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  1. #1
    Eric Laney's Avatar
    Eric Laney Guest

    Default strange voltage readings

    I wired in a light above a vanity in my bathroom. Ran basic 12/2 romex from the breaker to the fixture box in the wall, then a direct run from there to the light switch (standard one way switch). After hooking up one wire of the switch loop (black) to the romex coming from the breaker I decided to check for shorts in the switch loop. Using a sniffer I found voltage at the other wire (white with black tape), which was not hooked up at either end. I then measured 44 volts on the same white (with black tape) wire. To me this meant I had a short somewhere in the switch loop romex. I killed power and checked for any continuity between white and black of the switch loop and found none. I then checked the switch itself and found no continuity in the off position. Last I intalled a fresh piece of Romex to the vanity fixture, hooking up only the black wire. I once again had 44 volts on the unhooked white wire as well as chirping at the voltage sniffer. Again no continuity or shorts in the Romex. Is there always some voltage bleed into the white wire of Romex? I wound up installing everything and it all works fine, no stray voltage at the light fixture itself.

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  2. #2
    James Duffin's Avatar
    James Duffin Guest

    Default Re: strange voltage readings

    I bet you had light bulbs installed in the fixture when you took your voltage reading. If you remove the bulb you would not have had a voltage reading on the white wire. The reason you had a 44 volt reading on the disconnected neutral is because you were dropping some voltage across the bulb. Once you connect the neutral it is then grounded and you will not have a voltage reading on the white wire.


  3. #3
    Eric Laney's Avatar
    Eric Laney Guest

    Default Re: strange voltage readings

    The bulbs were not installed yet, nor was the fixture. Just had roughed in Romex coming out of the box.


  4. #4
    Join Date
    Mar 2007
    Location
    Dallas, Texas
    Posts
    4,112

    Default Re: strange voltage readings

    I'm with Fritz on this, induced voltage. This is the same principle that transformers work on, wires run in close proximity to a live wire will have voltage induced when the AC votage field collapses when the polarity field changes, 60 times per second in the standard US 60 cycle (Hertz) system.
    Connect the wires to the neutral and thus to ground and you won't measure anything.
    This is a much deeper subject if you care to pursue it, but that is one reason for bonding all the metal in a structure an pools together, to reduce voltage gradients.

    Jim Luttrall
    www.MrInspector.net
    Dallas, Texas

  5. #5
    Join Date
    Mar 2007
    Location
    Ormond Beach, Florida
    Posts
    25,304

    Default Re: strange voltage readings

    By the way, when you run a switch leg like that, the permanently re-identified white wire should be connected to the 'hot' wire, then the returning 'switched hot' which connects to the light is the black wire.

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

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