Transformer question

Baitjunky · Jun 25, 2015

I just bought a mazak vtc mill. I am feeding it with 208v main service line. I was told by the dealer it was 28 kva. By my calculations thru internet searches etc. I came up with need a 80 amp primary circuit. I feel very comfortable wiring etc. Just not real expirienced with transformers or 3 phase. Can anyone help me out on sizing the primary circuit.
Thank You
machine is a 99 mazak vtc 20b. Fusion 640 controller.

SAF · Jun 25, 2015

Not enough info to help you sort it out. Pictures help.

What does the machine nameplate say, what are the machine load requirements?

You say "208 main line" I'm assuming that would be your source voltage. Is it single Φ or 3 Φ source you have.

And is your load single Φ or 3 Φ and at what voltage and frequency?

SAF Ω

Baitjunky · Jun 25, 2015

I was under the impression the secondary had no bearing on the primary. Being the transformer is 28 kva. That is the primarys load correct? 208 3 phase.

JST · Jun 25, 2015

It does come out to 80A as a 3 phase circuit. Actually about 78A.

Since it is likely to be in what amounts to "continuous use" or "long term use", you would want to protect the circuit and wire it at 100A, if the 28kVA is what it draws when working. That's to keep the 80% on molded case breakers.

If the unit requires a lower current limit on its input line, that will probably be a fuse that you may have to supply in the box with the disconnect. And that might let you decrease the supply breaker. If they allow a breaker, you could do that size for the branch circuit itself.

Baitjunky · Jun 25, 2015

Machine has built in transformer. All the secondary stuff is built in.

Baitjunky · Jun 25, 2015

Jst that is exactly what i came up with. 78. So a 100 amp breaker with #3 thnn in conduit is adequate?

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