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Variable transformer #130728
11/11/06 02:14 PM
11/11/06 02:14 PM
ayrton  Offline OP
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!

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

Re: Variable transformer #130730
11/12/06 08:46 AM
11/12/06 08:46 AM
ayrton  Offline OP
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

Re: Variable transformer #130731
11/12/06 08:58 AM
11/12/06 08:58 AM
LarryC  Offline
Joined: Jul 2004
Posts: 783
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

Re: Variable transformer #130732
11/14/06 04:39 AM
11/14/06 04:39 AM
jraef  Offline
Joined: Jul 2004
Posts: 101
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.

Re: Variable transformer #130733
11/14/06 07:00 AM
11/14/06 07:00 AM
SteveFehr  Offline
Joined: Mar 2005
Posts: 1,213
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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