How to do the grounding when stepping up the voltage from 208V/y to 480V Delta?

[mathan1987]

Hi,

in one of our projects the power distribution is 208v. the pump(10hP) with 480v, 3phase motor is installed. We are thinking of using the step up transformer. 208V/y on the primary and 480V delta on the secondary. In this how do we do the grounding as it is a separately deriver system.

 

[LarryFine]

First, leave the primary X-0 floating; do not connect it to the supply neutral.

Second, you need to ground the secondary in some way. Some options are:

Corner ground (bond one line)
Zig-zag (neutral derivation)
Transformer w/ wye 480/277 sec.

A 208v delta-to-480/277 wye unit would be ideal.

 

[d0nut]

A 208v delta-to-480/277 wye unit would be ideal.

I agree. Buy the correct transformer for the application rather than trying to make something work by backfeeding a 480 delta to 208Y/120V transformer. Also, if your motor has a VFD, most of the drives are expecting a Y connected system. Using a delta connected system instead can cause issues since the L-G voltages will be different than in a Y connected system.

 

[petersonra]

Hi,

in one of our projects the power distribution is 208v. the pump(10hP) with 480v, 3phase motor is installed. We are thinking of using the step up transformer. 208V/y on the primary and 480V delta on the secondary. In this how do we do the grounding as it is a separately deriver system.

Might be simpler to change the motor.

 

[don_resqcapt19]

There is no requirement to ground the secondary of that transformer, but if you don't you must install a ground detector per 250.21(B). 250.30 will require a grounding electrode and grounding electrode conductor no matter if you ground the secondary or leave the secondary ungrounded.

 

[hillbilly1]

If it’s a dual volt motor, you could change the taps, but it would also require a larger starter, along with changing the control transformer taps. That is, if it can be field converted. May be cheaper that route, if it can be done.