I appear to have a short somewhere. Please help!

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  #1  
Old 05-22-04, 10:56 AM
Boot
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Angry I appear to have a short somewhere. Please help!

This is a long story, but I’ll try to keep it brief. Thanks for bearing with me as I REALLY need some advice. Some time ago the electric to my shop quit working. Whoever wired it (before I bought the place) ran regular house wire direct buried, so I figure a mole chewed through it or something. I decided to rewire it correctly using a heavy gage wire recommended by Lowe’s. I installed a new breaker box in the shop and ran the new wire to it from the breaker box at the house. Each wire is individually wrapped and all three wires are wrapped together in heavy-duty rubber. In addition, I ran everything in conduit underground. Everything worked fine for awhile, and then one day it just stopped working again. The breaker box at the house kept tripping, so apparently I had a short somewhere somehow. It has to be somewhere inside the conduit. It’s not at either end, but it’s definitely somewhere between the breaker at the house and the breaker at the shop. The house breaker will trip whether the shop breaker is on or not, but it won’t trip if I disconnect the hot wire at the house breaker.

Today, I tried switching the wires. I rewired the “hot” wire to neutral and the “neutral” wire to hot, and that worked. I now have lights at the shop again, and the breaker doesn’t trip. However, there’s a crackling sound coming from somewhere near the shop breaker box and it pops every now and then. Does that mean I still have a short somewhere? Am I going to have to dig up all that wire again? Anything I can do short of completely starting over again? Any idea why this is happening?

This is really frustration. Thanks so much for any input you can offer.

Boot
 
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  #2  
Old 05-22-04, 11:07 AM
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Yes, it sounds like you have an underground short.

You described the wire you ran underground, but can you give us the specs on the wire and the conduit? What kind of wire was it (some combinations of letters)? What gauge is the wire? What kind of conduit (PVC 40, PVC 80, EMT, ...) was it? How large was the conduit (diameter)? What is the distance to the garage? How did you get the wire into the conduit? Is it protected above-ground with conduit too? What is the amp rating of the breaker in the main panel?

As I'm sure you are aware, switching the hot and neutral created a significant hazard. So I assume that this is just an experiment. I strongly suggest you turn this breaker off until the problem is resolved.
 
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Old 05-22-04, 11:45 AM
Boot
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Thanks for such a quick response, John. I hope the following answers your questions:

The conduit is a ¾ inch PVC-40. The wire is encased in the conduit above and below ground the entire length of the run, which is approximately 100 feet. We enclosed the wire in the conduit by running it through one 10’ length at a time.

The wire is a 10 gage type NM-B 600 volt wire with a hot, neutral, and ground (3-phase I think it’s called?) I have hot to hot, neutral to ground, and ground to ground at either end. The breaker at the main box is 30 amp, and the breaker at the shop is 20 amp. I was afraid you were going to say I had a short, but I am stumped as to how I’d have a short unless I simply got ahold of some bad wire, which would be my luck.

The switching of the hot and neutral was a test, although I was hoping it might solve my problem. Correct me if I'm wrong, but my thinking was that it doesn’t really matter which wire you use for hot and which for neutral as long as you have them wired the same at one end as you do at the other. Aren’t both wires identical other than the color of the casing?

Here’s another thing that might come into play, and here is where my knowledge of the subject is lacking. The hot wire was too thick to fit the main breaker, so I had to isolate two of the strands to connect to the breaker. However, I have all strands wired to the box in the shop. Could this be a problem? If so, could I do a continuity test to determine which two strands I should run for hot?

Thanks so much for your help. It is GREATLY appreciated.
 
  #4  
Old 05-22-04, 12:40 PM
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You violated the National Electrical Code when you installed the wire in the conduit. The NEC prohibits you from putting the cable one conduit segment at a time. The problem is that the PVC glue dissolves the insulation on the wire.

You also committed a serious error by running NM-B. NM-B is prohibited in underground conduit. The conduit fills up with water by condensation, and the NM cable will not survive it. By the way, there's no "rubber" on NM-B cable.

You probably have to start all over. Run large enough conduit to allow you to pull the wire through after the conduit is complete. Use individual THWN wire (often sold as THHN but dual-rated THWN) rather than cable.

There are potentially many other things you did wrong. We should go over them before you do the job over.
 
  #5  
Old 05-22-04, 03:24 PM
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Use larger conduit, always oversize it. Its easier to pull the wire and allows an upgrade path later if you want a larger service in the shop. Use a rope to pull the wires through the whole run, doing it section by section is not good at all. Tie a large nut, deep sea fishing lead sinker, etc to the end of the rope to drop the rope through each conduit while glueing it. To make pulling the wire easier on you, buy a tub of wire lube.
 
  #6  
Old 05-22-04, 06:00 PM
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Boot, I know we've given you some pretty tough news to accept. Take a few moments to go through the seven stages of grief, and then come back and talk to us. We'll help you get out of this mess.
 
  #7  
Old 05-23-04, 11:17 AM
Boot
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I tell you, there’s nothing quite like taking two steps forward and ten steps back. I guess this is what I get for listening to Lowe’s “electrical expert” who told me “this is the wire and the conduit you need.” The wire ended up being total overkill anyway as I don’t think I need such a heavy-duty wire for what I plan to run out there. I didn’t glue the conduit, so that shouldn’t have been a problem, unless, of course, that in and of itself is another code violation. Is it possible that wire could have corrupted so quickly due to condensation (less than two months)? I guess it’s a moot point now, though.

Alright, so I guess I’m coming to grips with the fact that I’m starting over. I plan to run lights out there and fans from time to time. Plus, I have a 10hp table saw, miter saw, and various other shop tools. What exactly should I get to make sure I do it right this time?

Thanks for all your help. John, I appreciate the honesty. Tough answers are tough, but I do want to make sure I do it right. I’m a vastly underpaid teacher, so I don’t have the funds to hire an electrician, which is why I do it myself and learn as I go. This lesson was an expensive and time-consuming one. I should have come here first. I’ll know better next time.
 
  #8  
Old 05-23-04, 11:47 AM
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10 hp table saw?
 
  #9  
Old 05-23-04, 05:08 PM
Boot
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Oops. That's 10 INCH, not 10 hp. It's a max of 3 hp. My bad.
 
  #10  
Old 05-23-04, 07:30 PM
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Boot, you took away my first choice of a cause when you said you didn't glue the pipe. And it's unlikely that the NM has failed so quickly. Nevertheless, you have a short, and if it is underground, it is irrepairable. The short is apparently between the black and bare wires, which is why switching the black and white seems to make it go away (did you reverse the black and white at both ends?). But this is not an acceptable fix. You still have a short that is probably putting neutral current on your grounding wire. This can potentially electrify appliance cases, so it's not a solution.

First, verify that you can't find the short above ground where it might be repairable.

How deep did you bury the conduit and wire? Code requires 18" if in PVC conduit.
 
  #11  
Old 05-24-04, 07:37 AM
Boot
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John, I did reverse black and white at both ends. That brought power to the shop, but I was hearing a constant crackling and periodic popping somewhere near the box at the shop, so I killed the breaker at the house. That's where I stand right now.

I have inspected the wire at either end (as much of it as I can see), and I see no obvious places where the hot wire could be grounding out. So it must be underground somewhere. I don't know how in the world it could have happened, but it did. I found my original receipt for these materials, so I'm going to Lowe's to talk to a manager and see if he will agree to give me a credit for the wire and the conduit.

I actually buried the conduit a little over 18", so it looks like I at least did one thing right.
 
  #12  
Old 05-24-04, 07:46 AM
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This doesn't have anything to do with your problem, but I have two other questions:

(1) Did you electrically isolate the grounding and neutral at the subpanel? You should have This requires you to throw away the green screw that came with the panel, and it usually requires you to separately purchase a grounding bar kit.

(2) Did you install grounding rods at the shop? You should have.
 
  #13  
Old 05-24-04, 08:16 AM
Boot
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The box I bought for the shop has a grounding bar already installed in it. I have the ground and the neutral running to this bar. Yes, I did install a grounding rod at the shop. I didn't know if it was necessary since it's grounded at the main breaker, but I figured it couldn't hurt.
 
  #14  
Old 05-24-04, 09:11 AM
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If there is only a single bar, that's the neutral bus, not the ground bus.

You need to have a seperate ground bus in a subpanel, and the neutral bus cannot be electrically bonded to the panel itself (the green screw John refers to).
 
  #15  
Old 05-24-04, 09:33 AM
Boot
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Ahhhh...I see. I'll look into that.
 
  #16  
Old 05-24-04, 10:20 AM
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A couple books recommended by someone here might help you out before you burn something down or worse.

Wiring Simplified by H. P. Richter and Wiring A House by Rex Cauldwell are the two I bought, I think from Amazon.com.

Good luck,

Baldwin
 
  #17  
Old 05-24-04, 10:45 AM
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If you are a teacher then perhaps you can work out a deal. Take a residential electrical course at your local vo-tech. If it's in your district you can probably get it for free or close to it? You will save yourself a lot of grief over the long term. Several of the things you have described are not just bad workmanship but potentially life threatening. Very, very serious stuff. The people here are really helpfull but if you understand the basics then the suggestions you get are going to make that much more sense.
you might want to get this one insepected when you are done.
 
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