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#130728 - 11/11/06 03:14 PM Variable transformer  
ayrton  Offline
Joined: Dec 2001
Posts: 201
I am not familiar with these. Is it safe to assume it regulates voltage?
Running through the center of the three coils is a shaft, and attached to the top of the shaft are gears which drive a chain. I am not sure what it is doing.

The nameplate on the housing reads 'Input V 240 Hz 50-60 Ph3 Output V 0-280 Amps 250 KVA 121.

Now to calculate what this thing should be fed with, I usesd the formula KVAx1000/ Voltage (240) 121x1000=121000/240=504.16

They are feeding this with a Nema5 starter using 2/0 wire. If I am calculating right, shouldnt the ampacity of the wire be based on 504?

This variable transformer than feeds an oil filled transformer with output votages as high as 36kv.

It is in a drywalled room in an office suite. According to art 450 I beleive it needs to be in a vault.

But there are 7 exceptions. I do not know the KVA of the second tranformer.

Please help!

Tools for Electricians:

#130729 - 11/12/06 01:00 AM Re: Variable transformer  
Bob  Offline
Joined: Feb 2002
Posts: 182
Mobile, AL, USA

#130730 - 11/12/06 09:46 AM Re: Variable transformer  
ayrton  Offline
Joined: Dec 2001
Posts: 201
Thank you Bob. That was informative. I still am not sure on the feeder size.
Who can tell me if I am right when using the formula?
121kva @240v is 504amps. This is currently wired with 2/0

#130731 - 11/12/06 09:58 AM Re: Variable transformer  
LarryC  Offline
Joined: Jul 2004
Posts: 781
Winchester, NH, US
121kva @240v is 504amps

If you are single phase.
But since you are three phase

KVA = V x A x sqr rt 3

KVA/(V x sqr rt 3) = A

121000/(240 x 1.73)= 291

#130732 - 11/14/06 05:39 AM Re: Variable transformer  
jraef  Offline
Joined: Jul 2004
Posts: 95
San francisco, CA, USA
Yes, but if the nameplate says 250A, it is 250A. That needs to be the basis for your fusing, so the wire need only be sized for the fused circuit.


#130733 - 11/14/06 08:00 AM Re: Variable transformer  
SteveFehr  Offline
Joined: Mar 2005
Posts: 1,198
Chesapeake, VA
At the maximum voltage level of the transformer:

121kVA = 280V x 250A (3P).

To set the OCP for 291A @ 240V would allow the transformer to overload if it was adjusted to 280V. If this were a normal transformer and you could lug it to the 240V terminals, OCP can be sized for 291A, but in this case, 250A is required for safety.

[This message has been edited by SteveFehr (edited 11-14-2006).]

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