30kva transformer

[Sparky125]

So I have a 3 phase 30kva step up transformer
primary side is 120/208 to step up to 230/400v to feed a 3 phase table saw. My question is what size ocpd for primary said and secondary. The load for secondary is 25 amps.
30,000/208x1.73 = 83.3 amps
Do I multiply by 125% or %250 after?
Thank you so much.

 

[kwired]

400 volts is usually 50 hertz. Is your saw rated 50 hertz?

If so it likely won't like running at 60 hertz presuming that is what your supply is. It will run faster than it does on 50 hertz as well, like about 20% faster.

 

[augie47]

450.3 dictates the maximum overcurrent protection for the primary and secondary.
It is commonplace to size the primary OCP at 125% of the transformer current (next size up permitted) but, with proper secondary protection, the primary can be as large as 250%.

 

[Sparky125]

450.3 dictates the maximum overcurrent protection for the primary and secondary.
It is commonplace to size the primary OCP at 125% of the transformer current (next size up permitted) but, with proper secondary protection, the primary can be as large as 250%.

So can I put a 100 amp breaker on primary?

 

[augie47]

If you follow the 125% rule, you would have to use a 90 amp (240.6 standard size)