1

enter image description here

TLDR: The white wire is showing a current despite not being connected to power. Is that normal, and should I be concerned?

Long Version: In the picture is a white 14/3 wire running parallel and very close to a black 14/2 wire. They travel closely for about 8 feet to the crawl space below.

The white one used to be jumped off of the black wire in the box above this picture (but out of frame), but It is now wired only to a 3-way switch in that box. In the crawl space it is dead-ended in a junction box.

I was going to use it to operate a different light that hadn't been on a 3-way switch before by reconnecting it at that crawl-space junction box.

However, it is showing a current despite no known connection to a circuit. Is it possible that running so close to the live black wire makes it look like it has a current? And is it safe to use it as is, or should I crack into the wall more to separate them?

Improve this question
2
  • 1
    What are you using to check for current? A non-contact tester will read electric fields some distance away(3 to 6 inches) from wires/cables.
    – crip659
    Commented Jan 25, 2022 at 0:14
  • 1
    As @crip659 mentions, please provide details (picture perhaps) of what you are measuring and how. I doubt you are measuring current, and are you sure that what your device is showing comes from the white and not the nearby black?
    – P2000
    Commented Jan 25, 2022 at 5:51

2 Answers 2

Reset to default
0

Voltage and Current are 2 very different things. Voltage is like pressure in a water line, current is the flow rate. A tiny 1/4" at high pressure isn't going to flow a lot of water, a fire hydrant at relatively low pressure will flow A LOT!

Now, getting to your issue: You probably have an induced current , that's how transformers work. All you can do is put a load (like and old style incandescent bulb) on it and see if the power goes away. The non-contact testers or even the modern multi-meters are so sensitive they'll detect almost anything. If you don't feel safe working on the circuit, you'll have to turn off the breaker to the other cable as well.

Improve this answer
4
  • 1
    I agree the answer is yes, a hot conductor in parallel with another will induce a voltage, so this answer is correct , yup just like a transformer.
    – Ed Beal
    Commented Jan 25, 2022 at 2:08
  • @EdBeal Thanks for the comment. I wanted to jump into this before somebody blamed it on capacitance, vs. induction. Clearly you and I "get it"....wires aren't capacitors. Thanks again for your comment. +
    – George Anderson
    Commented Jan 25, 2022 at 2:52
  • 2
    @Trevor Note that you can't assume you have a relatively safe induced voltage. You need to prove that first. So as a minimum connect an incandescent bulb (not LED or CFL) with insulated clips or preferably turn off the breakers for both wires you're working with and use your tester to confirm all wires are dead before connecting a lamp. Then turn the breakers back on and check for voltage on your white wire.
    – Graham Nye
    Commented Jan 26, 2022 at 10:58
  • @EdBeal a transformer has windings on the primary side, forming a closed circuit from live to neutral. The OP has two wires (live and neutral) that are open, which do not form the primary side of a transformer. So it's not "just like a transformer". In my answer I provided several references to support my point (have a look). Perhaps you could cite a reference to better support yours?
    – P2000
    Commented Jan 27, 2022 at 3:00
-1

Is there really a voltage?

Is it possible that running so close to the live black wire makes it look like it has a current?

Assuming you mean that the wire is showing a voltage, then yes, that is very well possible especially if the wire is not connected at either end. It is not an indication of a problem.

And is it safe to use it as is, or should I crack into the wall more to separate them?

Yes you can use that wire but there are thermal limits as to how long bundles of parallel wires or cables can run close to each other. I don't think your case of two cables has a limit.

enter image description here

Capacitive and Inductive coupling

Some background on why there is a voltage even if nothing is connected:

A voltage can occur in a wire if

  • there is a nearby AC magnetic field caused by an improper pairing of live and neutral. Live and neutral wires should always be paired in the same conduit or same cable, and the neutral should carry all the return current from the live. If not, there will be time varying magnetic fields that may induce currents and voltages in nearby metal, such as nearby wires, or radiate an electromagnetic field and cause RF interference. This is akin to the transformer effect.

  • there are two wires running alongside eachother, close together and for some length. These pairs form a capacitor. Ineed a wire is not a capacitor, but closely spaced insulated pieces of metal (wire, metal plates) do have capacitance. The voltage (specifically the electric charge) in one will cause a voltage (charge) in the other with an electric field between the two.

This is often observed in 3-way switch circuits due to the 14/3 (or 12/3) cabling, or between relatively long runs of tightly spaced 14/2 (or 12/2 etc...).

For capacitive coupling to take place there does not have to be a current flowing. So these phantom voltages can originate in live circuit wires without active loads (breaker on but no loads), and they typically survive in other unconnected circuits or circuits with no or very small loads (perhaps a single LED light or wall wart will be exposed to 50ish V).

Is it a problem or hazard?

Capacitive coupling can occur in normal and legally wired circuits. Phantom voltage from capacitive coupling are generally not an indication of a problem.

So in your case, it is possible that the voltage in the black cable, even if the black has no loads using any power, is inducing a voltage in the white cable, even if the white is unconnected at both ends. In fact, connecting a high power load (kettle, incadescent bulb) in the white circuit, will make the phantom voltage go away, even though the load's circuit is effectively off.

You can also buy a "Stray Voltage Adapter" for your digital multi-meter.

enter image description here

This adapter is basically a safe way to load the circuit and confirm whether the voltage is supplied by a source might possibly hazardous amounts of voltage (e.g. a through a resistive short or strong coupling) or whether it's supplied by a weak source (e.g. short run of parallel wires)

enter image description here

Stray or ghost voltages occur from capacitive coupling between energized circuits and non en- ergized, non connected adjacent wiring. Because of this coupling effect and the multimeter’s high impedance, it’s not always pos- sible to determine if the circuit under test is energized or de-energized, and this creates confusion for the person performing the test.

Ref: https://www.fluke-direct.com/pdfs/cache/www.fluke-direct.com/tlk-225/manual/tlk-225-manual.pdf

For magnetic induction to take place there must be a current flowing in a wire (so there must be something drawing power) and its magnetic field must not be canceled by the same current in reverse direction in a paired wire. This special arrangement is quite rare, because the neutral -if wired correctly- always carries the same current as the live but in opposite direction. Therefore magnetic coupling is seldom the cause of phantom voltages.

Magnetic induction in wiring is often an indication of a wiring problem. Your situation does not suggest this is the case. You can use that wire for the purpose you indicate.

Physics and Math

This is a DIY site, not an electronics site, but for those who'd like a backgrounder on capacitive coupling between wires there's this:

enter image description here

And

Capacitive phantom voltage

Two parallel wires, insulated from each other but running side-by-side, effectively form the plates of a capacitor. If an alternating current (AC) is applied to one of the wires while the other wire is not connected, then AC voltage will tend to be capacitively coupled to the disconnected wire. If there are no other wires nearby this coupling will be nearly complete, and the voltage measured on the disconnected wire will be nearly the same as on the "live" wire. However, if there are other wires nearby (such as a ground wire in the same cable) then in effect a capacitor voltage divider may be created, and the voltage on the disconnected wire will measure somewhere between the "live" wire and the other wires.

While the voltage produced by capactive coupling may be startlingly high the amount of capacitance between the wires is typically quite low and incapable of supplying significant amounts of current

Ref: https://en-academic.com/dic.nsf/enwiki/4796365

Here's a schematic of the capacitive voltage divider, thanks to an earlier post by Pigrew in 2013:

enter image description here

It's that voltage divider, formed by capacitors, which brings the voltage to the 40V to 60V range, from 120V, depending on the co-bundled number of neutral and ground wires.

The specific count of wires affects the voltage divider ratio. The length of the parallel/adjacent run determines the "strength" of the voltage, i.e. whether it merely triggers a voltage detector or reading on a digital multi-meter, or whether it is a hazard to someone working on the exposed wire.

Images:

Improve this answer
13
  • 1
    @EdBeal if there's no current there's no induction, but we do see coupling in & between circuits that are off. You've never measured a phantom voltage in a hallway 14/3 with the lights off?
    – P2000
    Commented Jan 25, 2022 at 17:07
  • 1
    Do you realize that line lines of force cutting across a conductor create the the phantom voltage. the field going from 0 to a positive at 90 then back to zero at 180 then a maximum negative at 270 and back to 0 at 360 creates lines of force cutting through the other conductor INDUCES a voltage but who would want to get technical the amount measured will vary with how close and the field generated and the length of contact or parallel. Just like transformers that have output with zero current it is inductive coupling. The pulsating wave form it is why there can be voltage without flow.
    – Ed Beal
    Commented Jan 25, 2022 at 20:41
  • 1
    @GrahamNye Thanks for the question. Capacitors store power, often to backup electronic settings in case of a power outage or to "smooth out" a bridge rectifier to supply a clean DC supply to sensitive electronics. Wires can't be capacitors, the physics just aren't there. If they could, even if you turned off the main breaker to your house to work do electrical work, you could be shocked. Wires don't store power. Yes, there can be inductive coupling between parallel wires, but not capacitive.
    – George Anderson
    Commented Jan 25, 2022 at 23:14
  • 1
    @GeorgeAnderson "the physics just aren't there", we learn about calculating wire capacitance and capacitive coupling in advanced university physics or theory of electrical engineering classes, under electrostatics, field theory, and later electromagnetism (TE, TM and TEM) and transmission lines. I'm sure GrahamNye is familiar with these too.
    – P2000
    Commented Jan 25, 2022 at 23:55
  • 2
    @GeorgeAnderson You could not be more wrong. A capacitor is constructed from two conductors next to each other separated by an insulator. Two insulated wires running side by side for many feet fit this description. It won’t be a very useful capacitor but it is a capacitor. Please stop spouting misinformation.
    – nobody
    Commented Jan 27, 2022 at 1:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.