Transformer Sizing.

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You are starting on the wrong end of the circuit :)
You need a transformer large enough to carry the load + whatever future you wish to add.

If you truly want a three-phase 240/120 transformer, take care as you will have a high-leg and quite possibly a reduced neutral capability.
 
Backasswards way of doing it IMO, but if you have 400 amps of 480 volts 3 phase available you have roughly 332 kva. If you want to step the entire 400 amps up or down to any 3 phase voltage you are looking at a 300 kva transformer + or -
 
240 volts, 3 phase @ 200 amps is 83 kva.
The closest standard size would be 75 kva and that wold normally be sufficient but there are so many pitfalls it approaching it from this angle anything is possible.
As already mentioned, Step 1 is to determine to load. In your case, load per phase would be important as neutral loading could be pertinent.
 
FWIW, I'm in agreement with Gus' line of thinking thus far... and that is the reason I have not posted anything :sleep:
 
augie47 said:
240 volts, 3 phase @ 200 amps is 83 kva.
The closest standard size would be 75 kva and that wold normally be sufficient but there are so many pitfalls it approaching it from this angle anything is possible.
As already mentioned, Step 1 is to determine to load. In your case, load per phase would be important as neutral loading could be pertinent.
Click to expand...
+1.
Wirenut46' use of 200A all phases is still somewhat ambiguous, given that he specified 120/240 three phase.
If his load is balanced delta only, I wonder why he wants one center tapped winding. And if he then tries to combine some 120V loads, the loading will be higher.
If he has no 240V loads then he does not need three phase output in the first place. Based only on what he has said, there is no way a 35kVA will do the job.
 
Are you certain you want a 240 delta system and not a 208Y. As has been previously mentioned, the 240 delta will provide you with 2 120 volt phases and a 208 volt high leg. Take a look at this Wikipedia URL.

http://en.m.wikipedia.org/wiki/High-leg_delta

As far as sizing the transformer, you really should do a load calc and determine what your customers needs are. Once you do a load calculation the following link will help you size your transformer, the primary and secondary conductors, your OCPD, etc.

http://www.mikeholt.com/mojonewsarchive/NEC-HTML/HTML/TransformerInstallation~20020516.htm

I hope this is helpful if not informative, good luck.
 
Before givin an answer let's be sure of the question, primary 277/480 3 phase ( if you have 3 phase 480v, then you don't need the neutral to feed the transformer, so forget about the 277 this is only if you mesure to yor neutral or ground), then you want to go to 120/208v 3Phase (this is the most common) or you need 120/240v 3 phase( not to common)?
The 277/480v primary to a 120/208v Y a 75kva is what's common used, (common, very important word, check you load to confirm what you need)
 
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