Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I bought two 25' lengths of 14/2 wire for installing a light.

When the black wire is connected to live (say, of an outlet), with ground and neutral wires not connected to anything, the ground wire reads 60V with respect to the outlet ground.

The black wire reads 122V to outlet ground as expected.

I imagined that there is a short between the black and ground wire in the romex, but the multimeter says it is an open loop. (If there was a short I expect it would read a full 122V though).

Any idea what is causing this?

Cheers Ray

share|improve this question
3  
Why are you taking these odd readings, like between the grounds with the power on??? What prompted you to do this? ..... I'd all but guaranty you there is NOT any "short" inside the NM cable. – Speedy Petey Jan 17 at 18:45
    
@SpeedyPetey - Non-professionals that know a bit of electricity think that they are being safer by measuring all sorts of things, even as they create a safety hazard by, say, hooking up a hot wire without a ground or neutral. I do a lot of low-voltage electronics work, and I measure lots of things in circuits that would make no sense (or is even dangerous) to measure in a home electrical system. – Johnny Jan 17 at 19:42
2  
Be aware that digital voltmeters have very high input impedances. This means it takes almost no current for them to display voltages, so that any sort of coupling between wires can result in their showing a voltage even if there's no real "ooomph" behind that voltage. So, your 60V may be a spurious reading. – Daniel Griscom Jan 18 at 2:21
    
the rough narrative of why i needed to measure ground wire to ground is in my comment/reply to Ecnerwal. The idea was not to "...measure all sorts of things" but spurred by the high voltage shown by a regular voltage tester on the part of the ground-wired fixture which i expected would be dangerous. As Ecnerwal, Wolf and Griscom have explained this is capacitive without any current, and was corrected when I grounded the other end which was left open by oversight. thanks everybody. – Ray Jan 18 at 4:26

It's not connected to anything. There is non-zero capacitance between the lines in the cable, and your meter has a high input impedance. Capacitive coupling gives you a phantom voltage that will disappear as soon as anything is connected.

Connect the ground and neutral correctly and the non-problem will go away. Then there's the question of WHY you were connecting the hot wire without connecting the ground and neutral...

share|improve this answer
    
Thank you for precise answer. Checked out exactly as you suggested. Initially I connected the fixture with live and neutral, but overlooked grounding the ground wire. when i checked the ground wire at the fixture for safety it was hot - using the tester. I could not explain how it could be hot. It seemed to range between 60 to 100V and I could not explain it even when I noticed the ground wire wasn't connected to ground at other end. Thought it was some kind of internal short in the fixture or cable. Thanks! – Ray Jan 17 at 22:51

This is AC, so current can flow through a capacitor. Some capacitance in the wire would explain it, and a voltmeter never tells the whole tale. To delve deeper into the question, measure how much current wants to flow between your loose ground wire and your home's ground. Hopefully this will be too small to measure.

Next, what is the resistance between your loose ground and the other wires in the NM-B cable? This resistance should be megaohms (not infinity), outside the practical range of your common DVM. In an industrial setting, one might use a megaohmmeter or "megger", which applies up to 1000v. This not only gives more accurate megaohm readings, but tests insulation failure that may only occur at high voltage.

share|improve this answer
    
Thanks Wolf for the insight. Great diagnostic to check the current flow between loose ground and home's ground. I did not imagine the ground wire could be capacitively at 100V without being connected to anything. First instinct was to rage at China. – Ray Jan 18 at 0:38

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.