I have 3 plastic pipes in my backyard and one (two?) of them have 110V lines in them. All wires - hot, neutral and potentially ground are inside the pipe so I can't just use a clamp meter / current transformer. Image below, you can see two white PVC pipes and one grey PVC pipe.

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Without destroying the outer PVC pipes, how can I determine which plastic pipes are carrying the real 110V circuits and which ones are just empty? I can turn on the hot tub at the end of the circuit to start/stop electricity. I also have basic tools like voltmeters and a clamp meter.

  • If you can borrow a portable oscilloscope you might be able to use the probe to see which pipe gives you the most 60Hz (or 50Hz) pickup when you bring the probe close to the pipe.
    – John D
    Commented Sep 10, 2017 at 19:35
  • or try the same thing with a DMM lead wires closer to one pipe than the other with the other probe stuck on the ground. or grab the pipe with your hand and other hand to probe and other probe to earth on Vac mode. That will tell the difference. This ought to be metal conduit and was not done to our national code. Commented Sep 10, 2017 at 19:53
  • 1
    What you are looking for is a non-contact voltage tester. Use that keyword and you'll find what you need.
    – user64238
    Commented Sep 10, 2017 at 19:55
  • Can you see the wires going into the conduit on one or both ends? The wires should be in the gray electrical conduit rather than the white water pipe. Does water flow in the pipes that don't have wires? If so, those ones should be cold when water is flowing.
    – Mark
    Commented Sep 10, 2017 at 22:45
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    The gray conduit is the only one that is actually meant to carry electrical wires. That doesn't necessarily mean that somebody didn't run wires in the other ones, though. Commented Sep 11, 2017 at 5:50

4 Answers 4


I'd go full retard on this one, and have an assistant blow a hairdryer into one of the pipes from the open end, then put your hand on it down the hole and check which one gets hot. It may take a while to warm up, though.

  • Ha ha. When I wanted to identify the metal heating pipes in my home, the best advice was: Have someone else bang the known ends near the gas heater with a small hammer and listen at the unknown ends. Worked perfectly. OT, but proves that sometimes, non-electric (or thermal, for that matter) tests work best.
    – zebonaut
    Commented Sep 10, 2017 at 20:49
  • Yeah, well here they're buried in dirt so I don't think banging would work, but sounding an airhorn into the open end of the pipe would be a nice substitute...
    – bobflux
    Commented Sep 10, 2017 at 20:54
  • I actually like this idea but have access to only the hot tub end of the PVC piping ... the other end is inside the wall, behind the main circuit panel (!). Commented Sep 10, 2017 at 22:05

Use the electric field, imbalanced, into your multimeter. Make a metal pickup, wrap halfway around (so you are likely to have imbalanced Efields, even if +-120 is inside.

Signal strength? Assume 1cm distance between wire and external metal pickup. Assume pickup is 4cm (wrapped halfway around) and 10cm long. Assume air is your dielectric. We'll use the parallel-plate model of capacitance.

C = Eo * Er * Area/Distance

C = 9e-12 * 1(air) * 4cm * 10cm/1cm

C = 9e-12Farad/meter * 40 cm * (1meter/100cm)

C = 9e-12 * 40/100 = 9e-12 * 0.4 ~~ 4pF

We know Q = C * V, and differentiated with constant C, we get I = C * dV/dT

dV/dt = 377 radian/second[60Hz] * 120VACrms * 1.414 ~~~ 60,000 volts/second

I = C * dV/dT = 4e-12 * 60,000 = 240 e-12+3 = 240 e-9 = 0.24 uA.

Across a 10Meg Ohm DVM Rin, that 0.24uA produces 2.4 volts AC.

So, summary use a 4cm by 10cm piece of metal, to which you get a clean metal-contact (copper foil is good, whereas aluminum foil instantly oxides) with one wire of you meter.

Now, you need "ground" of the other meter lead.

  • Yeah, I didn't read the fancy math, but a piece of tinfoil wrapped around a multimeter probe, with the multimeter on AC and the other probe stuck into the ground, should work. It's a ghetto version of the "live wire detector"...
    – bobflux
    Commented Sep 10, 2017 at 20:35

Use the clamp meter, and an artificial return

Since you posted this on ee.se, I'm assuming you know a little about electricity. This requires some insulated wire and a little daring.

The goal here is to take power down the conduit, but return it via another route, so the magnetic fields no longer cancel. This is super bad in any other context.

De-energize the entire house while setting this up. Find the nearest point to each end of the conduit where you can access neutral and hot. On the house side location, find the neutral. (if it's on the LOAD side of a GFCI, tap there). String a wire from there to wherever, on the other end of the conduit, you can tap "hot". Cut the plug off a 2-prong extension cord, retaining the socket, and wire the neutral (ribbed end) to temporary neutral wire from the house. Wire the hot (non-ribbed) to a hot terminal fed by the conduit. For any load plugged into this extension cord, power will go in a huge loop. This is Very Bad normally, because it makes anything inside the loop the core of a transformer - and will cause eddy current heating and very high EMFs.

Clear enough roots you can get the clamp meter around all the pipes. Plug something double-insulated into the extension cord, and power up the house. Have an associate turn on the double-insulated device (tool etc.) Now current is flowing. Clamp the conduits until you get a hit. Don't do this anywhere near water, by the way. If that ground wire comes loose, it will be energized at 120 or 230 volts. This is much less of a concern if it's fed by a GFCI.

You might even install a GFCI on the house side for this test and then just leave it there.

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    That should work but if I did not have one I would rent a greenlee cs8000. Put a signal on the wire and it will show even through metal conduit which pipe has the live circuit.
    – Ed Beal
    Commented Sep 10, 2017 at 22:39

You can use a clamp meter / current transformer if you have any such size, Or you can wrap some wire around the pipe and measure the induced voltage with DMM when you switch on/off the load.

You will need a rod placed into the soil and you may wet the soil with salt water for better conduction. Then you connect the load from line to the earth from the rod bypassing the neutral or earth from the pipe. In such way, the current will flow just in one direction, no back flow through the pipe, so the magnetic field won't be cancelled.

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