Instructables

How do you trace electric wires in a wall, without tearing the wall apart?

I have a wall switch that I can't figure out what it is to. I have taken the front plate off and the switch works. There is electricity on the wires leading out of the box when the switch is on. Is there a way to trace the wires through the wall?

orksecurity4 years ago
Actually , I may need this answer myself. The neutral (only) of a ceiling light just decided to fail. It's on a different circuit than anything near it... so either I need to trace it through and try to figure out which splice has opened up, or I'll need to trace it in order to disable it for safety's sake and run an entirely new feed.

And of course it's not only on the second floor, but appears to be routed through some inaccessable spaces between the floors rather than up to the attic and back down. And it's BX rather than Romex, so it's shielded and my low-budget tracing tools can't follow it worth a darn.

GRUMP. If I have to, I'll hire a pro. But I'm still hoping to avoid that, especially since the guy who was here as part of the renovations I'm doing already took a quick look at it and established that it isn't going to be easy to trace down.

Phil B4 years ago
A fairly simple way to trace the path of a wire is to buy a 75 or 100 foot spool of single strand flexible (not solid) bell wire.  You also need an ohmmeter.  Turn the AC power inside your house "off."  Connect one end of the bell wire to the starting point in your investigation.  Go down the hall to the outlet or fixture you suspect may be connected to the wire you are investigating.  Connect one lead from your ohmmeter to the outlet's or fixture's terminal.  Turn on the meter.  Connect the other lead to the end of the bell wire.  If the wire you are investigating provides a path to this fixture, the meter reading will drop from infinity to a low reading.  If the reading does not drop, test other terminals on other fixtures and outlets.  A continuity tester may not work because those usually limit out at 300 ohms and the combined run in your circuit could have more than 300 ohms resistance.  It is also good to disconnect wires from fixtures before testing so you sure you are not reading a phantom circuit that feeds round about through something else, like a light bulb or a motor.. 
lemonie4 years ago
Correct wiring procedure is vertical, you ought to be able to see where the wires enter the box and go vertically from there. But it isn't necessarily the case. You can buy detectors - go looking for Wire/Pipe and Stud Detector

L

The _simplest_ wiring solution is up from the basement, or up to the attic and back down into the top-floor walls from there. Middle floors become more complicated. And older houses in particular may have some (ahem) creative wiring paths. For example, one wire in my house that used to go straight down through a wall now crosses sideways through the ceiling below (to avoid a new archway) before completing its downward run.
I saw a supply to a pair of sockets run from the basement up to 3 floors, held onto the wooden parts of the stairs with cable-clips. The old wiring was not trusted for a splice-in at the top of the house...

L
If it's a pier and beam floor it may go down into the crawl space.  L's idea of the wire/pipe detector is great.  Try to find one at a rental place to get the most sensitive one you can.  The ones you get at the builders store are not sensitive enough to detect thru much.
Jayefuu lemonie4 years ago
As Lemonie said, it probably goes straight up. Then it'll run along the gap between the floors, if you have access to the floor above you could lift up the carpet/floor boards to see where it goes.
seandogue4 years ago
For homes old enough to have tube & knob wiring (two single wires), there was a tendency to run wires to the top of a room, then drop them to their fixtures along stud spaces , at least in my area. while in the ceiling, there was a tendency to run across the "grain", driving tubes thru each joist to house an individual wire and sometimes along the joists...Sometimes, when joining outlets, they'd run tube thru the studs to connect strings of outlets.

With modern wiring, there's a tendency to follow the stud and joist spaces, especially since there's been more scrutiny by officials in how things are wired, although one can't be certain that it was done by the book.

Being who I am, I tend to think there was a purpose for the switch. And so, I'd be inclined to suspect it either at one time powered a ceiling light that has been capped over or (bad form) plastered in), or that it  once powered a "convenience" outlet.

For the first situation, is there some sort of a raised emblem in the center of the ceiling by any chance? If so, it's quite possible that it's just a cover plate and secretly houses those missing wires.

In a great number of homes built for returning veterans from WWII, they installed a convenience outlet, (often found in living rooms without any ceiling fixture) to which one would plug a lamp left on all the time, then use the switch to turn on and off current to the outlet. I personally find them more annoying than anything, but I guess they saved on wiring or otherwise reduced the effort/cost of building all the homes needed after the war terminated in 1945, somewhat needed with the flood of returning GIs..

Otherwise, see all the wonderful answers below.
paganwonder4 years ago

You may be looking at a switched outlet situation.  In older homes it was common to "split the outlet" (one half of duplex outlet wired to switch, the other half always powered).  I would plug a radio into each half of each outlet and flip switch up and down- an assistant would speed this process.  I suspect that in bedrooms the switched outlet is near the bed, in the living room the switched outlet is near the doorway.

Burf4 years ago
Tracing electrical wiring was a problem I often faced on remodeling jobs. The most effective tools I found were a hand held metal detector and a lighted borescope. Even having those tools available, there were times when the only option was to cut a small inspection hole and look inside.
As a general rule of thumb, wiring installed by an amateur or prior to 1960 was almost always the most difficult to trace.
orksecurity4 years ago
Another possible tool: There are gadgets that plug into an outlet (or a bulb-to-outlet adapter) and put a signal on the line which the matching detector can pick up. They're mostly intended for figuring out which circuit breaker controls that circuit, but I've found that at high sensitivity they can also sometimes be used to trace Romex cables. (Not conduit, though; that would block the signal.)
Depends on your local electrical codes. In the UK, wires can exit horizontally or vertically from a fixture.

Steve