Transformer Primary OC Protection Only. Delta to Wye.

[tsgelectric]

I need to hook up a machine at 208-240V 3ph using a transformer to step-down the voltage from 480V. There is not a convienient place for me to add Secondary overcurrent protection so I wish to use the primary OC protection only. The NEC code specifies that secondary overcurrent protection is not needed provided the transformer is connected Delt/Delta with a single voltage from the secondary. The only transformer I have lying around is a Delta/Wye with a 208/120V Secondary.

Does anyone know if this transformer could be used if I did not ground or use the XO junction on the Secondary? Would this cause any problems if the common of the windings were not grounded? What if the load is unbalanced? Does this violate code?

Thanks for any info if you have it.

 

[davidbeach]

Yes, it would violate code if there is not secondary protection. You could put CTs on the secondary and use a relay looking at the secondary current to trip the primary breaker; but you can not use primary current to protect the secondary.