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
Best Answer
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.