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Old 07-25-2010, 02:36 PM   #1
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Voltage Dropping from 120 V to 15 V


My line wire reads a consistent 120V. I have a wire run to the other side of the room that, when connected reads only around 15V at the far end. I'm not sure what the voltage reading used to but, but the fixtures on this circuit worked fine until a few days ago. The fixtures stopped working and I discovered the low voltage situation when I was tracing the problem.

Both wires are 12 GA, the wire I am loading onto the circuit is about 25 feet long. The wire in question runs through a damp crawl space.
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Old 07-25-2010, 03:02 PM   #2
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Are you reading these voltages with a digital multimeter?

Have you tried connecting an appliance, lamp, etc. to the line that reads 120V? How about the other line?
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Old 07-25-2010, 06:30 PM   #3
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Tried A Test Light


Quote:
Originally Posted by Proby View Post
Are you reading these voltages with a digital multimeter?
Yes. I am using a digital multimeter with alligator clips. It reads right at 120V anywhere else as expected.

Quote:
Originally Posted by Proby View Post
Have you tried connecting an appliance, lamp, etc. to the line that reads 120V? How about the other line?
I tried a test light. It didn't light at the far end. It did light when I connected it right to the line from the circuit panel. I suppose it is within the realm of possibility that I don't have the right wire, however I am quite confident this is not the case. I had labeled the wire in question and it only reads 15V when I have it hooked up (with wire nuts) to the line in from the subpanel. When I unhook it, the voltage reading is nil. -And there was obviously a problem before I unhooked everything as the outlet, etc. on that circuit had stopped working.
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Old 07-26-2010, 07:16 AM   #4
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You must have a loose connection somewhere.

What are voltage measurements
1. From hot to ground
2. From neutral to ground
3. From hot to neutral

With and without a 60 or greater watt light or other item plugged in and "turned on"?
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Old 07-26-2010, 10:48 AM   #5
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do you have a junctio in the crawl space?

my gues is corosion.
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Old 07-28-2010, 11:34 AM   #6
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Quote:
Originally Posted by sgall View Post
Yes. I am using a digital multimeter with alligator clips. It reads right at 120V anywhere else as expected.
Your 15V reading isn't real. It's a "phantom" voltage that shows up on your meter due to capacitive or inductive coupling between the wire you're measuring and other hot wires nearby. There's no need to understand that in detail, except to realize that very low voltage readings on digital meters are usually not to be believed. Try connecting a load (even just a night light bulb) at the same point you connect the meter, and the voltage will disappear.
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Old 07-28-2010, 11:50 AM   #7
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Originally Posted by mpoulton View Post
Your 15V reading isn't real. It's a "phantom" voltage
For a high input impedance DVM, 15v is awfully low for a phantom voltage. I'd expect this reading from a 1000 ohms per volt analog cheapie meter.
25' of Romex gives you 2500 pF of interconductor capacitance giving 1.1 Mohms reactance @ 60 Hz.

A 60 W incand. bulb has a cold resistance of ~20 ohms. If the voltage drops in half when loaded with this bulb the source impedance is 20 ohms, vs. a normal 0.1 ohms for this circuit.

This problem is pretty interesting.

Last edited by Yoyizit; 07-28-2010 at 12:12 PM.
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Old 07-28-2010, 02:26 PM   #8
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Do u have access to the crawl space ?
Anything else connected to the same line ?
Any junctions etc etc ?

I would definetly look at the physical shape of the wire to make sure there are no cuts, frays etc etc. If the line does have a connection point in between than it is most likely "pooched". May it be corrosion or loose connection etc etc. Definetly take an physical look at the line. Can u disconnect the line at either end? If so, do so and check for continuity...... I wouldn't count on the 15v reading from the meter....might be picking it up from an excess source or something.......I only say that b/c its happened to me before HOWEVER...I was NOT using the greatest of meters either......oopsy!

good luck and let us know

cheers!!
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Old 07-28-2010, 04:58 PM   #9
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Quote:
Originally Posted by Yoyizit View Post
For a high input impedance DVM, 15v is awfully low for a phantom voltage. I'd expect this reading from a 1000 ohms per volt analog cheapie meter.
25' of Romex gives you 2500 pF of interconductor capacitance giving 1.1 Mohms reactance @ 60 Hz.
That only matters if the voltage is resulting from parallel conductors in Romex. Unlikely, if there is a loose connection somewhere. Capacitance across the loose connection would be the source of the voltage.

Quote:
A 60 W incand. bulb has a cold resistance of ~20 ohms. If the voltage drops in half when loaded with this bulb the source impedance is 20 ohms, vs. a normal 0.1 ohms for this circuit.
I don't think he measured the voltage with a light bulb connected. Usually that makes a phantom voltage go away completely.
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Old 08-01-2010, 06:18 PM   #10
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I finally figured out the problem.

After finally deciding I would just have to run a new wire, I tore out a small section of drywall below the outlet on the far end. I immediately saw that the wire coming from the line under the crawl space did not go directly into the outlet box, like I thought. Instead it went out to an outdoor outlet box then right back in to the box. My immediate response, "Oh, BROTHER!" I went outside, pressed the Reset button on the outdoor GFCI outlet, hooked everything back up and all works perfectly. Basically it took me about 10 hours worth of work to press that stinkin' button.
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Old 08-01-2010, 08:01 PM   #11
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So it was a phantom voltage?

And if you had checked you would have found that neither the hot lead nor the neutral lead was connected to anything because the GFCI internal relay opened both leads [which kinda' makes me wonder where the phantom voltage came from]?

Last edited by Yoyizit; 08-01-2010 at 08:03 PM.
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Old 08-01-2010, 08:48 PM   #12
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I assume it was phantom voltage. I think the GFCI tripped due to moisture being in the outdoor box. (There aren't eves troughs on the addition yet.) Perhaps the moisture played a part in allowing some of the phantom voltage through. I did notice that the voltage kept dropping - perhaps as it dried out?
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Old 08-01-2010, 08:55 PM   #13
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Quote:
Originally Posted by sgall View Post
Perhaps the moisture played a part in allowing some of the phantom voltage through. I did notice that the voltage kept dropping - perhaps as it dried out?
I guess that's plausible.

In that case, with an incand. lamp connected the water would have evaporated more quickly, or maybe even boiled and so vapor would have come out of your GFCI.
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