X0 tap 120/208 480 trans

[Fordean]

Im not using the xo tap Was told while back to either ground xo Or not Ground
Cant remember Nnnnnnnmmnnnnnnnnnnm

 

[petersonra]

Im not using the xo tap Was told while back to either ground xo Or not Ground
Cant remember Nnnnnnnmmnnnnnnnnnnm

depends on the voltage and configuration. but mostly, it would be grounded.

 

[don_resqcapt19]

It you are using it in reverse, that is stepping the voltage, up, typically there would be no connection to XO.

 

[infinity]

Im not using the xo tap Was told while back to either ground xo Or not Ground
Cant remember Nnnnnnnmmnnnnnnnnnnm

As Bob and Don have demonstrated more details are needed to answer the question.

 

[Fordean]

It you are using it in reverse, that is stepping the voltage, up, typically there would be no connection to XO.

Were boosting 120/208 to 480

 

[petersonra]

Were boosting 120/208 to 480

Then the X0 on the primary side (120/208) would either be unconnected or connected to incoming neutral.

Keep in mind if the secondary 480 side is delta, you will need some kind of ground detect circuit.

 

[Fordean]

Then the X0 on the primary side (120/208) would either be unconnected or connected to incoming neutral.

Keep in mind if the secondary 480 side is delta, you will need some kind of ground detect circuit.

Not sure what your saying But a grounded conductor from ct cabinet will flow thru

 

[MD Automation]

Were boosting 120/208 to 480

I believe that people are guessing you are using a 480 Delta to 120/208 Wye step down transformer in reverse.

So your secondary output (480) is wound as a Delta and there is no real reference to ground. Which is why Petersonra is talking about a ground detect circuit - or maybe a corner grounded setup. And a delta secondary can be a problem, especially if your loads are devices like VFDs that expect to be sourced from a 277/480 Wye secondary.

But - back to your original question - if you are really backfeeding a typical Delta to Wye step down transformer, then I would not connect anything to the X0 terminal. Not a neutral - nor a bond to ground. Leave it float.

 

[don_resqcapt19]

Then the X0 on the primary side (120/208) would either be unconnected or connected to incoming neutral.

Keep in mind if the secondary 480 side is delta, you will need some kind of ground detect circuit.

From EC&M magazine:

Participants at one of my recent workshops had questions pertaining to reverse-fed transformers (back-fed). The input side was 480 volts (V), three-phase, and the output side was 120/208V. They wanted to know if the output side of 120/208V transformer could be used as an input side, and if so, how was the electrician to bond and ground the transformer?

In my opinion, when a step-up transformer is necessary, it should be designed and identified for such application. This article does not address such problems, but it addresses the general requirements that are outlined in the National Electrical Code (NEC).

The big question seemed to be whether the grounded conductor (usually a neutral) is required to be connected to the XO termination point. Of course, the answer is no. When the grounded conductor is used in this type of installation, it must not be connected to the XO terminal and must not be bonded to the enclosure or earth-grounded. For operational purposes, just connect the ungrounded phase conductors to the output terminals that are usually identified as L1, L2 and L3, respectively. Then an equipment grounding conductor must be routed with the supply circuit and used to bond and ground the noncurrent-carrying parts of the transformer enclosure.

 

[LarryFine]

Were boosting 120/208 to 480

Do not connect anything to what will be the primary neutral point. Wire it as if it was a delta primary.

 

[electrofelon]

Not sure what your saying But a grounded conductor from ct cabinet will flow thru

You cannot use the 120/208 neutral conductor as you neutral for the 480 side, if that is what you are proposing. You could if it was an autotransformer, but pretty sure its not.

 

[Fordean]

You cannot use the 120/208 neutral conductor as you neutral for the 480 side, if that is what you are proposing. You could if it was an autotransformer, but pretty sure its not.

We have no need for Neutral, This is for motor.

 

[electrofelon]

We have no need for Neutral, This is for motor.

Ok...I'm a bit confused what you meant in post #7 then.

If there are no electronics (like a vfd) you can corner ground one of the 480 lines. Supply the transformer with just three hots and an EGC, let's the XO float.