277/480v voltage issue

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Hcji

Hcji

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Indiana
I have 277v A-N, B-N, C-N
I have 480v A-B, B-C
I have 0V A-C
At MDP I have 277v all phase to ground and 480 across all phase to phase. This was a retrofit wire pull and has 4-4/0Al and 2/0 in a 2.5in pipe. Is it possible the wire was nicked or stretched enough on the pull in that would cause a break down? It worked for about a year. The neutral was not needed, but specced so it got pulled in also. I took neutral and made it the C phase and all is good. Any ideas here?
 
Where did you take those measurements? My first guess would be what you thought was C phase was actually a splice off A phase. Nicked wires causing A and C phases to short would cause a bolted fault and would not manifest as 0V A-C.
 
agree above. Where were the measurement ps taken?
what you have measured was two lines from the three phase.
 
Yeah, either you are not measuring what you think you are or two conductors are connected to the same phase. If the wiring were damaged you would not have gotten those voltages and the building might be on fire.
 
Measurements at the MDP were correct, Measurements at the subpanel 200ft away is were the odd voltages were. There is no splices, straight from MDP to subpanel. The conductors are colored B,O,Y,Grey so no way they can be crossed anywhere. Checked another subpanel and elevator and all is good.
 
Hcji said:
Measurements at the MDP were correct, Measurements at the subpanel 200ft away is were the odd voltages were. There is no splices, straight from MDP to subpanel. The conductors are colored B,O,Y,Grey so no way they can be crossed anywhere. Checked another subpanel and elevator and all is good.
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Are you seeing the missing voltage all of the time, or only wile under load?

It's possible there is a bad connection between a wire and its lug. Check voltages on the wires themselves.

Check for any voltage between each wire and its lug (aka fall-of-potential), while under load of some kind.
 
Yes. Missing voltage all the time, even pulled out of breaker inspected, then checked breaker with no wires in it at MDP and all good. Tested wire (not lugs) on subpanel side and same. Under load made 0 difference. Even tried 2 different meters
 
Hcji said:
Yes. Missing voltage all the time, even pulled out of breaker inspected, then checked breaker with no wires in it at MDP and all good. Tested wire (not lugs) on subpanel side and same. Under load made 0 difference. Even tried 2 different meters
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If you have voltage at one end of a wire, and no voltage at the other end of the same wire, then it cannot be the same wire; there must be a break somewhere.

Just to prove it to myself, I would next try to measure the voltage at both ends of the wire in question, perhaps with a long piece of wire for your meter or tester.
 
Hcji said:
Yes. Missing voltage all the time, even pulled out of breaker inspected, then checked breaker with no wires in it at MDP and all good. Tested wire (not lugs) on subpanel side and same. Under load made 0 difference. Even tried 2 different meters
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Somehow they are messed up in the MDP, IMO. Check the voltages P-P at the MDP on the actual breaker feeding the sub panel.
 
Hcji said:
As stated above I did take conductors out of breaker, turned breaker on and tested at MDP and all was good. It's the same wires from MDP to subpanel. I have even rang them out A to ground B to ground C to ground
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If that’s good then a junction somewhere in the 200’ run made up wrong.🤔
 
So now you have a supposedly unused wire in the pipe.

I'd suggest measuring the voltages between this 'unused' wire and the connected phases, and all phases (including unused) to ground.

Jon
 
Somehow it was phase A energizing C. It could have been a break in the C wire, with the C wire getting inductivly coupled to the properly energized A wire.

With the change in connections, what voltage did you measure on the 'unused' wire?

-Jon
 
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