Troubleshooting

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  #1  
Old 06-07-04, 09:16 AM
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Troubleshooting

I have a three wire fed sub panel in a barn. Occasionally the lights in the barn will dim, last night when they did I found:
L-1 to ground approximately 150 volts.
L-2 to ground approximately 75 volts.
L-1 to L-2 approximately 240 volts.

At the breaker in the main, feeding the sub, I got 120 volts to ground on each leg.

I used an analog meter, there are no multi-wire circuits from the sub, all 120V loads were unplugged except the lights and the breaker for the only 240V load was off.

This morning I went to check with the lights off but when I turned on the lights, to see if they were still dim, they came on bright and all voltages were normal.

I’m trying to figure out if I can isolate the problem to a hot leg, or neutral and to the feed to the sub or a branch circuit. My first thought was a problem with a hot feed to the sub but the more I think about it the more confused I get.

I would be thankful for any help in understanding and solving this problem.
 
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  #2  
Old 06-07-04, 09:57 AM
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Where did this ground you measured to come from? What is the voltage when measured from each hot leg to the return?
 
  #3  
Old 06-07-04, 10:58 AM
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I measured from each hot termination of the feeder to the neutral lug, which is grounded and bonded to the grounding buss.

Thanks
 
  #4  
Old 06-07-04, 11:05 AM
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Where is the neutral wire connected?
 
  #5  
Old 06-07-04, 11:13 AM
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Sounds like a bad neutral connection at one end of the feeder or the other.
 
  #6  
Old 06-07-04, 11:20 AM
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Bob,
It is connected to the neutral/ grounding buss, which is grounded via an 8 AWG GEC, and bonded to the enclosure.

Thanks



Thanks John,
I will double check the but I think they are good.
 
  #7  
Old 06-07-04, 11:29 AM
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At a sub panel the neutral and the ground must be separated. I doubt this is related to the problem, but it should be corrected.
 
  #8  
Old 06-07-04, 11:35 AM
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Bob, most likely this installation is correct with the neutral and grounding bonded at the subpanel. What you said probably doesn't apply to this situation.
 
  #9  
Old 06-07-04, 11:56 AM
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I doubled checked all the connections for this feeder at both ends, put a little more torque on one of the hot lugs but wouldn’t call it loose.

It may be a while before I know if that helped, as the problem is intermittent.

With no 240V loads would a loose, or open, neutral elevate the voltage on one leg if there is a load on the other? Would the load on one leg put it in series with the other leg through the branch neutral?

It is a 3-wire fed sub with no metallic paths (that I know of) other than the feeder between the two structures.

Thanks again
 
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Old 06-07-04, 12:24 PM
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With no 240V loads would a loose, or open, neutral elevate the voltage on one leg if there is a load on the other? Would the load on one leg put it in series with the other leg through the branch neutral?
If the neutral were loose or open, the voltages on a leg would vary in inverse proportion to the loads on that leg. And yes, the loads on one leg would be in series with the loads on the other.
 
  #11  
Old 06-08-04, 09:56 AM
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Thanks John,

With an open or loose neutral would the loads on each leg be in series with each other only on 3-wire circuits, such as feeders and multi wire circuits?
Could a loose neutral in a branch circuit cause the voltage changes on the feeder?



Sounds like a bad neutral connection at one end of the feeder or the other.
If the feeder neutral connections are tight, does that mean there is a problem somewhere along the conductor?
Is there something else that could cause this problem?






At a sub panel the neutral and the ground must be separated. I doubt this is related to the problem, but it should be corrected.
I thought a sub in the same structure as the main had to be fed with a 4-wire feeder, which would require the grounded conductor (neutral) and grounding conductor (EGC) to be isolated (a non service rated panel). But, if the sub is in a separate structure, that has no other metallic paths between them, then it could be fed with a 3-wire feeder and the grounded and grounding conductors would be bonded at the sub (a service rated panel). Personally, I would use 4-wire feeds for every thing beyond the main disco at the pole. Eventually I hope to have my property wired this way.

I don’t think the NEC defines or even uses the term “sub panel”, maybe to some it only applies to non service rated panels and to others it applies to any panel fed from a panel.
 
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