subpanel with an additional breaker, like a main breaker in the sub. See the feeder tap rules in 705.12.