I have installed a pair of 3-way switches in an incandescent lighting application (lights are not installed). After installation I used my handy voltage detector and found there was "voltage" present in any switch configuration at the light rough-in. The grounds are terminated in each box and at the main panel.

Troubleshooting: 1) disconnected wiring from switches and checked continuity. Switches check out fine.

2) wires still disconnected from the traveller pair, but 120V applied to the common terminal. Switch operates as expected. In the off-position, 120V is on T1 (arbitrary name for one of the traveller terminals), and 0V is on T2. Switch to on-position, and 0V is on T1 and 120V on T2 (measurement from traveller terminal to ground).

3) connect travellers and line voltage to one switch, other end of travellers left open (capped each off).

In the switch off-position: Common Terminal to ground = 120V (great) T1 to ground = 120V (great) T2 to ground = 40V (opposite of great)

In the switch on-position: Common Terminal to ground = 120V (great) T1 to ground = 38V (not great) T2 to ground = 120V (great)

When switch #2 was in the circuit it's common terminal connected to the light rough-in was 40V.

What to do? There is no access to inspect the run of 14/3, as the Drywaller came in and did his thing. Would a pinch in the cable cause this "inductive coupling"? Any thoughts on this would help greatly. Thank you

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    Do you have a low-Z range on your voltmeter? – ThreePhaseEel Aug 11 '16 at 0:54
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    What ThreePhaseEel is referring to is that modern voltmeters can be so sensitive that they see capacitively coupled voltage that has no real "ooomph" behind it, and isn't really there. If you add a load (e.g. a lightbulb) and then measure the voltage, what you see will be real. – Daniel Griscom Aug 11 '16 at 1:26
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    Sounds like inductive coupling to me when both T1 & T2 were not energized you had similar voltage. You can lay a few feet of wire on a another connected wire and measure voltage the longer the wires are in close contact the higher the voltage. Also a heavy load on the connected wire will create a larger induced voltage. – Ed Beal Aug 11 '16 at 13:31
  • Thanks for the feedback guys! I appreciate it - I will check on this tonight with a connected load. An electrical engineer freaked me out when he offered his opinion and said the cable may be pinched and causing resistance in the wire; which I believe would become a potential fire hazard. Gord. – user58627 Aug 11 '16 at 19:47
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    Minor technical nitpick - 'inductive coupling' requires that a current be flowing through the 'primary' wire. 'capacitive coupling' on the other hand only requires that there be a voltage present on the 'primary' wire. – brhans Aug 11 '16 at 21:14

Are you by any chance using lighted switches? If so they are likely the source of the voltage you are seeing. The 3-way switch is a "single throw double pole" switch that has a resister and light (usually neon) between the poles. This allows a small amount of current (milliamps) to flow to the load even when the switch(s) are off in order to light the switch. 40 volts sounds about typical for an open load i.e. no light installed but there is not enough current to light the light. This is true for incandescent lights, LED lights could light dimly or flicker.

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