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nymackem 10-30-2007 06:30 AM

Conduit issues - help needed
Hi - new to this forum.

I am installing a run of approx 40 ft of 1/2 inch EMT conduit carrying one 10/3 cable from my service panel to the new location of my clothes dryer.

I have installed about half the conduit and decided to connect power to two of the wires at one end (by pigtailing them to a 2-conductor extension cord) and test the circuit at the other end to check everything is OK with the job so far.

Clearly it's not OK because when I connect the power I get a blown fuse on the circuit it's connected to. When I test the voltage on the circuit, the hot and neutral wires have a voltage drop of 120V as expected - but there is also:

1. voltage drop between the hot wire and the third wire of the 10/3 cable that is not connected to anything of about 90v

2. and voltage drop between the neutral and the third wire of about 25v

.... which is odd because the third wire is not connected to anything!

Does anyone have any insight/experience with this. Is there some property of conduit or of runs of closely run cable that causes this? I am pretty sure the cable is not damaged in the conduit (not certain) but thought that if it was I would get a 120V reading, not something odd like 90v or 25v.

Any help appreciated.. Thanks.

NateHanson 10-30-2007 06:48 AM

I think a wire that's not connected to anything can have a floating charge like that, induced by the neighboring wires. It's like a floating neutral or open ground. You'll get strange voltage readings, and they're not really accurate.

If this thing blew the breaker, how did you test the voltage on the wires?

I assume you've already hooked up your 10/3 cable to the dryer receptacle, right?

nymackem 10-30-2007 07:28 AM

Nate thanks for the quick response.

Basically I ran a 15ft length from dryer receptacle to a junction box. I tested that length by applying power at the junction box - first to the black & white wires by hooking them up to a power source, and then to the red/white wires. With the black/white hooked up I got a 120V reading across them and some "floating" voltage by testing the red/white and red/black wires. Same thing when I hooked power up to the red/white wires. No fuses blown. At this point every thing looked OK since I was getting 120V readings on the connections that matter and no fuses blew.

Then I ran another 15 ft from my junction to a 2nd junction box (leaving 10ft to go to the service panel). When I tried to repeat the above and applied 120v to my black and white wires at junction box 2 and test at the dryer receptacle, the circuit blew (quite noisily actually) as soon as I powered it up.

To try to isolate the problem I have since tested each piece of the cable:
1. From dryer receptacle to junction box 1
2. From junction box 1 to junction box 2
3. From dryer receptacle to junction box 2

I have done each of these by hooking power at one end and a light bulb at the other and test 1 and test 2 are fine (120v across hot an neutral) - test 3 is what blew the fuse) But using my voltage tester on tests 1 and 3 I get the odd readings like 25v and 90v.

Since the problem occurred after I connected junction box 2 to junction box 1, I thought that there may be some cable damage on that part of the run, but that part of the cable on its own tests just fine.

At this point I don't know enough to know if the odd voltage readings are connected to the problem or something entirely unrelated. I am assuming this has something to do with the issue in the absence of any better knowledge.

Any help greatly appreciated!!!!! Thanks, D - this is driving me c-r-a-z-y : )

47_47 10-30-2007 07:45 AM

I do not believe that you can run 10/3 cable through a conduit. You should be running individual conductors (thhn). To test this wiring, I would remove the 10/3 connectors from the main power and use an ohm meter. You can check wire continuity and isolate the shorted wire much easier and safer. Do not forget to check for continuity to the EMT conduit.

NateHanson 10-30-2007 08:35 AM

My impression is the same as 47's (although I'm an amateur) - I don't think romex goes in conduit.

However, I'd say the most likely place for the short is in junction box 2. Check there first, maybe a wire came out of a wirenut, and is touching the metal box or something.

If it's still testing bad, maybe it'd be best to pull it out and pull THHN through there instead. It'll be easier, and also meet code.

mdlbldrmatt135 10-30-2007 08:37 AM

Yep........... they need to be individual conductors........ no Romex in Conduit......

nymackem 10-30-2007 10:03 AM

1 Attachment(s)
Thanks for the great info - clearly I do not know totally what I am doing... so here's a picture that might help contextualize my questions. If you look at the graphic below, can anyone see a problem with this layout?

Right now I have wired part A and part B. Part A is wired with Romex through framing in the finished area, and part B is through conduit (using Romex which I now know I will have to replace with THHN).

Part C I have not done yet (neither conduit nor cable), and I will do with THHN.

I encountered the problem testing lengths A & B. Do you think this is because I did not use THHN? Or because the conduit is not yet attached to the service panel and therefore has no ground? The ground cable from the Romex part in the finished area incidentally is attached to a grounding screw in junction box 1. I also kept the grounding wire on my wrongly installed Romex in part B and attached that to the grounding screw also in junction box screw.. could that be an issue.

Sorry to be such a dumbass. I am actually quite a bright guy - just trying to learn.

47_47 10-30-2007 10:45 AM

I am unclear as to where you are getting your floating voltage readings of 25 and 90 volts. As the problem seems to only exist when checking your wire from the dryer to box 2, through in box 1, the bad connection should to be in box 1. Did you twist the wires together first :yes: or put them straight into the wire nut and twist them together with the wire nut itself :no: ? Double check your connections and the clamps on the romex, you may have cut the insulation. I would also eliminate box 2 and keep a continuous wire from box 1 to the panel.

47_47 10-30-2007 11:05 AM

Junction boxes must also be accessible and not in finished walls.

nymackem 10-30-2007 11:12 AM

4747 - when I completed wiring part A, I applied power at junction box 1 to the black and white wires, while the red wire was just hanging out in the air.

When I tested voltage across the black and white wires at the dryer receptacle, I got 120V. I also tested voltage across the black and red (90v) and the white and red (25v) - which is odd since the red wire at junction box 1 was just hanging there.

When I completed wiring part B I did the test again and that was when the circuit blew. Although testing only part B by applying power at junction box 2 and testing at junction box 1 is OK.

Junction box 1 is accessible in the garage and not inside a finished wall.

Is your point about reducing the number of junction boxes one of aesthetics/economy, or is there some sort of code/integrity issue about having junction boxes? I used them because my garage to basement has a really convoluted cable run and I wanted to make pulling easier.

EDIT - just read your part about wire nuts again - I guess I did that wrong too as I twisted them together with the wire nut.

Jeez - OK lots to consider. Think I will finish installing the conduit and then rerun the cable using THHN.

J. V. 10-30-2007 11:38 AM

You CAN put NM in conduit. But it must be calculated for the cable and conduit size. Just like you calculate conduit fill, except its one cable. Not allowing NM in conduit is an old myth.

I personally would not do it. Seperate wires are the way to go.
He probably scratched or nicked that cable when he was pulling it in. Pull it back out and check for the black spot on the

HouseHelper 10-30-2007 12:32 PM

Check the cable clamp at JB 1. It is very easy to overtighten **/3 cable and cause a short. Also, it is better to check wiring using an ohmmeter instead of an extension cord. A lot less fireworks and much safer.:thumbsup:

Andy in ATL 10-30-2007 03:02 PM

I don't think you're a dumbass.:)

Pull out what you think is the bad wire and check for nicks. Go ahead and pull everything and connect to the breaker and to the recp. Quit doubting yourself and quit testing every little piece with an extension cord. Take your time and MAKE SURE every thing is right and TIGHT. I bet it works after you check everything.:yes: Confidence is part of the magic ju-ju that will make it work.

No circuit needs a ground to work. It is strictly there to provide a low impedance path back to the SOURCE (NOT TO GROUND, DESPITE WHAT YOU MAY THINK) in the event of a line to case fault. It is critical it is hooked up for safety. Google "phantom voltage". Digital meters are notorious for giving"false" readings. Once a load is applied, this phantom voltage goes away.

nymackem 10-30-2007 09:00 PM

OK first let me say that continuity testers rock. Don't know why I didn't know about that sooner... the voltmeter I was using was also an ohmmeter so I had it there, just didn't know what to do with it - sooo much better/faster for pinpointing the problem and of course safer. The article on phantom voltage helped a lot too.

Second you guys edge out the continuity tester in my estimation. After testing the cables I was able to find:

1) continuity on wire run A between the black wire and the ground wire that was remedied by loosening the cable clamp where the Romex entered Junction Box 1.

2) continuity in run B (the Romex inside the conduit) - again between the black and ground wires. I have not been able to fix this yet, but I will at the weekend since I will be replacing this run with THHN.

Not sure what the deal is with black and copper wires seeking one another out but I will see how I get on at the weekend and let y'all know.

Yours brimming with confidence-inspired ju-ju, D

NateHanson 10-31-2007 09:43 AM


Originally Posted by nymackem (Post 70830)
1) continuity on wire run A between the black wire and the ground wire that was remedied by loosening the cable clamp where the Romex entered Junction Box 1.

You need to replace this cable. The insulation was damaged by the cable clamp or hammer or something, and this will continue to short-out if power is applied to it.

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