AIC Ratings

[Nick]

I have a situation where we have an existing 480/277V 3000A switchboard that we are adding two buckets to. Except for the 3000A main all over current devices are fused switches. We have the option of adding fused switch buckets or circuit breakers. The existing gear does not have a label telling us what AIC it is rated. The main is rated 100KAIC and the fused switch buckets are rated 200KAIC. The existing plans call for 100KAIC bus but state the available fault current at 42KAIC. I have not received the real available fault current from the utility yet.
My question has to do with the rating of the board. It appears to be rated 100KAIC. The available fault current will probably come in around 20KAIC. If we install 42KAIC rated breakers, are we violating anything in a remodel condition like this?

 

[bwilson]

Re: AIC Ratings

As long as the breakers have a higher AIC rateing
the the avalible fault there is no problem, unless
this is a series rated system.

 

[ron]

Re: AIC Ratings

Go by the existing plans minimum. They might know of a utility, generation change/possiblity or distribution change effecting the available, and if you go to 42K, it is not going to serve the client in the long run.

 

[templdl]

Re: AIC Ratings

Just for grins for 3000a @480v would require a 2500kva transformer. With an unlimited available fault current on the primary with a common 5.75%Z the available fault current on the secondary would be a bit over 52ka plus 12ka if you and 100% motor load providing a total of almost 65ka the way I look at it.

 

[Nick]

Re: AIC Ratings

Thanks for the replies,

bwilson, It is not a series rated system.

Ron, the plans do state available fault current 42KAIC. For some reason there is a note on the plans for the gear to be 100K. I am researching the gear to see what it was manufactured for. It does not state on the gear what the entire assembly was rated for.

Templedl- Per a walk today with the utility rep the transformer is 1000KVA, I don't know the impedance yet, there is a 190Ft run of 6 paralleled 700 Aluminum to the service gear. Using the value you gave (5.75%) which is pretty typical for this utility I get just under 19KAIC available. (Not taking into account motor contribution which is minimal)

 

[gwz2]

Re: AIC Ratings

Not knowing the distribution system, You say it is not a series rated system. Then all OCPD's will need to be fully rated for the Available Short Circuit Current (ASCC) at the point where the OCPD is. Are there any 10Kair CB's which will be seeing that 19K you mentioned?

 

[jerryb]

Re: AIC Ratings

Don't forget to account for the motor contribution that will be added to the available fault current from the utility. It sounds like the 190 foot run will help you with this part.