Tranformer taps and voltages

[louis1]

This is my first time in this forum but have a question on a specific aplication i am working on. I have a situation where 480 is not available, only 3 phase 120/240. We need to add an additional 30 hp to the existing 30 hp circuit and pulling larger wire is not an option. So i have decided to use a transformer to step up to 480 and create the capacity to add the additional load. My question is this: I have a 75 kva xformer in stock that is designed for 480 primary to 120/208 all 3 phase. What voltage will i get on the primary taps if i connect the transformer backwards and input 240 v 3 phase on the secondary taps. (i dont have 3 phase available at my shop to bech test this and dont want the customer to watch me test it in the field

 

[iwire]

Besides NEC violations you would have about 554 on the output.

 

[louis1]

I was reading in some other forums that back-feeding is allowed by nec although i havent pulled out my book to check. And as far as the calculation, how do you come up with that voltage?

 

[defears]

Because 208 is diffrent than 240. 480/208=2.3x240=553.8 volts
You can use a delta to Y trans backward, and be code compliant, but then you have to corner ground it.
You are better off getting the right tansformer.

 

[louis1]

because 208 is diffrent than 240. 480/208=2.3x240=553.8 volts
you can use a delta to y trans backward, and be code compliant, but then you have to corner ground it.
You are better off getting the right tansformer.

corner grounding? Please excuse my ignorance and explain. Its gonna take too long to get the exact right xformer

 

[iceworm]

corner grounding? Please excuse my ignorance and explain. Its gonna take too long to get the exact right xformer

iwire is dead on - get the right transformer. For example when you say it wil take too long to get the right transformer - my question is, too long compared to rebuilding the installation after the incorrectly applied one burns up?

I don't see you have a choice. Tell the customer you don't have the correct transformer and you will have to test this one to see if it can be made to work or you will have to order the correct transformer.

When you go to test the 208 Wye winding connected to 240D, expect the 208 winding to saturate and the unloaded primary current will be way high. Expect the transformer to overheat without any 480V secondary load. Should be able to fry eggs on it after a couple of hours.

ice