Step up transformer calculation

[Nietz001]

I have an unusual request to step up power from 208 to 480volts. If anyone cares to help I will give you my answer and hopefully I'm correct.
208 3 phase to 480 volts 3 phase
load at secondary - 480 volts is 125 amps
My math tells me I need a 415kva transformer at 208 to get the power I need.
Does this sound right, that's a big transformer

 

[david luchini]

125 Amps at 480V, 3ph is 103.9kVA.

 

[jdcpe17]

this isnt that unusual. kva rating would be the same looking at the primary or secondary. the volts and amps both change. now something else you could consider, the transformer can be basically 125% over used... meaning you could potentially go as low 83.14 kVA. but youre not going to find a 83 kva transformer... make sure you get wye and delta orientation correct for whatever you need...

 

[templdl]

Also, If I may suggest that you specify a 208 delta to a 480Y/277 transformer. It is not all that unusual for 208Y primary to be specified based upon the source voltage of 208Y/120 and a 480 delta 3w secondary based upon the neutral not being required but with little regard to just how you are going to ground it.

 

[Nietz001]

I guess I knew that the transformer would be sized by the load at the secondary, I guess what I was thinking is what size would the primary need to be. Assuming the transfomer has a turns ratio of 4:1 wouldn't that be about 600 amps on the primary or am I mising something?
Jeff

 

[david luchini]

I guess I knew that the transformer would be sized by the load at the secondary, I guess what I was thinking is what size would the primary need to be. Assuming the transfomer has a turns ratio of 4:1 wouldn't that be about 600 amps on the primary or am I mising something?
Jeff

You are missing something...the turns ratio is 1:2.3. The primary current would be 288 Amps with a load current of 125 Amps.

 

[kingpb]

I guess I knew that the transformer would be sized by the load at the secondary, I guess what I was thinking is what size would the primary need to be. Assuming the transfomer has a turns ratio of 4:1 wouldn't that be about 600 amps on the primary or am I mising something?
Jeff

KVA is KVA! it doesn't matter which side of the transformer you are on.

(sqrt 3) x 480V x 125A = 104kVA (load)

Standard size transformer will be 112.5KVA; at 208V that's 312A, at 480V that's 135A.