NEC compliance for Backfed breaker

[Pjw88]

Hello im currently installing an eg418k which is fed by a 40A breaker at the end of a 200A main panel from the grid side. the load side supports a 100A subpanel/critical loads panel and after moving the critical loads I’ve discovered most of the remaining non essential circuits are either nonfunctional or completely unneeded to the point that only 100amps of circuit breakers will remain in the main 200amp panel. If the total draw in the main panel could ever be 100amp would I be able to increase the 40amp backfed breaker without overloading the bus in this situation and remaining code compliant? Thanks for helping everyone!

 

[400bird]

The loads in panel don't factor into the calculations.

It is just main breaker + backfeed breaker <= bus bar rating +20%

So, if you want to increase the backfeed breaker, your only options are to downsize the main or increase the rating on the bus bar

 

[Pjw88]

The loads in panel don't factor into the calculations.

It is just main breaker + backfeed breaker <= bus bar rating +20%

So, if you want to increase the backfeed breaker, your only options are to downsize the main or increase the rating on the bus bar

Thank you so much for the advice! I’ll keep it at 40 or downsize.

 

[HighTechLab]

The loads in panel don't factor into the calculations.

It is just main breaker + backfeed breaker <= bus bar rating +20%

So, if you want to increase the backfeed breaker, your only options are to downsize the main or increase the rating on the bus bar

I think there is some exception, need to look up, that if the sum of all load breakers in a panel is less than the bus rating, the 20% rule is out the window. I'll dig through my code book when I get to my desk in the morning.

Think about it.

The 20% rule is there to prevent an overloaded bus fed from two sources - but if your breakers are all at max load, they can't exceed the rating of the bus even if fully loaded since they would all trip before the bus became overloaded.

EDIT: I did recall that you can downsize your main breaker to increase your PV breaker size - for example if you have a 200A main, you can downsize it to a 150A and now your PV breaker could be 90A

EDIT2: NEC 705.12(B)(3) -

 

[Hedges]

I think there is some exception, ...

EDIT2: NEC 705.12(B)(3) -

Which I was just reading in some training last night.

I think with 200A busbar and 200A main breaker, this alternate rule lets you stuff the breaker panel with 200A total of branch circuit breakers. That's backfed PV + load breakers. So you could have a 100A PV breaker if sum of loads is 100A.

Likely your hybrid doesn't need 100A breaker. Mine could use 70A breaker for 56A continuous load to critical/backup panel. That would let you have 130A total breaker load on each phase.

 

[Pjw88]

Which I was just reading in some training last night.

I think with 200A busbar and 200A main breaker, this alternate rule lets you stuff the breaker panel with 200A total of branch circuit breakers. That's backfed PV + load breakers. So you could have a 100A PV breaker if sum of loads is 100A.

Likely your hybrid doesn't need 100A breaker. Mine could use 70A breaker for 56A continuous load to critical/backup panel. That would let you have 130A total breaker load on each phase.

Which makes sense considering the sum wouldn’t exceed 200. I don’t plan on back feeding and im targeting zero export which if configured with a 60/70A backfed breaker only for supplmental ac power should be compliant and include wiggle room for incidental backfeeds should they occur.