In the basement I have 19 recessed lights connected to 8 3-Way switches. There were dimmers on the 4 switches located in the main 4-gang box, and they were making a lot of buzzing noise. Not only that, the lights (CFLs) would flicker quite a bit and burn out a lot more quickly than they should. So I decided to replace the dimmer switches with standard 3 way switches. The new switches are working properly. However, I noticed some strange voltages while I was checking things out.
There are 2 circuits feeding the 4-gang box with, as far as I can tell, one 14-3 wire which goes to the panel. The red and black wires are the "hots" from the 2 circuit breakers at the panel and the white wire is apparently acting as the neutral for both circuits (I know that's a no-no, but I'm not sure that's related to the 50V). The ground is tied to the other grounds. The red wire is feeding the power to 2 of the switches, and the black wire is feeding the other 2 switches. As far as I can tell by sticking my head above the drop ceiling, the white wire in the 14-3 is the only neutral going back to the panel and there are no breaks or junction boxes in between.
To rule out the possibility of something backfeeding voltage into the red, black and white wires, I disconnected them inside of the 4-gang box. At the panel, I turned off the breaker for the red wire and still read 50V between the red and ground and 120V between the black and ground. If I turn off the black breaker and turn on the red breaker, I'll read the 50V between the black and ground and 120V between the red and ground. If I turn on both breakers, both read 120V to ground. If I turn off both breakers, I read 0.0V for black and red.
So, theoretically, all I have is a 3 conductor + ground wire going out to a box with the ends completely disconnected, yet I'm reading 50V between one of the wires and ground if the other circuit is powered. The only thing I can think of is that the red and black wires must be touching somewhere. I tried a continuity test between the black and red wires, and found no connection.
I traced the wire from the panel to the 4-gang box and couldn't see any obvious kinks or cuts, etc. Is there some electrical phenomenon which is causing 50V to appear on the other wire? Was this mystery voltage causing the CFLs to flicker? Is there anything else I should check out?
Best Answer
What you are describing is called a Multi-wire Branch Circuit, and it is completely legal (given that it's done properly).
The first problem, is that it doesn't should like the breakers feeding this circuit are handle tied together. It should be feed by a double pole breaker, or the two breakers handles should be tied together. You should not be able to shut off only half the circuit (red only or black only).
As for the voltage. I'd suspect it's phantom voltage (inductive coupling), and so should not cause any problems (especially if the breakers were properly tied together).
If the breaker for the "red" conductor is off, and current is flowing on the "black" conductor. Because the conductors are parallel, and in close proximity, a voltage may be induced on the "red" wire. There should be no current, however. So if a load was introduced, the voltage would drop to 0.
You can determine if it really is phantom voltage, simply by using a low impedance voltmeter. The low impedance meter will read 0 volts, while a high impedance meter will read the phantom voltage. An analog meter is one example of a low impedance meter, though some digital multimeters are able to operate in both modes.