Some time back I had some folks who needed me to take a subfeed from a subpanel. That is to say, they had a 2-pole 50a breaker at the main house panel feeding a subpanel in the shop. From this subpanel they wanted me to take a 2-pole 30a to a second subpanel. The problem was that when I checked voltage at the subpanel in the shop, I found 120v on one phase to neutral but only ~46v to neutral on the second phase, giving me phase-to-phase voltage of ~166v. The best I could determine (i.e., guess) at the time was that one of the phases was grounded somewhere in the underground run between house and shop - only just not enough of a ground to trip a breaker.

Does anyone out there have any other ideas of what the problem could have been?

Sounds more like a bad connection or a gopher bit wire.

You used a digital multi meter to test I bet?

Take a couple of minutes to read this thread about Phantom voltage and DMMs then come back here.
https://www.electrical-contractor.net/ubb/Forum1/HTML/006361.html

Open all the breakers in the sub panel, then use a old fashioned wiggy to test from each hot to ground.

I bet you will find that the wire that showed 46 volts is actually dead.

If one of the phases was grounded enough to lose about 80 volts the breaker would trip.

Thanks, Bob. I've read some of the phantom voltage posts on this forum before, but didn't think to apply the concept here. I will try what you suggested with my wiggy.

In the meantime, if i assume the phase is dead, i think i'm in the same boat either way. Don't know what caused the "death", but know i have to pull a new feed. Yes?

I would agree Tripp unless the feeds not the problem. What about the breaker on the feed end? The 50 amp one. Maybe half of that is bad or the connection is lose? Or even the terminal in the panel that feeds that 50 amp. Maybe the prong has broken off there? (Although I doubt it). Alot of times the wire where it is connected to a breaker or lug or whatever when it was installed they scored the wire too hard and after time with heat and vibration has come lose. And you can even see it with your naked eye unless you stick your hand in there and try to move the wire and the whole thing comes right off in your hand.

In my opinion before you pull a new feed check for a list of these things and more before you go to the trouble and expense of pulling in a whole new feed.

Good luck.

Thanks, Galway. I remember I did a DMM check at the original panel in the house where the 2-pole 50a is, and the reading was fine. Is there any other way to check if the problem is there at the source?

Open neutral/ degraded neutral connection anyone?

It certainly could be, it depends if his measurements where really hot to neutral or hot to ground.

Never mind I take that back, it can not be a neutral problem.

The voltage is correct on one leg and off on the other leg.

A bad neutral would show up on both legs, one would be high and the other low.

You beat me to it, Bob: if the reading was 120v on one phase to neutral, then neutral's not the prob.
But for what it's worth, my DMM measurements were Hot to Neutral and Hot to Hot. As a matter of fact, there was no Ground pulled in the original subfeed - something i plan on remedying when i pull my new subfeed. But first i must get back out there with my wiggy.

But Bob - tell me more about this bad neutral thing: why would one leg show high and the other low? Is that the only possible scenario?

Thanks in advance for elaborating.

Morning Trip, when you try to measure line to neutral on a feeder that lost the neutral connection the meter is not in parallel with the feeder. You actually are in series with part of the load on the circuit.

Here is one of Rogers drawings, it may help to explain the problem.



As the drawing is now if you place your meter leads between the neutral and the hot you will read 120 volts.

Leave you meter connected but 'disconnect' the neutral at the source, now what is the meter reading?

That would depend on the size of the load on each of the hots.

The meter could read anywhere from 0 volts to just short of 240 volts.

It will also if one side reads 25 volts high the other side will read 25 volts low.

Many times a lost neutral will end up releasing smoke from appliances that do not like an extra 50 or more volts.

Sorry if I can not explain it well, perhaps Jon will step in.

Thanks, Bob. I will have to save this post and the graphic to refer back to several times, I'm sure.

ok guys i will set myself up for this one.
what if you reversed the connections at the 2 pole breaker(move the red wire from L1 to L2 and the black wire from L2 to L1) and then took a reading at the sub panel now you would have power on the leg that was zero and zero on the one that had power or perhaps like someone else said about loose connection at the 2 pole hence woolahh 240 appears.
just a thought good or bad?
h20