Transformer calculations

[Evilsinge]

Hello. I was hoping someone could help me with the wire sizing on the secondary side of a transformer. I want to use a 277 volt single phase to 120/240 volt 3 kva transformer. The question is do I use the lower or higher voltage for my calculation.

3 x 1000 / 240 = 12.5 amps
3 x 1000 / 120 = 25 amps

I fill I should go with the higher amperage and 10 gauge bx wire.

Any help would be much appreciated.

Cheers Paul.

 

[GoldDigger]

Hello. I was hoping someone could help me with the wire sizing on the secondary side of a transformer. I want to use a 277 volt single phase to 120/240 volt 3 kva transformer. The question is do I use the lower or higher voltage for my calculation.

3 x 1000 / 240 = 12.5 amps
3 x 1000 / 120 = 25 amps

I fill I should go with the higher amperage and 10 gauge bx wire.

Any help would be much appreciated.

Cheers Paul.

The transformer will produce 12.5A at 240V, not using the neutral wire on the secondary.
Or it will produce 12.5A out of each line output independently, using the neutral wire as the return. So if you have two maximum sized 120V loads, you could say that you are producing 25A at 120V.
What you cannot do (because of the size of the wire in the secondary windings) is put a single 25A 120V load from one side of the secondary to the neutral.

If you operate the transformer within its rating, no conductor will be carrying more than 12.5A. (When you have two 12.5A 120V loads, the currents in the neutral will actually cancel out, not add. )

 

[charlie b]

Short answer: you always use the larger number. For example, if you have a three phase, 30 KVA transformer with a secondary rating of 120/208 volts, you divide the 30,000 VA by 208 (and then divide again by the square root of 3).