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How can I work out where an electrical cable runs through walls/floors?

I am building a circuit map of a 1941 house with unfinished basement. I shut off all circuits at the main service panel, switch on the circuit I wish to trace, and track the cable from the panel and through the basement till it disappears into the floor above, then run upstairs and see what's powered and tag it. 2 cable runs I have tracked to where they disappear into the wall/floor leaving the basement and lost there. Checking outlets (switches/receptacles/luminaires) throughout the house indicated none were powered by the circuit.

Is the only option to start cutting holes in the wall and hope I can pick up the path of the cable run from there?

Would an analog tone and probe kit work? I guess I would switch off main breaker, pop the service panel, and hook up the probe directly to the start of the circuit within the panel, then chase the tone through the walls?

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    NM in a 1941 house means that some rewiring was done before this. Is it possible that they don't power anything and are just capped off in the walls or in the attic?
    – Niall C.
    May 15, 2012 at 18:06
  • Some rewiring was definitely done. It looks like a service panel upgrade for washer/dryer hook-ups followed by somewhat more recent work when most of the attic was finished. Ground floor is still serviced by fabric-wrapped 2-wire without ground, with the old service panel serving as an impromptu junction box (ugh). May 15, 2012 at 18:11
  • Given the state of the electrical wiring, I can't imagine them taking the trouble to pull cable they weren't going to use. Even if capped off, I'd still like to trace the cables to be sure. Same technique/tool (if it exists) would also come in handy when chasing wandering circuit paths between outlets. May 15, 2012 at 18:13
  • You answered your own question in your edit. I have one from Klein that works well.
    – Tester101
    May 15, 2012 at 19:19
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    @JeremyW.Sherman You likely won't need a top of the line model, just grab a middle of the road version from a manufacturer you've heard of.
    – Tester101
    May 16, 2012 at 16:52

2 Answers 2

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Pick up a middle of the road tone generator and probe kit from a company you've heard of. No need to buy the most expensive model you can find, it will likely have features you'll never use.

I've used this one Gardner Bender Wire-Tracker Wire Tracer, which can be purchased from Home Depot for ~$40.00.

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It's not the best model on the market, but it worked very well for me. It was sensitive enough to accurately trace cables through both walls, ceilings and floors, and was able to attach to a variety of cables.

Make sure the power is off before connecting the tone generator. Some models can work on hot lines, but it's still safer to kill the power.

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I plugged an old electrically noisy computer power supply in the outlet of interest and used a portable AM radio to detect which circuit breaker it was on. In my case, the nasty power supply was very noticeable around 550 KC.

So basically the AC filtering section capacitors are undoubtedly shot. I was listening to my radio one day in the kitchen area of my house when I came across the noise. It drove me crazy trying to locate the source. Turned out it was an old pentium computer running in my shop in the backyard.

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