RPC transformer sizing ratio or rules of thumb

Greetings, I have a 15hp rpc and am in need of a step up transformer for a 480v application.

There is a used 30kva transformer available fairly locally, which is bigger than necessary, but I'm concerned about the inrush.

Is there a rule of thumb and/or ratio available for the maximum kva a given hp rpc can handle?

Thanks in advance

how do you intend to ground the 480v side?

the issue with transformers is the reactive current drawn by the transformer will reduce the voltage a bit. this can be compensated for with capacitors. inrush isn't a problem except for the upstream circuit breaker.


one method to "mostly ground" a floating 480v delta transformer secondary in this application is to connect 3 equal sized capacitors in Y, and ground the common point.

then get 3, 480v rated indicator lamps and wire them up from each phase to ground. if your 480v system gets grounded somewhere, one of the lamps will go out, the other will get a lot brighter.

note that this will not meet "code" without a ground fault detector wired up.. yes they exist. floating systems with ground fault monitoring is used in a lot of high reliability systems.. any phase can be grounded while the system remains operational, but at increased voltage stress levels.. submarines for example, where pump failures are not acceptable.