# transformer



## crodrigues45 (Oct 19, 2011)

Wiring a step up transformer 208/480v.Three phase
question is, I am feeding a three phase compressor . Do I do anything with XO or just let it float?


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## micromind (Aug 11, 2007)

Leave it float. 

Also, disconnect the bonding jumper, if there is one.


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## nitro71 (Sep 17, 2009)

micromind said:


> Leave it float.
> 
> Also, disconnect the bonding jumper, if there is one.


Which bonding jumper are you refering to? He still needs to bond XO to ground.


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## buddhakii (Jan 13, 2011)

definately do not let xo float. Bond like you would any other time and you still need a gec.


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## Big John (May 23, 2010)

_budhakki, _I think he means he's reverse-feeding a step-down transformer, so the X0 is on the primary. 

Float the primary X0 because the feeder is already grounded at another point, then on your secondary 480V circuit, ground one phase.

-John


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## jmsmith (Sep 10, 2011)

Big John said:


> budhakki, I think he means he's reverse-feeding a step-down transformer, so the X0 is on the primary.
> 
> Float the primary X0 because the feeder is already grounded at another point, then on your secondary 480V circuit, ground one phase.
> 
> -John


John- 
Would you ground a phase if it was feeding a motor only, or run an EGC and bond it to the transformer bonding connection and the frame of the motor itself? Was just wondering, because I have had to utilize ground-fault detection on equipment, and am trying to wrap my head around this one! What would intentionally grounding a phase do to you in this instance?

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## micromind (Aug 11, 2007)

nitro71 said:


> Which bonding jumper are you refering to? He still needs to bond XO to ground.


When backfeeding a 4 wire wye connected transformer, the X side is the primary. XO cannot be bonded to ground. To do so will cause current to flow in the grounding system.

When the X side is the secondary, then XO must indeed be bonded to ground.


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## nitro71 (Sep 17, 2009)

micromind said:


> When backfeeding a 4 wire wye connected transformer, the X side is the primary. XO cannot be bonded to ground. To do so will cause current to flow in the grounding system.
> 
> When the X side is the secondary, then XO must indeed be bonded to ground.


Yeh, guess it would be called HO then.


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## Big John (May 23, 2010)

jmsmith said:


> Would you ground a phase if it was feeding a motor only, or run an EGC and bond it to the transformer bonding connection and the frame of the motor itself...?


 You always need an EGC regardless of whether the system is grounded or ungrounded. The only thing that changes is whether the neutral is bonded to it.


> ...What would intentionally grounding a phase do to you in this instance?


 It provides voltage stabilization and a fault current path.

I'm 98.6% sure you can legally get away with an ungrounded delta, but I don't like ungrounded systems so it's not a route I would chose.

-John


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## jmsmith (Sep 10, 2011)

Big John said:


> You always need an EGC regardless of whether the system is grounded or ungrounded. The only thing that changes is whether the neutral is bonded to it. It provides voltage stabilization and a fault current path.
> 
> I'm 98.6% sure you can legally get away with an ungrounded delta, but I don't like ungrounded systems so it's not a route I would chose.
> 
> -John


Makes sense.... It was most of the motor power I dealt with ran ungrounded delta. Of course all the phases were ground-monitored.

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