Transformer from existing feed.

[laketime]

I have a customer that wants to utilize an existing 3phase 480v 100a feed. They want to use a transformer to reduce voltage to 120/208v. The maximum amperage out of that feed we can get is 100 amps correct? We establish the neutral for the 120v on the load side of the transformer with bonding to building steel correct? Sorry for the basic questions but I have been out of the commercial game for too long.

 

[augie47]

Looking at standard size transformers: (and if my math is correct)
A 75 kva 3 phase transformer would have a 90 amp input and a 208 amp output at 208v.
A 45 kva is closer to 54/125.
(The NEC actually allows higher currents)
I would be a little reluctant to operate the 75kva on a 100 amp breaker taking inrush into account

FWIW: Most of the 45 kva transformers I see are supplying a 125 or 150 amp panel.

Bonding the neutral on the 208/120 side to building steel is one of the acceptable means of establishing a grounding electrode system as detailed in Art 250.30

 

[laketime]

Thank you so much.

 

[laketime]

The customer just called and said they are purchasing a 50kva transformer. Could you please supply me with the formulas. I dont mind doing the math. Or if you are looking for something to do please do the calculations but I would like to see the formulas. I used to know this stuff 20 years ago. I would like to know the output if fed from 480/3 phase 100 amp.

 

[augie47]

A 50 kva transformer would normally be a single phase unit.
In that case it's simply 50000/480= 104 amps for the primary and 50000/240 = 208
(IMO that will be pushing the 100 amp breaker)


If you are using 3 phase it would be 50000 /(480 x 1.732) or 60 amps 50000 / 208 X 1.732 or 138 amps