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Resistance of 1 foot of Romex cable?

Discussion in 'General Electronics Discussion' started by MatthewM, May 25, 2013.

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  1. MatthewM


    May 25, 2013
    Here's the problem I'm trying to solve --

    I have a 250-foot Romex cable (normally used for interior cabling for electric boxes) that I've turned into an exterior extension cord. I'm using it to provide power 24/7 to my garage (on a temporary basis, don't worry).

    Last night, after 2 solid days of rain, a circuit breaker tripped. NOTE: We've had rain a couple times since I had this cable strewn across my yard, but no problems. But maybe the constant, persistent drizzle, alternating with periods of light, medium and heavy rain, allowed moisture/water to get in some small crack or hole, somewhere along the cable, and eventually shorted out the two prongs.

    Getting to the point --

    Right now, the 2 prongs measure 92.3 ohms. I wonder if there's any way to calculate WHEREABOUTS in the cable the hole might be? It's the kind of hole that could be fixed with duct tape -- just something to keep the water out. It wouldn't take much, since it's not high-pressure like a water hose or something.

    Obviously the multimeter is measuring X feet into the cable -- going in one side, and coming out the other. So however much resistance ONE STRAND of Romex has per foot -- that would be quite helpful. Unfortunately, I can't measure it myself since I have plugs on both ends. I don't have any other Romex cable handy.


  2. BobK


    Jan 5, 2010
    That resistance will not help you. If the wire is 14 gauge it is about 2.5milliOhms / foot if it is 12 gauge it is 1.6mlliOhms / foot. So 250 ft of 14 gauge wire would only be about 0.625 Ohms.

    So first, let's get it straight what you are measuring. I am guessing you are measuring across the two conductors with nothing connected to either side. Is this correct?

  3. MatthewM


    May 25, 2013
    I took the multimeter, set it on the lowest Ohms setting, and put one probe on each of the 2 prongs of the plug on one end (I skip the Ground terminal at the bottom).

    I choose to measure at that plug because it's right here in my room -- it goes out the window (yes, my window is permanently open a small amount) and out to the garage on my acreage. All excess wire (about 100 to 125 of the 250 feet) is inside the garage.

    On the other end, there's another plug and that has a surge strip plugged into it.

    So normally I should read "infinite" resistance between the two prongs of the plug (hot and neutral). But right now there's continuity -- water has shorted the two wires out somewhere.

    Also, Romex has 3 wires inside. Hot, neutral, and ground. Ground is non-insulated, just bare copper. The other two have white and black insulation covering them.

    I'm thinking the insulation and/or outer sheath has been violated somewhere along the line (cat bite? etc.) and it's letting water in when we get a LOT of rain.

    It's possible I'm interpreting the multimeter reading incorrectly. It might be 92.3 milliohms for all I know.
  4. duke37


    Jan 9, 2011
    If there is water in the cable, it is scrap. You cannot get it out.

    You could cut the cable in half and keep the good half, cut the bad part in half etc.

    Or you could make a pulse generator and use a fast scope to look at the time that the pulse gets to be reflected.

    Another way is to connect the cable to the mains and run your hand down the cable. My nephew did this, he was lucky to survive I unplugged him!

    If the cable is lying on the ground, it will get cut if trodden on or run over.
  5. davenn

    davenn Moderator

    Sep 5, 2009
    ir you were measuring some low resistance fault with the circuit breaker/ power strip still on the end of the cable
    measure the resistance between the blacj and white again when there is NOTHING on either end of the cable

  6. brevor


    Apr 9, 2013
    Romex is really tough cable I don't think water would penetrate it. And a cat could not bite through it. Check the end for a short, or your power strip.
  7. MarcD


    May 26, 2013
    Hi Matt, Like one other post mentioned, romex is not suitable for wet conditions. best to remove it and let it dry out for a LONG time. The paper inside the romex will be wet for quite some time. consider using type UF romex even if temporary. safer and tougher stuff for direct burial. code is 18'' deep. 6'' if you have 2 inches of dry mix concrete put on top of UF wire or a PVC run.
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