120/208v voltage troubleshooting

jklr

Member
Location
Miss
Occupation
Electrician
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?
 

CoolWill

Senior Member
Location
Alabama
Occupation
Electrical Contractor
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.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
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

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
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

Member
Location
Miss
Occupation
Electrician
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.
 

jklr

Member
Location
Miss
Occupation
Electrician
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

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
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

Member
Location
Miss
Occupation
Electrician
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

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
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

Member
Location
Miss
Occupation
Electrician
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!
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
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

Member
Location
Miss
Occupation
Electrician
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?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
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.
 
Top