Breaker vs bus - Short circuit kA ratings

[powerpete69]

Background information:
I have an existing factory that has a switchgear that is rated for 65 kA for its bus. However, some of the breakers are only rated 25 kA inside this switchgear.
That might not be a problem, but with easypower arc flash software, I show a 27 kA short circuit on the switchgear bus in a faulted situation. The main transformer is 500 KVA with 2% impedance. Infinite bus calculation would be around 30 kA. so the 27 kA sounds reasonable since all the utility short circuit info was entered correctly from the utility company.

Actual question:
Is it against code to put a breaker that is rated less than 65kA on a bus that is rated 65 kA?
My guess is that a 35kA breaker would work just dandy, but I prefer not to guess.

Incidentally, first time I've seen a 2% impedance on a main transformer. Super efficient, but high kA ratings for your equipment.

 

[mayanees]

Actual question:
Is it against code to put a breaker that is rated less than 65kA on a bus that is rated 65 kA?

No, as long as the breaker can interrupt the available fault current.

 

[steve66]

Is there a main breaker or an upstream breaker? It could be a series rated combination.

I think the lower impedance transformers are becoming more common with the DOE energy efficiency requirements. It seems like lower %Z equates to fewer losses in the transformer.

 

[Jraef]

There is a difference between bus bracing and interrupting capacity, the issues are independent of one another. Bracing of the bus just has to meet or exceed the available fault current. Interrupting capacity also has to meet or exceed it as well, but just because the bracing exceeds it doesn't mean the IC has to, it just has to meet it, meaning they can be different so long as BOTH at least meet it.

It's also however probable that, as steve66 said, if there is a main breaker or fuses, those 25kA branches may be in a series rating with that main OCPD at a higher value than 25kAIC and that would be the value you would want to plug into your software..

 

[powerpete69]

The main breaker is 1600 amps and its rating is 65 kA.

The calcs were in easypower and all other electrical devices are included in the calc.

 

[powerpete69]

Sounds like 35 kA will do the trick and meet code.