Interrupting capacity of sub breakers

[PCEC]

I have a 400A 120/208V MLO switchboard with 18kAIC bracing. Do my breakers also need to be 18 kAIC? I assume it is 18kAIC for a reason but haven't seen a calculation and do not know the actual available short cct current.

 

[augie47]

Awfully low bracing for a switchboard. I would double-check.
Your branch breakers would need to be rated for the available fault current although they would likely series-rate with your main.
In any event, the available current should not exceed the bracing.

 

[JoeStillman]

Awfully low bracing for a switchboard. I would double-check.
Your branch breakers would need to be rated for the available fault current although they would likely series-rate with your main.
In any event, the available current should not exceed the bracing.

It's a MLO switchboard so there can only be a series rating with a certain upstream fuse.

 

[augie47]

missed that. Tnks