Resistance of 1 foot of Romex cable?

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

  1. MatthewM

    Thread Starter New Member

    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. doug08


    Jan 30, 2011
    How about what gauge? That is the most important factor. Any run over 100', you must use one size larger wire to make up for the voltage drop to carry the same current. If your looking to supply 15A over 100', then you need to use 12ga wire. 20A over 100', you need 10ga....and so on. As for the break, it is most likely near the ends/connectors, especially if no sign of damage is done to the cable. Also try using an inexpensive voltage sensor and run it along the cable.
  3. gerty

    AAC Fanatic!

    Aug 30, 2007
    Was that reading taken with anything plugged into the other end of the cable ?
    If that answer is no, you may have some cracks/abrasions in the wire insulation. Most brans of romex use a paper filler to keep the jacket from bonding to the conductors. That filler will hold the moisture, which would show up as a high resistance short. If that is the case, when the cable dries out it'll work. This is one of the reasons you don't use romex outdoors,

    edit: Without knowing the gauge of wire we cannot answer your question about resistance, but it should be a small fraction of an ohm per foot. Wire resistance is usually measured per 1,000 feet.
  4. tubeguy

    Well-Known Member

    Nov 3, 2012
    Here's a wire resistance chart:

    Even 250ft of 14 gauge shorted at the opposite end (500ft of conductor) would have less than a couple ohms resistance.
    So as gerty said it'd just wet, or you could have a partial short somewhere assuming nothing is connected to the other end.
    If you had a very accurate meter, you could compare readings from each end.
    Last edited: May 25, 2013