How to tie in neutral bar

[Mine electrician]

We have just installed 3 new pad mount 12470 Delta / 480 Wye transformers. We have no neutral load, it's all 3 phase motors. I have a ground grid we just installed, how should that tie into my transformer and should anything be tied to the neutral bar in the low side of my transformer? Right now it goes to the ground spot on the transformer and then to the first disconnect, but it seems like there is no way for any fault current to get back to the transformer.

 

[ActionDave]

250.30, you either bond the XO and earth it in the transformer or in the first disconnect, either or, not both.

 

[Smart $]

This is not a service transformer?

Do you want a grounded system? In heavy industry it is common to run systems ungrounded, especially where a trip can present a more dangerous condition than a line to ground fault. After all, one line to ground fault on an ungrounded system is not much different than the intentional ground of a grounded system. A ground detector is required, along with it being monitored and faults corrected as soon as can be safely managed.

If you want a grounded system, Dave has directed you to the appropriate section (if not a service transformer). There are a lot of details to that section so it is difficult to cover all the possible specifics here. So read up if that's the route you want to go, and come back if you have any additional questions.

Welcome to the forum. :thumbsup:

 

[Saturn_Europa]

This graphic may be helpful

 

[rlundsrud]

I'm only asking this as I'm curious, I understand that it isn't practical to change anything now. Wouldn't it have been better to use a Delta secondary since there are exclusively motor loads? If so, then you could corner ground the Delta secondary.

I'm hopeful that some of the other members can weigh in on this as I would like to know what is considered the convention way of doing it.

 

[ActionDave]

This is not a service transformer?...

His handle says Mine electrician. I would not be surprised that those are their own transformers.

 

[ActionDave]

I'm only asking this as I'm curious, I understand that it isn't practical to change anything now. Wouldn't it have been better to use a Delta secondary since there are exclusively motor loads? If so, then you could corner ground the Delta secondary.

I'm hopeful that some of the other members can weigh in on this as I would like to know what is considered the convention way of doing it.

Wye systems are more familiar and have the added advantage of reducing the voltage to ground.

 

[Smart $]

His handle says Mine electrician. I would not be surprised that those are their own transformers.

I would not be surprised either. But I had to ask because the grounding details are a little different (as is the section that covers such).

 

[texie]

I'm only asking this as I'm curious, I understand that it isn't practical to change anything now. Wouldn't it have been better to use a Delta secondary since there are exclusively motor loads? If so, then you could corner ground the Delta secondary.

I'm hopeful that some of the other members can weigh in on this as I would like to know what is considered the convention way of doing it.

There are a number of reasons why a Y system would be better than a delta (even when there are only line to line loads). Among them, the phase to ground voltage is the same as line to line in a corner grounded system and thus requires equipment rated for this. In this day and age, corner grounded delta rated equipment such as panelboards, etc. is becoming less common. Also much load equipment these days, such as VFDs are not suitable for use in a corner grounded systems.

 

[GoldDigger]

There are a number of reasons why a Y system would be better than a delta (even when there are only line to line loads). Among them, the phase to ground voltage is the same as line to line in a corner grounded system and thus requires equipment rated for this. In this day and age, corner grounded delta rated equipment such as panelboards, etc. is becoming less common. Also much load equipment these days, such as VFDs are not suitable for use in a corner grounded systems.

The NEC also specifically instructs us to consider the effective phase to ground voltage in an ungrounded system to be equal to the line to line voltage. That will also affect what equipment you can use.

 

[Smart $]

There are a number of reasons why a Y system would be better than a delta (even when there are only line to line loads). Among them, the phase to ground voltage is the same as line to line in a corner grounded system and thus requires equipment rated for this. In this day and age, corner grounded delta rated equipment such as panelboards, etc. is becoming less common. Also much load equipment these days, such as VFDs are not suitable for use in a corner grounded systems.

Given the same 3Ø voltage as a corner-grounded delta, a wye system will have 57.7% (1/√3) line to ground voltage.