In an existing building the utility is replacing transformers (for an addition which will have a new separate main disconnect). The new transformers have a much higher short circuit let through current. The original building service is a 50 year old ITE switchboard. The let through current is over 80,000A. The short circuit rating of the main circuit braker is 50,000A. (the transformers are about ten feet away from the switchboard indoors in a room next to the electric service room) Any suggestions (other than replacing the entire switchboard)??? The main circuit breaker and switchboard are rated at 1600A.
Anything put between the transformer and the existing equipment would have to have a tested series rating. It is all but impossible to get a calculated series rating unless the existing equipment are "power circuit breakers".
Have you thought about running the incoming cables around the building a few times? This will add impedance and lower the available fault current.
Re: New Transformers/New Short Circuit Current#64366 04/06/0608:43 PM04/06/0608:43 PM
You only have a few choices. By mearly adding a current limiting device between the transformer and the 50kAIC breaker doesn't help. They must be series rated together, which they generally are not for older equipment. You can add a reactor downstream of the transformer, and it will limit fault current, but be careful of voltage drop problems. Validate the fault current that you are being told, at least with an infinite primary calculation, and don't forget to add contribution from downstream motors. Wire can reduce current significantly, so run lots of wire as mentioned by JBD.
Re: New Transformers/New Short Circuit Current#64367 04/06/0609:11 PM04/06/0609:11 PM
Check the fault current as given. If you were given a fault current with an infinite source the actual value will be less. You will need to recaculate the fault including the utiltiy system impedance. The utility may be reluctant to give this information.