120/208v voltage troubleshooting

[jklr]

We've got a subpanel that we ran a new circuit in and while checking voltage initially we were getting:

A-B phase 208
A-C phase 208
B-C phase 208
A-Ntrl 120
B-Ntrl 120
C-Ntrl 120
A-Gnd 100
B-Gnd 127
C-Gnd 134

The subpanel is fed from another subpanel and the first panel is fed from a step down transformer. Any ideas?

 

[hillbilly1]

Easy one, neutral not bonded to ground. Should be a jumper in the transformer from XO to ground.

 

[CoolWill]

Yeah, I'd be looking at the X0 bond jumper in the transformer. Where ever the GEC comes in is where the system bonding jumper should be located. I usually see it in the transformer but it is legal to do it at the secondary disconnecting means as well.

 

[ptonsparky]

Your question was answered but did you happen to take a N-G reading? Curiosity on my part.

 

[jklr]

Your question was answered but did you happen to take a N-G reading? Curiosity on my part.

Yes nuetral to ground was about 19-20v

 

[infinity]

I agree missing SBJ. I've seen this many times but I'm not sure why. Could be some guys incorrectly assume that the X0 is factory bonded.

 

[hillbilly1]

I agree missing SBJ. I've seen this many times but I'm not sure why. Could be some guys incorrectly assume that the X0 is factory bonded.

The most common scenario I’ve seen is transformers for isolated ground panels. They seem to think isolated ground is truly isolated! LOL!

 

[jklr]

Thanks everyone for the replies. I have been mostly residential and now slowly getting into commercial work. I will be going back out there next week as I told the manager that there was an issue somewhere upstream.

 

[LarryFine]

Yes nuetral to ground was about 19-20v

And what should it be?

 

[jklr]

And what should it be?

In a perfect world, 0. But less than 2v is what I was taught. I have been trying to learn about separately derived systems as I have not had a lot of experience with them being mostly residential.

 

[LarryFine]

In a perfect world, 0. But less than 2v is what I was taught. I have been trying to learn about separately derived systems as I have not had a lot of experience with them being mostly residential.

Are you asking why the neutral and the EGC must be solidly connected?

 

[jklr]

From what I gathered after doing some research is that the reason the EGC and Neutral are bonded at the transformer is to give the secondary a reference to ground since there is no "physical" connections between the conductors. I was confused honestly when I was reading voltages the other day and was getting the correct Line to Line and Line to nuetral readings but then getting different readings Line to ground.

 

[LarryFine]

From what I gathered after doing some research is that the reason the EGC and Neutral are bonded at the transformer is to give the secondary a reference to ground since there is no "physical" connections between the conductors. I was confused honestly when I was reading voltages the other day and was getting the correct Line to Line and Line to nuetral readings but then getting different readings Line to ground.

Okay, to put it simply, the goal is so that a line-to-ground(ed surface) fault will cause the OCPD to open just like it would react to a line-to-neutral fault.

The only thing that creates fixed line-to-ground (earth) voltages is intentionally grounding one of the other source conductors.

Us grounding an SDS is really no different than the POCO doing it.

 

[jklr]

Okay, to put it simply, the goal is so that a line-to-ground(ed surface) fault will cause the OCPD to open just like it would react to a line-to-neutral fault.

The only thing that creates fixed line-to-ground (earth) voltages is intentionally grounding one of the other source conductors.

Us grounding an SDS is really no different than the POCO doing it.

Makes sense. Thank you for explaining!

 

[roger]

I agree missing SBJ. I've seen this many times but I'm not sure why. Could be some guys incorrectly assume that the X0 is factory bonded.

I think many see the core bonding strap and think it is actually the XO bond

 

[LarryFine]

I remember decades ago, when I was a helper, we set up a transformer and lighting panel in a warehouse for rows of 8' fluorescent lighting. We energized the installation quickly so we had some light to work in.

When we installed the jumper in the transformer, there was some arcing and one tube went out. I immediately deduced that a ballast wire was shorted to the housing, and the jumper shorted the ballast output.

 

[jklr]

I think many see the core bonding strap and think it is actually the XO bond

Since the EGC will be bonded at the transformer to the grounded conductor, the bonding screw in the panel should be removed, correct?

 

[ptonsparky]

Since the EGC will be bonded at the transformer to the grounded conductor, the bonding screw in the panel should be removed, correct?

Yes.
You can do it at either place, not both.

 

[LarryFine]

If you bond in the transformer, you run separate neutral and EGC, and don't bond like in a sub-panel.

If you don't bond in the transformer, you run only the neutral, and you do bond like in a main panel.

 

[ptonsparky]

If you bond in the transformer, you run separate neutral and EGC, and don't bond like in a sub-panel.

If you don't bond in the transformer, you run only the neutral, and you do bond like in a main panel.

Don't you need an EG back to the transformer?