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Thread: Common Wire

  1. #16


    How about adding a common neutral to the mix?

    The NEC does not define a common wire.
    My answers are based mostly on the ICC codes. Advice given is my personal opinion and every person performing work should acquire a permit from his/her jurisdiction and get the work inspected. My opinions are not directions to follow for DIYs or professionals

  2. #17
    Electrical Contractor/Instructor jwelectric's Avatar
    Join Date
    Jun 2007
    North Carolina


    A real life story that will fit well in this thread as well reflect on another thread.

    I just got back from a neighbors house helping him get his holiday lights up and running.

    He needed four places to plug in some lights he had on his fence and two small trees at the end of his driveway. He had bought a 50 drop cord and a 4 inch square box for two duplex receptacles.

    He cut the end of the cord off and installed the cord to the box with the receptacles and the breaker tripped.

    When I opened the box he had put the white wire in the right place but reversed the green and black. When I told him what the problem was this is what he said.

    “I thought that black was the “common” wire. We always use black as the “common” wire.”

    Jerry works on dump trucks for a living. He does electrical on these trucks.

  3. #18
    Licensed Electrical Contractor Speedy Petey's Avatar
    Join Date
    Jun 2007
    NY State, USA


    Jerry needs to lay off the 120v AC wiring and let you handle it!
    Answers based on the 2008 & 2011 NEC. If you're on the '14 already I feel sorry for you.

  4. #19
    Electrician Chris75's Avatar
    Join Date
    Aug 2007
    Litchfield, CT


    Quote Originally Posted by jwelectric View Post
    When I opened the box he had put the white wire in the right place but reversed the green and black. When I told him what the problem was this is what he said.

    “I thought that black was the “common” wire. We always use black as the “common” wire.”
    Seems pretty "common" to me when someone does not know what they are doing.

  5. #20

    Smile The Common wire is the hot black wire.

    Here's a well written, thoughtful article explaining the common wire in a three-way switch, that should lay to rest the tentacled monsters these things can appear to be to DIYs:

    According to The Natural Handyman, the common wire is the "hot" wire leading to the lamp fixture, or fed from the breaker box, and in modern buildings, is usually black. The other wires are called "travellers," usually white, and once the common wire is identified, and the green "ground" wire attached, the others can be hooked up to whichever connections are left. He strongly advises replacement of the both switches, as both will have suffered from the failure of either, and it's better to be safe, than sorry.

    I found the advice on using a multimeter (inexpensive and commonly available) very useful in identifying the common wire. Trip the circuit breaker, remove the switch, widely separate the wires, switch on the circuit, set the multimeter to 110v, then touch one of the probes on the box's ground wire, and the other to each of the remaining wires. Only one of them should register a current, which makes it the "common" wire.

    Turn off your breaker again, and install your master switch. Repeat for the slave switch, this time setting your multimeter to "infinite resistance" or "continuity". It will beep when you've found the correct wire.

    If you find more than one hot wire, it's probably because one of the switches in the gang box controls an outlet (for table lamps, etc.), while the other outlet is always hot. Refer to the link for a very detailed explanation, and the fix.

    Hope this was helpful!


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