3-Wire Circuit Voltages

altus

I have a question about unexpected voltages with a 3-wire circuit. Here is the scenario:

50 ft. of 12-3 w/ ground NM cable without any devices, connected only at the panel. When I energize both hots I get the expected voltages: 125V from each hot to neutral (and ground) and 250V across the two hots (my meter may be reading high). When I turn off one of the breakers I get these readings (with black hot energized, red hot not energized):

Black to Neutral: 125V
Black to Ground: 125V
Black to Red: 55V
Red to Neutral: 40V
Red to Ground: 40V

I was surprised to see a voltage involving the non-energized red. If I reverse the energized and not energized hots I get the mirror image of these (red at 125V, black at 40V, etc.)

I thought it might be something with the cable so I substituted another length of cable, about the same length, and got the same results. Then I substituted a two-foot length of cable and the Red to Neutral voltage dropped below 5V.

The only thing I can think of to explain it is that there is an electro-magnetically induced voltage in the cable caused by the slow winding of the four conducters in the cable; as if it was a transformer coil stretched out very long.

Is this a normal condition or is something amiss?

P.S. By the way, I intend to switch out the two single pole breakers with a double pole, I just didn't have one on hand today.

Celtic

It's an induced voltage....


Here are some pic.s from a very scientific test I performed a short time ago.

Testers used:
Ideal Circuit Tracer (Just the one piece)
Ideal Vol-Con (about 21 years old)
Fluke T2
Fluke 87
AWS Snap 8
GreenLee GT-11

....shown also is one partial roll of 14/3 NM



I stripped back both ends of the NM and jammed the black and white wires into a working outlet ~ the red and EGC are not connected to anything...using the Ideal Tracer to show that the recept is HOT:







Next...
We did NOT detect any voltage with the Vol-Con:


Next....
We sensed a voltage using the Fluke T2:


Next....
We sensed a voltage using the Fluke 87:


Next...
We did NOT detect any voltage with the Snap 8:


Finally....
We sensed a voltage on the red wire...on the end of the NM that was inserted into the recept. using the GreenLee GT-11:



The fact that the NM is in a coil has little to do with the results shown.
The test was performed when I found a "voltage" on the red wire of a smoke alarm circuit - with NO alarms installed (why I was doing all this is another story ).

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altus

Thanks, Celtic. After posting I came across this link about phantom voltage that mdshrunk posted in another thread:

nema.org/stds/eng-bulletins/upload/Bulletin-88.pdf

I have no idea if I have a low-impedance meter, but suspect that I don't since it's an ancient analog. Make sense that the longer cable would affect the impedance. Meter impedance might explain why your different testers gave different results.

I guess the next step will be to put a load (like a clear light bulb) across the unexpected voltage and see if there is any current to speak of.

Nothing new under the sun...

Celtic

I used a bunch of meters there...what meter did you have?


I doubt it......here is a link to a similar thread in another forum:
"Hot" wire...

You'll see some familiar names there



I dunno about that...


(Quite possibly the greatest book ever illustrated )

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frenchsparky

Celtic :

that is very good expaination about the induction voltage.

that why i always carry at least min of 2 testers with me [ normally i have about 12 diffrent testers with me on hand it varies depending what i need to do ]

and for other as well always test all the meters with knowen power source each time before you use it.

Merci, Marc


P.S. sorry for other day i gave ya hard time with the pie in french lol sorry about that

altus

Celtic,

I don't have any of the meters you used. Mine is a 20+- year-old VOM from Radio Shack. Not a real electrician's tool! But then I'm not a real electrician - just stayed at a Holiday Inn Express last night!

Your links were insightful. The comment by one participant about capacitive coupling meshes with what I read in NEMA Bulletin No. 88.

Any hypothesis on why you got different readings from different meters? I don't dispute your inclination that it's induced voltage. That was my first thought - what with the wires wrapped so tightly in the cable. It's just that the cable length affecting the voltage reading throws me. Your differing meter readings lead me to believe there's something more at work here than just inductance, the meters seem to be affecting the observation - very Heisenberg-like.

Since the meter is the only load at the moment, the intention with the light bulb load was to see how much current was really available there, not just what the voltage drop across the meter load was.

I should say that I get the same readings at the panel terminals as I get at the end of the 50 ft. cable.

In any case, is this something to be considered normal with 3-wire circuits using NM cable and not a problem? My voltages were normal between all conductors with both breakers on. So if I make sure I have a double-pole breaker instead of two singles to cut everything off I'll be safe?

Finally, what can one say about Calvin and Hobbes? I stand corrected, there was something new under the sun. But alas, not recently. Had a great one to share but I guess I'm too new here to get a link embedded and I don't know how to embed a jpg into this text.