Hi folks
Still completing wiring of my new construction home. I have installed s verbal 3 and 4 way light circuits. All function and test correct. Then today as I installed another 3 way-after connecting both switches I went to the conductors where the light will eventually connect and tested the voltage as my wife toggled either switch. On one throw I would get 124v which indicated light on and then when the switch was toggled in the other direction thus shutting off the light-the voltage read 20v...instead of zero as I expected. This was the same with either switch. I spent 2 hours checking and re-checking my wires and switch terminal connections. I even changed out the switches with another new pair. Got the same results. I am stumped! I also checked the grounds and neutrals and hot at the fuse panel. If anybody has advice on what's happening here..I really appreciate it.. thanks. Jim

 

Are you using a digital meter an an old fashioned analog meter to check voltage? For simply house wiring I find analog gauge much more useful as it does not display phantom voltages like a digital can.

 

Thanks. I am using a Fluke digital. But out of the 5 separate 3 way circuits in my house...this is the only one displaying [email protected] when the light is off. It's driving me crazy no understanding why.
If you have an ideas...I'm all ears. Jim

 

Just connect a small light bulb first and measure the voltage across it. I suspect you will now see close to zero when the switch is off. Stray coupling is very dependent on wire routing and proximity to other live wires, so it would not be unusual to see phantom readings on some circuits and not others.

 

Thanks for your help. I appreciate it.
Jim

 

With 3 way circuit, one of the wire will be floating (not connected to anything) depending on the switch position.
There is electromagnetic field around a wire carrying electric current. This electromagnetic field will induce current to a wire running parallel to it. That is why you get that 20V reading.

But this field is very weak, therefor current is also weak and a small load will bring that voltage back down to 0.

Analog meters works by running electric current through a coil, thus putting some loads. This will make induced voltage unreadable by analog meters.
Digital multi-meters have very high impedance in voltmeter mode (ideal voltmeter will have infinite impedance), that is why you are able to read voltage.

Basically, digital meter are too good.

You can put 1Mohms resistor across leads of the digital meter to keep the meter from reading induced voltage. Or simply ignore voltages that don't make sense.

 

Or buy a cheap ($8-$15) analog multimeter.

 

Yeah. That is a option too. I forgot to mention.

Only thing with analog meter is if you drop them, the reading can be off quiet a bit because it will damage coil spring on the needle.
I had 2 analog meters broke like that and never bought another because I'm fine with digital meters I already own. But for the price, you can just buy new one.

Also, with analog meters, being in a wrong range can really damage them. Make sure to start from highest range.

 

Thank you all for your very helpful insight. Especially lambition for explaining the parallel electromagnet field behaviors. Since the lights and switches work exactly as designed.....it sounds like I can quit getting high BP over this one. Jim