480v transformer kva size

[rcrabbe13]

Hi all,

Im trying to figure out what size transformer I will need for a 480v(primary) to 240 secondary with 2 200amp breaker panels. I found a couple forums with misleading information. I did my math and found I would need at least a 150kva but stumbled across a thread that stated a 100 kva will work for 400amp? Just need some clarification.

Thanks!

 

[augie47]

There are some variables to consider, but the basics would be 400 amp X 240 or 96 kva.
In many cases a 75 kva would work as you are allowed 125% on the secondary current but a lot depends on the nature of your load (motors, continuous, etc) and the actual load.

 

[topgone]

I suggest you draw your circuit and start from there. That's how it is with me. It ain't hard, compared to looking at the ceiling seeing numbers flashing.

 

[rlundsrud]

I would like to see a load Calc so that we know what your actual load is, but here are a few numbers that might help.

The secondary output (240v) for the following 3 phase transformers would be:

150 kVa= 150,000/(240?1.732)=360 amps
112.5 kVa= 100,000/(240?1.732)=270 amps
75 kVa= 75,000/(240?1.732)=180 amps

I hope that helps, if you want your primary amperage, simply substitute 480 for 240 in the above formulas. There is a very nice article that Mike Holt wrote some time ago that covers all aspects of transformer calculations, the link is below.

http://www.mikeholt.com/mojonewsarchive/NEC-HTML/HTML/TransformerInstallation~20020516.htm

 

[augie47]

Taking into account the OP's profile, adequate information has been provided to answer his question.
I am closing this thread.