Transformer Sizing

[A/A Fuel GTX]

I've got a machine with a FLA rating of 125A @ 3 phase 120/208. I must install a transformer that will be fed from a 480V source to power the machine. How do I determine the transformer size needed to provide the required power?

 

[A/A Fuel GTX]

Can I just use the formula:

45000/208/1.732= 124.9A and assume a 45KVA is adequate and size my wire and OCPD @ 125% of 125A?

And for the primary would I go:

45000/480/1.732= 54.13A and size the wire and OCPD @ 125%?

 

[kingpb]

Maybe, maybe not. The issue is what is going to happen on starting of the machine. If the load, i.e. machine, has a starting current higher than the FLA, or a low starting power factor, then the transformer size, including impedance, needs to be adequate to handle the necessary starting current and VARS.

As an example, a single motor started DOL that is connected to a dedicated transformer, often times will need to be as much as 2X to 2.5X the size of the motor just to handle the reactance needed by the motor on starting.

 

[A/A Fuel GTX]

This machine basically has a soft start so the inrush is really not a factor.

 

[kingpb]

Soft start does not mean that it will not have any inrush, it will just be reduced.

Without anymore details to go by, my gut feeling is that the 45KVA is probably cutting it too close.