Voltage loss in underground cable.. FOUND IT!!!

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Chuck Shaughnessy

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I have power going from my house to my barn (100 amp service). One leg is solid 120v. The other is roughly 5V. Measurements are without load on either leg. At the house end both are 120V.

Can this be a connection, corrosion, or breaker problem or is it a problem in the cable below the ground? Is this dangerous?

How do you go about finding the problem and can it be "patched"?

I have virtually no current draw in the barn - sometimes lights, sometime circular saw, fence charge, nothing major. If the voltage drop is not repairable is it a problem if I just disconnect one leg and only run 120 (instead of 240) to the barn?

Any help will be appreciated.

Chuck
 

Reach4

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I would suspect the problem is with a splice. I don't know if there are some electricians that use a time domain reflectometer (TDR) to figure out where the discontinuity is. Maybe one who does a lot of signal wiring would have one.
 

WorthFlorida

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It be of help if you mentioned when did you notice that one leg was bad? Did you own the home when the barn run was put in? Any idea where the where and how the cable is buried. It is a conduit? How old is this run? Is there a breaker panel in the barn?

How are you reading the voltage, across the black (red) and white? Where you read the 5 volts try it across the black and ground (green wire). It is possible that the neutral opened.

If you cannot find the source it is OK to abandon the bad leg but you must disconnect the wire at the breaker, cap it off and tag it so the next guy knows what is going on. Switch off the breaker and tag it so it can be read with the panel cover on. This will leave you with an some non usable outlets or lights but any 220 volt outlet must be completely disconnected, wires capped off, outlet removed and a blank cover installed in its place. Or use the good line and convert it to a 110 volt line.

If all of this sound greek to you, then it be wise to get an electrician. At that same time have the outlets in the barn changed to GFCI it they are not.
 
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Chuck Shaughnessy

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Nope - I get it all. It went in about 15 years ago. No conduit. Direct buried cable. I don't think there are any underground splices - just a cable about 100 feet long. House breaker box has a 100 amp breaker - this feeds into a subpanel in the barn. One leg in the barn measures 120V between EITHER neutral or ground. The other leg measures 5V also between either neutral or ground. There are only 3 breakers in the barn - one powers 4 outlets and the other 2 fluorescent lights. I noticed that the lights stopped working sometime in the last year - no digging was taking place so probably no new damage. Just yesterday I was trying to fix the lights - I thought that all of the ballasts were bad, but ... no voltage, which took me to measure the voltage at the breaker box and ... 5V. Moving the breakers to the 12oV leg has everything working but I don't like it leaking somewhere (if that is what is happening.) I wish I had an inductive ohmmeter so I could see if there is any current trickle in one leg and not the other but ... don't have one. There must be some current leak as the 5V is with no load and no apparent current flowing. Or, even better, a TDR. I would like to find it and fix it, but given the low power needs in the barn it doesn't justify the expense of laying a new cable when I can probably make it work with just 120.

There are no 220V outlets in the barn so that's a non-problem. And, I understand the capping and labeling requirement ... I would still rather find this and fix it.

Is there any chance that the connection between the wire and the buss at the breaker box is corroded (I can't see it) and that is causing an issue?

Thanks for your help. I will keep thinking and maybe try a few things to see if I can isolate it a bit.
 

WorthFlorida

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You can buy a clamp meter fairly cheap. $40 at Lowes. http://www.lowes.com/pd_464290-295-21010N___?productId=50125835&pl=1&Ntt=clamp+meter. Its good for around the house and I bought one to get me out of a jam. It's good but not something that I would use everyday but that one time per year that you need one, it's perfect.

Is the 5v at the breaker or at the feed wire in the barn subpanel? Have you ruled out a bad breaker? Have you removed the cover to inspect the subpanel? PICS would be nice.
 

Chuck Shaughnessy

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I am traveling until Thursday but will post some pictures when I return. Definitely measuring from the buss lug and not on the other side of the breaker. I did remove the cover and it would be cleaned up in there a bit - some bugs and stuff - but I think I got that stuff out of the way in the process. I have a couple more things to try when I get back and will post some more. Thanks much for your help.

One more interesting fact ... I think ... I didn't spend a lot of time looking at this but the voltage BETWEEN the two legs seemed to be 115 - suggesting that they were more "in-phase" than out of phase. This would suggest some sort of phase noise, or stray capacitance and/or inductance moving the phase relationships around. More to do to investigate that as well, but kind of strange.
 

Reach4

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Is there any chance that the connection between the wire and the buss at the breaker box is corroded (I can't see it) and that is causing an issue?
With the covers off of the breaker boxes , you may be able carefully probe the wires and terminals separately. If you cannot see the wire to probe, you could try just retightening the screws. Or you could unclamp the wire to measure. I presume the wire is copper.
 

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The wire may be aluminum, I am not certain. I will have to check when I am back home. It may be that there was no anti-oxidation stuff put on the wire lug and there is some oxidation in there causing an impedance problem. Definitely something to check on.
 

Jadnashua

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Aluminum wire doesn't need much to cause it to become a bad conductor, so cleaning it off, coating it with the required antioxidant paste and retorqueing it may be all that is required. But, in the process, it may have arced, and ruined the breaker. Consider pulling the main breaker in the subpanel and verifying the bus structure underneath is still good on both ends.
 

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Back from travel. Picture attached this time. The wire seems to be aluminum but appropriately coated with the anti-oxidant goop. I have moved all of the breakers onto one side of the line so there is an effective 120V that is working fine. The other side is not loaded and measures 5V between both neutral and ground. I had hoped that we had some oxidation/corrosion on the line that, if cleaned up, would clear up the voltage problem, but it appears that this may not be the case.

Other than digging up and looking for a fault ... any ideas?
BarnPanel.gif
 

Reach4

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Measure the voltage at the other end of the big wire.

If the wire a the other end does not look identical, there must be a splice. If they look the same, it is undetermined.
 

Chuck Shaughnessy

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At the other end it is 120V. There is no splice - at least no an intentional one. Perhaps a rock has pierced the sheathing and created a current leak and/or corrosion - but, no idea where.
 

Reach4

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A current leak could not explain your symptom.

Are there markings on the cable that you can read?
 
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Jadnashua

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There are specialized tools that can locate where a cable has failed, but your average electrician probably does not have one, or know how to use it. If you want to use that, you'll probably have to call around, and it may take some work finding someone that can do it, should you decide. The cost of the service might make just replacing the cable look like a better choice! Hard to say what it would cost, as rates can vary considerably.
 

Reach4

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I would maybe inquire at a local radio club who might be willing try using a TDR on your isolated power circuit. I expect a radio hobbyist might be willing to try and agree to getting paid only if the effort located the problem.

http://www.megger.com/common/documents/TDR500-6172-726_UG_en_V02.pdf is the manual for that Megger unit. Some people buy equipment, use it, and then sell it used on an auction site.
 

Chuck Shaughnessy

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A probably silly question, but... How is it wired at the source, and is the voltage between the two hots 240V?
The source is a main breaker box at the house and the two hot wires come out of a 100A breaker and ...yes, at the source they are 240V.

Today I disconnected the line from the box just to ensure that there wasn't a low impedance path to ground in the box and ... no change in the 5V. I suspect that I have a cable that has been penetrated with water and it is creating some impedance issues and paths to ground (currents leaks). The only way to fix that is to find it and repair it or to run another line. It is rocky just outside the barn so when the rain stops I will dig that up and see what I can find. If it is not there, I will try to find someone w a TDR and if I can't do that ... then I can just disconnect one side and cap it and just run the barn on the 120V. That will suffice, but I hate having a problem cable in the ground and not knowing what is going on there.
 

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The reason that current leak could not explain your symptom is that as long as the breaker did not blow, that would not cause the voltage at the far end to be so low. A short strong enough to cause that kind of voltage drop would blow the breaker.

The reason I asked about the markings on the cable is that if that cable is not what somebody would bury, that would imply splices. Are the wires entering the ground and coming out of the ground in conduit?
 

Chuck Shaughnessy

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The reason that current leak could not explain your symptom is that as long as the breaker did not blow, that would not cause the voltage at the far end to be so low. A short strong enough to cause that kind of voltage drop would blow the breaker.

The reason I asked about the markings on the cable is that if that cable is not what somebody would bury, that would imply splices. Are the wires entering the ground and coming out of the ground in conduit?


There is no conduit, and it is direct bury aluminum cable. I can try to read the markings tomorrow and see what is there, but it was done by a licensed electrician and permitted and inspected so I am guessing its fine.

I do think a current leak could explain it. If there is some impedance in the line due to water entry or other corrosion, creating impedance and a voltage drop ... and another impedance to ground, there is plenty of resistance to cause less than 100A to flow but enough to create a voltage drop, and my 5V reading could be coming off of something between the 2 impedance loads. If the impedance to ground is, say, 120 ohms, then you only have 1A flowing and no breaker trips. You can get a lot of voltage drop with very little current if the resistance is high enough. I can draw the diagram if you want but its easy to model a path that creates a voltage drop, and path to ground, drawing something less 100A. I wish I had an inductive ammeter to measure what current is flowing - that would help understand but wouldn't isolate the problem.

Next step is to dig where it enters the barn (rocky there) and look for damaged cable. If there is any, I will have to repair/splice it. If that's not the problem, then a TDR is the only answer short of a complete replacement. Looking for a TDR as you suggested with radio guys in town. If no luck there, and if my suspected rocky spot doesn't pan out, then I will just disconnect one side, and cap it, and leave the barn at 120V.

I appreciate your help and thoughts on this. It is quite frustrating, but one of those things when you have stuff underground I guess.
 
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