Voltage drop in repaired underground line

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  #1  
Old 02-27-05, 05:28 AM
Greenguy
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Red face Voltage drop in repaired underground line

I purchased a farm with an equiment shed about 135 ft from a entrance service pole with a mounted 200 amp outside panel box feeding a 100 amo panel box in the shed. The underground line was broken during past septic work and I repaired the line with underground repair couplers and all wqs ok until about two weeks ago when the system failed. I checked at the pole up and downstream of the breaker in the 200 amp box, got 120/240. At the panel box in the shed, got only like 40/60. Also just noticed no ground rod at panel box in shed. What might be causing the voltage drop this time, another break in the wires? But why would both wires go bad at the same time again? What purpose does the ground rod at the shed panel box serve? Thanks
 
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  #2  
Old 02-27-05, 05:51 AM
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For openers I would shut off power and check all connections.

Second, a few questions...did you check the voltage in the shed panel phase to phase or phase to ground? How did you repair the underground break, please describe what you used and how you did it. Was the wire run enclosed in conduit or just direct buried? How deep? How many wires were there and how many were damaged that needed repaired? Also, are you certain that nothing in the recent past happened that may have contributed....a storm, excavation work on the property, anything out of the ordinary? Since you have been on the property has there been constant traffic over where the wire is buried, ie. new road, any chance of stone being driven down into shallow buried wire, that sort of thing?
 
  #3  
Old 02-27-05, 06:37 AM
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I have shut off the power. The line consists of three aluminum cables direct buried type, two hot, one neutral. The initial break was caused by prior septic service wherein a backhoe apparently cut through all three cables (they then twist spliced the wires, wrapped in electric tape and reburied without any moisture protection). When I discovered where the break was, the aluminum cables had deteriorated so I cut back to good solid lines, used underground connectors and replaced the wire back to the pole. All worked fine after that. The original line was buried about 18 " deep. There has been no traffic over the area and I checked for possible damage from frost "heaving" where a line might have been pulled, found no indication of any damage or loose wires. As in 95% of my past experience with electrical problems, I suspect grounding is the culprit. I tested phase to phase and phase to ground. Thanks
 
  #4  
Old 02-27-05, 07:31 AM
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Grounding may be an issue but I lean toward a partial open of some kind. I'll share this with you....

Got a call to a very very similar situation. This one was a 200 amp disc with a 100 amp disc nippled to it out on the service pole. The 100 amp disc fed a 100 distr panel in a new bld about 130 ' away. Only had intermediate and erratic voltage in the bldg even though it was correct at the mains. All conventional checks revealed nothing. Know what I found? In the 100 amp disc on the 100 SD breaker-the underside plastic clip that snaps onto the bar holding it in place was cracked and didn't provide a firm connection. Whoever put it in broke it and prob let it go..........
 
  #5  
Old 02-27-05, 07:32 AM
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Grounding is not your problem. Grounding will not affect phase to phase voltage or phase to neutral voltage. I would suspect the splice. There is equipment that can determine the distance of the problem. If it appears to be the same distance as the splice then you know where to dig.
 
  #6  
Old 02-27-05, 07:57 AM
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I used a silicone filled underground repair splicer to repair the line initially, do these fail, looked like a good water tight splice when I made them. With the absence of a ground rod at the panel box in the shed, I am suspicious of the entire installation. I will check the 100 amp breaker in the shed for damage. Please keep the suggestions coming, many thanks.
 
  #7  
Old 02-27-05, 08:47 AM
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I know there are several kinds of underground splice kits out there. I kinda always leaned toward butt splice/heat shrink or split bolts/mastic pads/with 3m sealer route. The facts are it was the wire that was damaged but who knows how far stretched. Can you free up both ends of all 3 wires and check the resistance of each?

Hate to think about running new wire but if you have access to equip to bury it, enough confidence in reusing the switch gear and distr panel, and do it yourself- you may take a lot of the sting out of the cost...
 
  #8  
Old 02-27-05, 12:41 PM
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Thanks for your help, I suppose I will eventually replace the line and hope that was the problem. I hate to think that after replacing it, I discover the problem still exists and the problem was something else unrelated to the line itself. It just seems strange to me that both hot wires have gone bad at the exact same time. Any further thoughts will be appreciated. I'm going to dig where the break was and test up and down stream of the splice to see what might be the condition there. Keep the ideas coming, thanks
 
  #9  
Old 02-27-05, 01:07 PM
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You might try using another breaker as a test to check the voltage just to verify the main is bad you know?
 
  #10  
Old 02-27-05, 02:26 PM
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Are you referring to the breaker on the pole or in the shed? I checked it up and downstream of the breaker on the pole and found it ok both places there. I unhooked the line upstream of the shed panel box and it was there that I tested and found it dropped, so I think the breakers are probably not involved, but thanks for the idea. Am I thinking correctly there?
 
  #11  
Old 02-27-05, 04:18 PM
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Open the breakers on both ends so the line is isolated. Then measure the ohms between each cable. Better yet put a megger on them.
 
  #12  
Old 02-27-05, 04:38 PM
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Farm wiring maybe something different.
If you have a 200 amp panel feeding a 100 amp sub panel.
99 % of the time your going to have a set of 100 amp breakers inside the main 200 amp panel feeding the sub panel.
and a second set. (100 amp disconnect inside the sub panel.(because the sub is at a remote location))

I think you should try turning off all-all the breakers then turning them back on to reset the breakers.
did you over look a set of 100 amp breakers in the main panel ?

Use a regular ohm meter to check for open wires and lose connections.
 
  #13  
Old 02-27-05, 05:05 PM
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I'm thinking that before I'd want to replace the wire I'd want to make sure all other potentials have been eliminated, as I'm sure you are.

In your first post you say after the splice repair all was well until 2 weeks ago when the system failed-what happened? How long ago was the splice repair done? What wire size? You say the shed panel is rated at 100 amps so I just assumed (and maybe wrongly so) it was supplied from a separate 100 amp breaker or disconnect with correctly sized wire from the main 200 amp service. Is there a separate 100 amp breaker or disconnect located there that shuts off power to this shed? If not there needs to be. Is there a 100 amp main breaker in the panel in the shed or is it just main lugs?

My last post suggested the 100 amp breaker/disconnect AT THE MAIN SERVICE may be the culprit if it possibly was defective, lose, or damaged somehow. Is it possible to substitute another breaker at the main service area (realizing this is a test and you won't be drawing ampacity) and check the voltage right on the wires themselves at the shed panel?

Maybe the correct question is to ask how the shed panel is supplied power, what is the configuration at the 200 amp main? How is the farmhouse and/or any other buildings supplied power? What are the panel sizes there? Are there any problems there?

Sure don't want to chew this to death Greenguy, but if that wire run to the shed is sized at 100 amps and is not protected, it has the potential of the 200 amp service there. Not knowing what else is being used on the property, you MAY have a very real hazard here potential and need to be aware of it.

Please let us know ...
 
  #14  
Old 02-28-05, 06:02 AM
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The wiring to this equipment shed is very simple really and that may be why I can't discover what went wrong. Let me go back and detail the flow on this and please forgive my layman's terms for various parts. The electric supply comes from the road pole (where transformer is located) to a service pole near the house. At the top of this service pole is a remote disconnect switch and then the current comes down the pole in two hots and one neutral to an outside panel box containing one 200 amp breaker. From there it goes under ground to the equipment shed and into service panel with one 100 amp main breaker...its a regular square D box. Each hot hooks to a lug running down each side and the neutral hooks to the neutral lug at the top. Breakers for each circuit are mounted below the main. I would have expected a ground rod attached to the neutral lug, but there is none, something I suspect needs to be there.
To answer what went wrong, simply put, nothing that I'm aware of happened unless lightning struck the building, because no traffic has been over the buried line which is I think No.6 wire, direct bury type.
I have tested the 200 amp breaker at the service pole by testing the current up and down stream of the breaker and it is registering the same on both sides of the breaker.
At the building, with the wires disconnected from the panel box, low voltage is present. Hooked up, nothing on any circuit works and because of course with low voltage, that should be correct.
I guess I just can't get it through my head that both these hot wires could go bad at the same time.
I think the answer is replace the underground wires, install a grounding rod on the shed's neutral lug and move on to the next problem, what do you think?
Based on what I've explained about how the wiring runs, ie. 200 amp breaker feeding the line going into a 100 amp main breaker in the shed, is there something dangerous there that should be changed? Thanks for your help.
 
  #15  
Old 02-28-05, 06:44 AM
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Open the disconnect and meg the wires as preiviously suggested, especially between neutral and hot. I have troubleshot aluminum feeds before where the wiring was almost completely deteriorated inside conduit, which had filled with water, neutral and 1 hot were completely deteriorated through, with just trace amounts of wiring left inside what was left of the insulation. I couldn't believe the breaker hadn't tripped and that the system was even working a couple days previously. This wire was underground triplex. I would seriously look at the wires as source of trouble, even going as far as unrolling a piece of #12 from the breaker to the subpanel and ohming out ea wire.
Also yes, there is a problem with the 200 amp main feeding the 100a subpanel UNLESS the wires are rated for 200 amp.
 
  #16  
Old 02-28-05, 07:09 AM
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Greenguy,

You didn't say what else comes off the 200 amp breaker at the main service area but my guess would be the house. What size panel is in the house?

Now:

Since there is only one 200 amp main breaker you need to understand that means there is a 200 amp potential present on all the wires connected to it-those going to the house AND the shed. Turning the breaker in the shed OFF only stops the potential up to the breaker inside the shed. It is still present in the wires running TO the shed-and since your best guess is they are #6 wires, even THAT is still too small for 100 amps, let alone 200 amps. It is therefore presumable that if you are using enough electricity in the house, even though the wire feeding the house panel may be sized properly, the amperage is more than what your wires going to the shed are rated for. Understand? You have parallel "service" feeds here, one on under-rated wire.

Without seeing this with my own eyes, my recommendation is this:

Contact a reputable electrician to look this over. He will tell you (wise eyes on) if this setup is correct or if it needs improvement. My guess is changes need to be made which will include the proper grounding solution. You need a professional present here to help you. I bet you can work with him on any part you may be able to do yourself to save some bux.

Meanwhile I would turn off the power at the MAIN 200 amp breaker and take off the wires going to the shed. Then turn it back on. That way you can still have power at the house until he can get there to help.

Please let us know.
 
  #17  
Old 02-28-05, 07:22 AM
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Please describe in more detail the voltage tests you did that you reported in your first post. What instrument did you use to do the test, what points did you probe, and what were the results?
 
  #18  
Old 02-28-05, 07:59 AM
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Actually the feed to the house is separate to equipment shed in that at the top of the service pole, the incoming service goes to the house. The incoming service is tapped there and goes down the pole to the outside panel where the 200 amp breaker is located and continues through that breaker down the pole underground to the equipment shed. I checked and the direct burial wire underground is 2-2-4 direct burial. I think I need to change the breaker in that line to 100 amp based on your posting. I think also, I need to just bite the bullet and put in a whole new line. Am I correct that the panel box in the shed needs a ground rod attached to the neutral lug?I'm going to try to draw the wiring here:

service pole--------------------transformer pole
/ \
/ \
/ \
/ \----------------House-150 amp main
/
/
panel on pole with 200 amp main breaker only
|
|
| 2-2-4 buried line
|
|
shed panel box with 100 amp main breaker



Hope this shows the wiring better than my description.
 

Last edited by Greenguy; 02-28-05 at 08:15 AM.
  #19  
Old 02-28-05, 08:08 AM
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John- I tested up and downstream at the 200 amp breaker in the panel at the pole and at the disconnected ends of the 2-2-4 line in the shed. The tests were done with a Commercial Electric digital multimeter model HDM4100. At the pole I got 120/240. At the end of the 2-2-4 line I got varying readings but mostly 40/60.
 
  #20  
Old 02-28-05, 08:10 AM
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Thanks for the patience displayed with this post.....believe me, it's appreciated!
 
  #21  
Old 02-28-05, 08:51 AM
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Varying readings in the 40-60 range almost certainly indicate an open circuit. But you didn't answer my question about what points you probed with your meter. Your meter has two probes. You put on on one thing and one on another. We need to know what those two things are. And I suggest you not only measure between every pair of wires, but between each wire and ground.
 
  #22  
Old 02-28-05, 11:56 AM
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Farm service laterals are special.

Greenguy

Because it is an agricultural property your farm has an isolation switch in the form of a pole top switch on the yard pole. The enclosed breaker or panel on the yard pole is a bit of a mystery to me. It can be used as an isolation switch or as a transfer switch but I imagine you would have mentioned it if it was fitted with the necessary accessories to serve as a transfer switch. Just so that we are clear when you open that breaker is it only the shed that losses power?

I suspect you are looking at a failure of the underground service lateral to your shed. Unless you open the breaker at the yard pole and the breaker at the shed and conduct a ground resistance test with a megohmmeter you won't know the nature of the failure.

The most likely point of failure is near the place it was cut. That does not necessarily mean your splice but you will have to check it. When a backhoe cuts an underground line it stretches it quite a bit first. This can leave many broken strands in the cable that will serve as future failure points.

If I understood your previous postings you have a sight isolation switch at the top of the yard pole. The output of that switch is tapped to supply the lateral to the shed. If the tap is on the line side of that pole top switch then the enclosed breaker on the pole is just an element of the sight isolation switch.

One question I can answer for you right now is that there has to be a grounding electrode system at your shed. The absolute minimum is two eight foot driven rods located at least six feet apart. That is the easiest electrode system to install but it usually has the highest impedance to earth. If you are going to trench for the new service lateral anyway you should consider installing a ground ring at the shed consisting of a number two or larger copper conductor buried directly in the earth at a depth of thirty inches or more, encircling the entire building. A less labor intensive substitute is to bury the equivalent length of bare number two in the lateral trench and bonding it to two rods driven ten feet out from the shed and at the end of the bare number two. By equivalent length I mean you lay just as much bare number two in the lateral trench as it would take to encircle the building. Even if you bury that thirty inches or more deep you add the ground rods to comply with the letter of the code since it would not be circling the building. Circling the building really is better practice because the rain and snow from the roof will tend to keep the soil near the shed wetter than elsewhere. The best practice with any grounding electrode is to install it so as to be below the permanent moisture level but in many soils and locations that is impracticable. One very important question in all of this is were is your well and is it a drilled well with a metal well casing?
--
Tom H
 

Last edited by hornetd; 02-28-05 at 12:15 PM.
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