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Old 02-08-2012, 08:42 PM   #1
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Hot green wire


I'm replacing a lighting fixture in my home. There are 3 wires coming out of the conduit: a white, a black, and a green. I assume the white should be the common, the black should be hot and the green should be a ground wire. However, both the black and the green are hot: both show 120V with a voltmeter. The white shows as 0 which is what I expected.
1) Am I correct that there is something wrong?
2) What are my next steps to diagnosis/fix the problem?
Thanks very much.

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Old 02-08-2012, 08:47 PM   #2
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Hot green wire


Between what two point are you checking for voltage? Black to green? Green to white etc.

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Old 02-08-2012, 08:53 PM   #3
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Hot green wire


In each case I was measuring the voltage with a voltmeter between the wire (black, white, or green) and the common (i.e. non hot) connection in a nearby outlet. Additionally I have a probe type Klein voltage tester which shows the same thing: i.e. it lights up showing that the black and green wires are hot (while the white is not).

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Old 02-08-2012, 09:02 PM   #4
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Hot green wire


So far there does appear to be a problem somewhere. Ordinarily black to white measures 120 volts, black to green measures 120 volts, and white to green measures 0 volts. Black to the common reference* in the nearby outlet should measures 120 volts, white to reference measures 0 votls and green to reference measures 0 volts.

We next want to find out why the green wire "is hot". You will need to do some tracing to find out how the wiring is routed from the light fixture box down to the panel namely what other outlet boxes it goes through and in what order.

* You and I are jumping to the conclusion that this common in the nearby outlet is equal to neutral (bonded to the neutral bus) in the breaker panel. If you feel uneasy about jumping to conclusions, a surer reference is had by stringing a wire from the neutral bus (terminal strip with white wires) in the panel up the stairs and across the room to where you are working (under the light fixture) and measuring voltage from this to the black wire etc.
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Old 02-08-2012, 09:02 PM   #5
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Hot green wire


Your assumptions are correct. The green should only carry current in the event of a fault until the breaker clears the fault.

Something is not kosher somewhere.
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Old 02-08-2012, 09:24 PM   #6
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Hot green wire


Thanks. What do i do to trace the green wire? What should i be looking for?
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Old 02-09-2012, 03:13 PM   #7
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If you can, it may be safer to use a cold water pipe as a ground reference so you don't work in the panel any more than necessary.

At 120v it's not too likely that it is a phantom voltage but with more than one fault present it is possible.

If the green wire cannot light a 100W incand. bulb to full brightness the wire may go to 120v through a load of some kind.

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Old 02-09-2012, 03:59 PM   #8
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Hot green wire


Since only 3 wires show up at the light fixture they likely go straight to the switch. I'd take a look in that switch box but turn the breaker off first and while it is off recheck your voltages.

Be careful with the power on testing as a hot ground can be very dangerous, any metal that is part of the fault path can be at line voltage ... so focus and don't touch any metal till you figure this out.
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Old 02-09-2012, 05:48 PM   #9
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Hot green wire


Using the water pipe as your reference also requires jumping to conclusions (or verifying that there is a fat ground wire, usually #6 copper as a grounding electode conductor from the pipe to the panel neutral bus).

Having a small incandescent bulb (7 watts such as a night light) across the two meter probes is enough to dissipate phantom voltage and cause the meter to read the real voltage across those two places.
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Old 02-10-2012, 04:06 PM   #10
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Quote:
Originally Posted by AllanJ View Post
Having a small incandescent bulb (7 watts such as a night light) across the two meter probes is enough to dissipate phantom voltage and cause the meter to read the real voltage across those two places.
Agreed, but 100W or more can quash phantoms and check for resistance in the line.
And a 1 kw load and a DVM can do the above plus check for bad connections or bad wirenut installs.

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