3-way switch - voltage on 'off' wire

Old 11-29-02, 06:41 PM
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Angry 3-way switch - voltage on 'off' wire

Is it normal to observe some voltage (25-50 volts) when a three way switch connection is off (perhaps as a result of some electromagnetic or some-other phenomenon)?

Or is the presence of such a voltage an indication of some type of short or other problem in my wiring that needs to be fixed?

Here's more detail on my problem in case it's needed or helpful.

I went to install a new light fixture on a 3-way circuit and when I went to make voltage checks (initially just to verify that the fixture was powered down), I was surprised at what I measured. I expected to measure:
120 Volts - w/ both switches up or
both switches down
0 Volts - w/ one up and one down

Instead, I measured:
120 Volts - w/ both up
45 volts - w/ two of the other
up/down combinations
25 volts w/ the third combination
(I actually don't remember for sure which switch combinations pertain to which voltages, but I don't think the exact switch settings are too important).
Since switches are cheap and the old switches didn't fit the new paint color in the room anyways, I decided to install new switches and hope it would fix the problem.

So I could check things out along the way, I installed the first switch to hot end of the circuit. I first hooked up the hot wire from the panel to the appropriate terminal. I then turned on the power and checked the voltage at each of the switch output terminals. As expected, the voltage measured 120 V at one, and 0 Vat the other. And these both switched, as expected, when I flipped the switch.

Satisfied that the switch worked properly, I then hooked up the wires to the output terminals. Once hooked up, I then measured voltages at the ends of these wires at the 2nd box (the ends that were to conned to the 2nd switch). I measured 120 V for one of these wires, but instead of measuring 0 V for the other wire, I measured 25 V. Flipping the switch merely switched these measured voltages between the different wires, but the same values (120 V and 25 V) were observed.

Before tearing walls out rerouting wire, etc., I need to confirm that this really is a wiring problem and not a typical characteristic of 3-way circuits.

One more interesting observation. I did find a junction box between the two switch boxes. When I disconnected the wires here, I measure 15 Volts for the 'Off' wire.
This is why I thought there might be some type of Electromagnetic effect which would show greater effect the more wire there was. I'm reaching for explanations of why I'd pick up 15 Volts in part of the circuit and 25 in the entire circuit between the switch boxes.

Any insights you can provide would be greatly appreciated. I hope to hear from you soon?
Old 11-29-02, 07:09 PM
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Are you using a digital multimeter for your voltage tests?
Old 11-30-02, 09:14 AM
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Yes, I'm using a digital voltmeter (multimeter).
Old 11-30-02, 09:52 AM
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Phantom voltages

A digital multimeter will measure voltage without loading the circuit under test. In electronic work were the available current is often small that is an asset. In electric work however it is a better idea to use a solenoid tester of the "Wigington" type. The Square D "Wiggy" is the simplest and usually least expensive version of this type of tester. The virtue of solenoid testers is that they place a small load on the circuit under test. On 120 volt circuits they will draw approximately seven milliamperes of current which coincidentally is just enough to trip a Ground Fault Circuit Interrupter. This prevents the tester from showing voltages that are merely induced into adjacent conductors of the circuit by capacitive effect.

Last edited by hornetd; 11-30-02 at 06:07 PM.
Old 11-30-02, 03:21 PM
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Thank you very much for your very helpful response. I had actually already convinced myself that the observed 'phantom voltage' was not an issue by bypassing the wall wiring and connecting my roll of romex cable to switch and observing that sure enough, the phantom voltage was observed again (this time even higher.....approx. 105 Volts).

Convinced that this was a normal observation, I then proceeded to finish the job and everything works fine. I just wish I hadn't wasted hours trying to figure out what could be wrong with my circuit. Oh well.

Thanks again for the tip. I'll definitely remember it next time I do any wiring.

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