Tracing wiring in finished walls

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  #1  
Old 01-04-16, 03:54 PM
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Tracing wiring in finished walls

I recently bought a house and need to add some new receptacles. It would be very helpful to know where the wiring is inside the walls. Can anyone recommend a device that isn't too expensive that would help me avoid cutting holes in walls just to find wires? I'm guessing an inexpensive non-contact voltage detector won't do the trick, but I'm hoping I don't need to buy one of those very expensive wall scanners (or, not buy one and then cut holes in the walls). Thanks.
 
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Old 01-04-16, 04:22 PM
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Not the way you add new devices. Cables in the wall have no slack and it is difficult to connect to them. The correct way is to either run from a nearby unrestricted receptacle or run all the way from the main panel.
 
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Old 01-04-16, 04:22 PM
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You can use what's called a Fox and Hound. They even make a junior version. I'm not sure which version depot carries now.

You would take the wire out of the breaker and clip one lead to that black and the other lead to the panel ground. You can follow the line thru the walls. Needs to be used on NM (romex) type wire. Won't work on MC (BX) or conduit.

One thing..... cabling will be fastened to the studs and there will probably be no slack available to work with.

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Old 01-04-16, 08:06 PM
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Thanks for the tip on the fox and hound.

Not the way you add new devices.
All I said is that it would be helpful to know where the wires are!

Here's the situation and what I am thinking: I have two outlets on an exterior wall (no outlet on the other side to tie into) and I'd like to add an outlet between them. Spacing of existing outlets is to code, it just so happens that they are not in the right place for my needs. If I cut a hole for a new box and if I know where the existing wire is, maybe I can get away with cutting only two other access holes to be able to cut the existing cable or remove the staple, use the existing cable to connect from one existing receptacle to the new one, and then fish new cable from the new receptacle to the now-disconnected existing receptacle. I don't see what would be wrong with this approach, but there's a lot I don't know so please tell me if I'm missing something. Running new cable all the way from the panel is impractical.
 
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Old 01-04-16, 08:15 PM
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If you have two receptacles on a wall...... the most logical way to have run the wiring is straight thru each stud. If you cut into the wall between those two receptacles you may see the wire but there won't be any extra slack in the wall.

Is there a basement below ? You could possibly add a wire to one of the existing receptacles.
Also, you may find that wiring between receptacles down there.
 
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Old 01-05-16, 07:14 AM
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The basement is finished so that probably doesn't make things easier.

I understand that there won't be any slack, and that I will have to fish new wire through the existing holes in the studs to connect the new outlet to one of the existing ones (using the existing cable to connect the new outlet to the other existing outlet). But if I can tell where the wire is before I start cutting drywall it will make things easier.
 
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Old 01-05-16, 08:10 AM
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I'm not critical of your technique, but there are easier ways.

You don't need to know how the existing cable is ran. You only need to get a cable to your new box from either the left or right receptacle, whichever is easiest (easy meaning getting a new cable thru only one stud if possible)
This can usually be done with only two holes and no patching:
- the hole for the new receptacle, old work box hole
- the hole for the old receptacle, you will cut out the existing box and replace it with an old work box.
 
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Old 01-05-16, 08:35 AM
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I see your point, thanks, that makes sense. I was thinking of "daisy chaining" the receptacles with two cables attached to the two sets of terminals; your technique would involve (if I understand it) three cables entering one of the boxes (the two cables that are there now and then a new one to feed the new outlet). What is the best way to make the connections inside that box (wire nut all the cables together and pigtail to the receptacle?)? I can look up how to do the box volume calculation, but as a rule of thumb is a standard outlet box big enough to add an extra cable? Thanks!
 
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Old 01-05-16, 08:48 AM
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Yes, pigtailing is the approved method. A box fill calculator: Electrical Box Fill Calculations - Construction Monkey
 
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Old 01-05-16, 08:54 AM
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Yes, the best connection is to connect all wires together with a single pigtail to the receptacle screws.

You should be OK on box fill, like you said, two existing cables and one additional.

Let us know if you run into a receptacle that has both black and red wires (a 3 conductor cable with ground is present).
 
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Old 01-05-16, 09:01 AM
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Great, thanks!

One other question: if I do it this way I will be running cable through studs with no nail guards. Is that ok as far as code is concerned? I think the rules are different for "old work" vs. new construction but I'm not sure of the details (just starting to figure out how I'm going to do this work, obviously.). Don't want to do it and then have problems with the county inspector.
 
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Old 01-05-16, 09:33 AM
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I was able to trace some wiring inside a wall with a non-contact sensor - the one I used was a Sperry, with an adjustable dial. $15 at the local hardware store. With tweaking of the sensitivity I could trace pretty accurately through a plaster wall.

I would have to put a load on the wiring in order to find it however - ie: plug a lamp into an outlet and trace from there. Or another example - I couldn't trace the wiring from an overhead light to a switch until I flipped the switch and turned the light on.
 
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Old 01-05-16, 10:49 AM
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if I do it this way I will be running cable through studs with no nail guards. Is that ok as far as code is concerned?
Yes it's OK, keep your holes in the center of the stud. The cable doesn't need stapled inside the walls.

If you were to ever notch a stud for cable, that would require nail plates.
 
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Old 01-05-16, 02:39 PM
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The simplest and easiest solution by a wide margin is a "Wiremold" extension box , X feet of wiremold , and a Wiremold oulet box.
 
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