Isolation transformer sizing per the protective circuit breaker

[Stacker]

Hello,

Question:

If I have a 480Vac 200A circuit breaker service feeding a 480A/200A machine, machine is also has a molded case inverse time circuit breaker of 200A, if I place an isolation transformer on the load side of the feeding circuit breaker, line side of the machine circuit breaker can I size the transformer to the FLA of the equipment, in regards to conforming to the NEC?

The FLA of my machine is 160A.

a 145KVA XFMR @ 480Vac is rated for 174A
a 175KVA XFMR @ 480Vac is rated for 210A

What is confusing for me is if I use the same transformer as a drive isolation transformer inside of the equipment I would size this transformer to the FLA of the branch circuit, per UL508A, with respect to the NEC.


Thank You,

Todd Evans

P.S. - This forum has given me much, I've came here often for advice and have learned many things. If I can answer anything with UL508a, I'm not bad. I just don't claim to be a master electrician so I defer to those that are.

 

[Smart $]

I didn't do the calc's, but it appears the smaller would be compliant. The only doubt stems from whether the machine has any large motors. If yes, the transformer should be sized large enough for the voltage not to sag lower than what the motor needs to start... and even if sufficiently sized for that, there may be some shortening of transformer life expectancy. There are some websites which give more adequate information on sizing transformers for motors than I can provide here in a few short words...