Backfeeding a 3ph 75kva step up transformer

[electrical16]

Hi I am trying to backfeed a 75 kvs step up transformer originally it steps up from 240/480 I need it to go 480/240 the transformer is already hooked up to the building but the owner want a 480v 3ph backup generator to run through the transformer to power the building so my issue is when I hooked up the generator to the panel it feeds in 480 between the terminals 240 each terminal but when it comes out of the transformer I get 240 between the terminals but when I test each terminal by itself I get 22v on one 240 on the other 2 when it's supposed to be 120 on each and 240 between them can someone explain why this is happening thnx

 

[ptonsparky]

Draw out what you did and give a model and make of transformer.

Details.

 

[CoolWill]

No bonding jumper on the secondary? I agree that a diagram would be useful. And a transformer nameplate pic would be awesome.

 

[infinity]

I agree is this Wye/Delta, Delta/Delta, Delta/Wye? Photos will help with your questions.

but when I test each terminal by itself I get 22v on one 240 on the other 2 when it's supposed to be 120 on each and 240 between them

How are you testing each terminal?

 

[ptonsparky]

I've never fed a three-phase step down transformer in reverse, to step up voltage. Not a recommended procedure FWIU.
Guessing the same can be said about reverse wiring a step up transformer.

 

[Tulsa Electrician]

Your 240 coming into the building what type of system. High leg, corner grounded or ungrounded delta.

Your transformer may not support a high leg.

Show us a pic of the transformer label and let us know what your system on the 240 side.

 

[kwired]

Hi I am trying to backfeed a 75 kvs step up transformer originally it steps up from 240/480 I need it to go 480/240 the transformer is already hooked up to the building but the owner want a 480v 3ph backup generator to run through the transformer to power the building so my issue is when I hooked up the generator to the panel it feeds in 480 between the terminals 240 each terminal but when it comes out of the transformer I get 240 between the terminals but when I test each terminal by itself I get 22v on one 240 on the other 2 when it's supposed to be 120 on each and 240 between them can someone explain why this is happening thnx

If originally intended to step up 240 to 480 there is a pretty good chance the 240 side is delta connected and there is no neutral terminal. If so all you will be able to use this for is 240 volt ungrounded or corner grounded delta if reverse feeding it.