I have three sets of 3-12 underground wire running from the house to the garage. It is not connected at this time to either house fuse box/electrical or garage (basically its buried with approx 6-8 feet of wire coming out of ground for eventual hook up).

Since they are unconnected dead wires, I’m looking for a tool that could generate low voltage power to one end of each wire so that I can then check the opposite end and determine which wire is which.

Any ideas would be appreciated.


7 Answers 7


I would go for the tone generator, because I already have one anyway and use it for business, so it's paid for. But if I didn't have one and did have a multimeter (everyone should have a multimeter), then I would actually test as follows, knowing that there are 12/3 cables rather than random individual wires:

  • Connect Black/White/Red together on the end of one cable. Mark this cable 1.
  • Go to the other end. Test until you find the one where black/white/red show continuity. Mark this cable 1.
  • While you are at that end, connect black/red/white of another cable together. Mark this cable 2. Mark the other cable, cable 3.
  • Go to the first end. Test until you find the one where black/white/red show continuity. Mark this cable 2. Mark the last cable, cable 3.

If you have some resistors around of known values, you could do everything in one batch, but with only 3 cables, this is about as easy as it gets. Actually, thank you @cube for a way to do essentially that, by treating different combinations of wires in each cable differently, which allows you to differentiate 3 cables at one time without any resistors. This could actually be extended up to decoding as many as 5 cables at a time:

  1. Black/Red/White
  2. Black/Red
  3. Black/White
  4. Red/White
  5. None

Extending to more than 5 would actually become 4-at-a-time because you wouldn't know the difference between any of the "None" cables.

  • 2
    This is even faster than a tone generator. You only really need a tone generator when there are considerably more cables. It doesn't save time on a three-cable job like this. I'd just use a multimeter, personally.
    – J...
    Dec 28, 2020 at 10:30
  • 2
    I'd suggest using the multimeter to double-check for current (against separate ground) before binding the wires together. It's all too easy to think you've disconnected a cable on both ends, only to later realize you disconnected 2 different cables. Dec 28, 2020 at 20:03
  • 1
    In theory, you could extend this to far more cables by allowing connections between the cables. For example, if you connect the red lead of one cable to the black lead of another, you could identify both on the other side. Or you form a chain of all cables, where black of each cable is connected to red of the next, which will identify any number of cables. But it's really just theory, in practice I'd be afraid of making mistakes and arriving at wrong conclusions.
    – wrtlprnft
    Dec 29, 2020 at 19:37

Kind of improvement on the continuity checking answers:

On one side:

  • Connect all three strands of cable 1 together
  • Connect red and black of cable 2 together, leave the third unconnected
  • Leave all three strands from cable 3 unconnected.

On the other side measure for continuity between black and all other colors on each cable.

  • if it tests as connected to two, other colors, you have cable 1
  • if it tests as connected only to red, you have cable 2
  • if it doesn't test as connected to any other, you have cable 3

Needs only a single trip (+ possibly one more to undo the connected cables).

  • 1
    As I commented on manassehkatz's Answer, the OP should double-check for current before connecting the cables together. Dec 28, 2020 at 20:06

You're not allowed to run multiple circuits to an outbuilding, unless those circuits have different voltages (120V vs 240V vs 120/240 vs MWBC) or specialty use (e.g. one of them is a light that is switched from the house).

Since you're only going to use one of them anyway, it's a simple matter of picking a cable at the garage end and just using it.

To identify it, plug or wire in a heater or incandescent light load (which is a big resistor). Then at the house end, check between hot and neutral with an ohmmeter. The defunct wires will read infinity (or 0 if you shorted them), and the live circuit will read a resistance in the 10-1000 ohm range.

If you can find a basis to use another one (nobody said the switched light line CAN'T have receptacles on it)... then once the first circuit is complete, start in with the second.

  • 1
    Given the three wires are already installed, and presumably have been used in the past, would OP's property have had multiple voltages to the outbuilding? Or the lights were remote-switched from the home? Or would it simply be a janky old-time install?
    – Criggie
    Dec 28, 2020 at 9:07
  • 1
    @Criggie my guess is the previous installer did not know about the rule. If there were subtle signs of amateur work (NM cable outdoors, improper stub-ups, too shallow depth, etc.) then I'd say definitely that. Or it could be that the original installer quite wisely threw down fat conduit and another actor put in multiple cables. Dec 28, 2020 at 19:24

I use tone generators for that kind of thing. The tone generator clips onto a single conductor or two conductors using aligator clips (there are other attachment types like RJ45). The small fat pencil like detector then can be used at a fair distance from the conductors (4-6" away) as they pass through walls or come out into junction boxes. You can detect through drywall and through the wire sheathing.

The cheaper ones work fine ~$25.

Example at Amazon.

  • 2
    This answer would be more useful if you explain how and why you "use tone generators for that kind of thing"
    – Alaska Man
    Dec 27, 2020 at 20:07

While a tone generator will work, it is a tool you are likely to use once. A multimeter can be used for a wide variety home tasks. An inexpensive multi-meter and a spool of small gauge wire (@ about 10 cents a foot) will do the trick providing you are not dealing with hundreds of feet between ends. Spool out the small gauge wire so you can reach both ends of your unidentified wires. Attach your small gauge wire to one of your unknown wires. Go to the other end of your wires. Set you meter to the ohms setting. The needle should read zero or "OL" if your meter is digital. Attach one lead to your small gauge wire, then touch the other lead to each of the unknown wires You are looking for any movement of the needle or a value other than "OL". Mark both ends of that wire and move the small gauge wire to another unknown wire. Go to the end and repeat the process until all the wires are identified.


i would just supply current through all one by one and check with the screwdriver tester

  • 1
    At first read, I though you were suggesting to test the current by bridging the wires with a screwdriver. You might want to add a link or other reference to the type of tester you're talking about. Dec 28, 2020 at 20:09
  • 1
    This requires connecting 120V AC. That needs to be handled very carefully. Toner or multimeter is much safer Dec 28, 2020 at 20:17
  • 1
    By pretending every question asker is a rank novice (unless really obvious otherwise) and answer accordingly, but without being patronizing, is a great way to provide really good answers. Assuming every user has your level of knowledge can lead people to potentially catastrophic consequences
    – FreeMan
    Dec 28, 2020 at 20:55
  • STANLEY 66-120 178mm/7.5'' Spark Detecting 2-in-1 Screwdriver 5mm, 100-500V AC amazon.in/dp/B00LJYGHPI/ref=cm_sw_r_cp_apa_i_6dR6FbNNYEY62 Dec 29, 2020 at 4:33
  • That screwdriver, like a typical non-contact tester, works only for mains-level voltage (in this case it says 100+). Which likely means connecting 120V to an unknown, open, cable. Not a good idea, unless you are trying out for the Darwin awards. Dec 30, 2020 at 20:09

I'd connect a battery to two wires on one end and measure with a multimeter on the other, it should show the correct voltage for the battery only if you test the right two wires.

Check for shorts between wires first, with the continuity testing tool in the multimeter.

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