# Sizing conductors for 208v to 480v 30kva transformer



## Bfath (Dec 12, 2018)

Hello, I need to install a step up transformer in a cabinet shop. The new machine is 480v Y and runs at 31 A at full load. I have already figured out that the size of transformer needed is a 30 KVA. I have determined the secondary side of the transformer needs a 8/4 copper teck cable. What I am mostly wondering about is the wire size for the primary side of the transformer at 208v. If the machine is running at 480v 31A, what size of teck cable will I need for the primary side of the step up transformer? Thanks for your help!


----------



## varmit (Apr 19, 2009)

If there are any across the line motor starters of any sizable horsepower, a 30 kva transformer could be to small to handle the inrush demands.

That being said, #8 fused at 50 amp would be minimum on the load (480) side.The line (208) side would need to be minimum #3 fused at 100 amp.

Be sure to make no connections to the transformer "XO" terminal.

The wire sizing and fusing are the same size as if the transformer was used 480 line to 208 load. The transformer losses will be a few percent more, so do not size a reverse fed transformer too closely to it's normal maximum load.


----------



## varmit (Apr 19, 2009)

Also, the 480 side of this transformer will be an ungrounded system. It would require a ground fault detector or be corner grounded.


----------



## Cow (Jan 16, 2008)

I would use a 208D x 480/277v transformer.

https://equotesv2c.hammondpowersolu...ectUrl=https://www.hammondpowersolutions.com/

Hammond Part Number: SG3A0030BK
Description: 3ph 30kVA 208V-480Y/277V 60Hz AL

Lead Time:Typically stocked*


----------



## joebanana (Dec 21, 2010)

Kva/(voltage x 1.732) No smaller than 125% of continuous load.


----------



## micromind (Aug 11, 2007)

The current on the 480 side of a 30 KVA transformer is 36 amps. On the 208 side it is 83 amps.


----------

