Voltage loss in subpanel - advice needed

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  #1  
Old 08-23-05, 08:25 AM
tkboys
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Voltage loss in subpanel - advice needed

I had a shop built on our property last fall and a 100 amp service professionally installed in the shop. The contractor buried a 2ga aluminum 3 strand wire in ground approx 3.5 ft below grade and approximately 200 ft of wire. I went into the shop yesterday and none of my 220 volt equipment would start nor would some lights or outlets work. Checked voltage from source panel and the 100 amp breaker in the source panel is showing 220 volt via my tester. I also checked the inbound voltage before the 100 amp breaker on the subpanel and I show reduced voltage on 1 side (50 volts) and 110 volts on the other.

What is causing this decline in voltage? There's obviously a voltage drain in the 200 ft of line between my home and the shop. Why and what are the potential root causes. How can this be repaired?

Thanks in advance for your responses.

TKB
 
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  #2  
Old 08-23-05, 09:09 AM
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If you are using a digital tester, don't. The readings won't be reliable for this purpose; the 50V reading is probably 0V in reality. Instead use a lamp-type tester.

The first thing to do is turn off the breaker at the main panel and check the lug tightness of both hots and the neutral feeder to the subpanel. Check at both the main and at the subpanel.

Next thing to do is try a new breaker in the main panel. It's rare, but breakers do go bad. EDIT: Just realized you already tested 240V at the breaker, nevermind that suggestion.

Is there any possible way the wire got cut recently?
 
  #3  
Old 08-23-05, 09:09 AM
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The low voltage is phantom (meaningless). Pretend that it is zero. One of your two hot lines is open, either at a connection at one of the ends, or (hopefully not) underground.
 
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Old 08-23-05, 09:10 AM
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If these are Aluminum conductors,first check for a corroded connection at the terminals of the circuit-breaker in the Service-panel, and the terminals in the shop panel where the conductors connect to the panel.

AS best you know, was a "Direct-burial" cable installed?

Good Luck, and Enjoy & Learn from the experience!!!!!!!!!
 
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Old 08-23-05, 09:51 AM
tkboys
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Originally Posted by ibpooks
If you are using a digital tester, don't. The readings won't be reliable for this purpose; the 50V reading is probably 0V in reality. Instead use a lamp-type tester.

The first thing to do is turn off the breaker at the main panel and check the lug tightness of both hots and the neutral feeder to the subpanel. Check at both the main and at the subpanel.

Next thing to do is try a new breaker in the main panel. It's rare, but breakers do go bad. EDIT: Just realized you already tested 240V at the breaker, nevermind that suggestion.

Is there any possible way the wire got cut recently?
The only way the in ground wire would get cut was if a critter such as a meadow vole or mole severed the wire sheathing under ground. The wire was buried deep for this reason according to the contractor. The only place I can image a critter severing or effecting the line is at the outbound and inbound connection points for the wire. There has been no construction near the underground wire.

Thanks for your comments. Any other ideas?
 
  #6  
Old 08-23-05, 10:03 AM
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Underground wires can fail for many reasons. Sometimes the fill was not screened and a rock got pressed up against the wire. Sometimes the incorrect cable was used and it eventually fails under the wet conditions. Sometimes the ground shifts.

But don't worry about that yet. Check all the connections at both ends. This is easy (but not necessarily safe unless you know what you're doing).
 
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Old 08-23-05, 10:16 AM
tkboys
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Originally Posted by PATTBAA
If these are Aluminum conductors,first check for a corroded connection at the terminals of the circuit-breaker in the Service-panel, and the terminals in the shop panel where the conductors connect to the panel.

AS best you know, was a "Direct-burial" cable installed?

Good Luck, and Enjoy & Learn from the experience!!!!!!!!!
Thanks for your response. Yes they are aluminum conductors. I'll check for corrosion on both ends. The cable buried was a cable rated for direct contact underground. I'll let you know what I find on the corrosion front.

TKB
 
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Old 08-23-05, 07:53 PM
tkboys
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Originally Posted by John Nelson
The low voltage is phantom (meaningless). Pretend that it is zero. One of your two hot lines is open, either at a connection at one of the ends, or (hopefully not) underground.
Thanks for your feedback Mr. Nelson. I further checked the issue this evening. I used my tester to check for continuity. I'm getting continuity between one of the hot lines and the ground line (using Ohms function on tester). My interpretation of this is there is a break in the line or something punctured the sheathing on the wire and the hot line is grounding itself. Would this explain the considerable reduction in voltage on one side? This may also explain why some outlets and lights are working and others not. The way the breaker panel is configured, each hot side alternates in the fuse panel with the circuit breakers installed. When I check each breaker (power off) I am getting continuity on one hot side of the breaker and the ground in the breaker panel. When I check the other side of the 100 amp breaker there is no continutity between the hot side and the ground. Do you agree it would appear one line is grounding out somewhere in the cable?

TKBoysen
 
  #9  
Old 08-24-05, 02:41 AM
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IF one leg was grounded, the breaker in the main panel would be tripping.
 
  #10  
Old 08-24-05, 02:41 AM
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Just a note.
If you have lights or something plugged in down the line you maybe checking the continuity in a light bulb.
 
  #11  
Old 08-24-05, 06:49 AM
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I'm getting continuity between one of the hot lines and the ground line
Continuity checking has limited usefulness in home electrical work. This result is not useful. Zero resistance continuity would of course be a dead short and trip the breaker. Non-zero resistance continuity is necessary for anything electrical in your house to draw electricity and function.

If you want to conduct a meaningful continuity check, shut off the feeder breaker in the main, and all breakers in the subpanel, and check for continuity from one end of each hot wire to the other end. Of course you'll need 200 feet of wire to extend the probes on your meter.
 
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