SCCR/AIC of a subpanel

[Electromatic]

I know there is a lot more to fault current calculations than what I wish to get into here, but I'm curious if the following is at all reasonable.

When installing a subpanel, could one take the AIC of its feeder breaker and simply use the impedance of the feeder conductor run to calculate what rating the breakers in the subpanel should have?

 

[jim dungar]

Assuming the upstream AIC is accurate, your method would likely give a highly conservative answer. This would be similar to having an infinite source.

I have seen many boiler plate engineering with breaker AIC specified by voltage rather than considering conductor impedance. The result is 65kAIC breakers installed where the available fault current is less than 10kA.

 

[winnie]

I agree with Jim.

If you know the available fault current at the supply breaker, you can use the feeder impedance to calculate what is available at the load end of the feeder.

The interrupting rating of the supply breaker tells you something about the actual available fault current, but not the whole story.

Jon

 

[augie47]

Keep in mind, if your subpanel is the same manufacturer as the supply panel, it's likely your subpanel breakers will series rate with the supply breaker allowing you a lower rated breaker than the calculation allows.

 

[petersonra]

I know there is a lot more to fault current calculations than what I wish to get into here, but I'm curious if the following is at all reasonable.

When installing a subpanel, could one take the AIC of its feeder breaker and simply use the impedance of the feeder conductor run to calculate what rating the breakers in the subpanel should have?

You have no way to know if the feeder breaker AIC is appropriate unless there is some documentation that shows it is. What if someone installed a 22kAIC CB at a place there is 30kA of available SCC.

 

[Electromatic]

Thanks, all. I realize there are other considerations to my OP--mainly the assumption that the feeder breaker is correctly rated (could be underrated, could be overly conservative). I just wanted to check that it's reasonable, generally speaking.

 

[steve66]

Well, what if the feeder breaker is series rated with an upstream breaker? Then the fault current at that breaker might be higher than the breaker rating. For example, many panelboards with higher ratings than 10KAIC use a higher rated main breaker that is series rated with 10KAIC branch breakers.

So I would say no. You would be more likely to be OK using the feeder breakers upstream main breaker, but even that is no guarantee.