Common Wire

Discussion in 'Electrical Forum discussion & Blog' started by Giles, Dec 6, 2008.

  1. Giles

    Giles Retired tool & Die and Mechanic

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    I have heard several definitions of "common wire". I have heard some people refer to the common as the hot and others refer to the neutral as being common.
    What is the definition of common when used in wiring?
  2. Cookie

    Cookie .

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    I am not an electrician but, I define a common wire by what I am connecting to it. I am not sure if this is what you meant.
  3. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

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    There is typically one place for the term "common" in AC building wiring. It is the odd wire in a 3-way switching scheme. The other two wires are typically called the travelers.

    A neutral is NOT a "common".

    Some HID fixtures use the term "com" in ballast wiring, but this can be a neutral or hot depending on the feed.
  4. maintenanceguy

    maintenanceguy In the Trades

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    Location:
    Pennsylvania
    nobody calls either of these wires a "common" wire.

    A neutral wire can be "common" to several different circuits in your house and you can run a special circuit called a "multiwire branch circuit" where both halves of the circuit share a neutral but it's not called a "common" wire by anyone I've ever known or worked with.

    A common wire usually refers to control circuits such as the low voltage circuits for a lawn irrigation system that run out to each electrically operated valve. In these arrangements, one common wire loops from valve to valve. Each valve also gets its own power wire to turn the valve on and off.
  5. Thatguy

    Thatguy Homeowner

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    Common (to more than one load).

    I guess it could be said the hot and the neutral are common to all the outlets connected to them.

    The "wiper" terminal in a 3 way switch (a SPDT switch) could be called common because at one time or another it is connected to each of the other two terminals.
    Same with the wiper of a rotary switch.

    http://www.google.com/search?hl=en&q=define: common&aq=f&oq=
  6. Cookie

    Cookie .

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    That guy,

    I read this in your link, I am sitting here laughing, it reminds me of something, I used to tell my kids, LOL. I love it. I would tell my kids, to " go pick up your room" they would ask why. I would say, "because you are common." One son, came back to me with a dictionary, and read to me the definition of common. He asked me, " am I an unwashed mass?" :D

    So, a few days went by. I told him, "go pick up your room, ok?" He said, " Why?" I said, " because you are special." An hour later here he comes again with the dictionary, this time I RAN.

    Today he is a teacher. :D

    Thank you for the wonderful memory! Cookie

    "the common people in those days suffered greatly"; "behavior that branded him as common"; "his square plebeian nose"; "a vulgar and objectionable person"; "the unwashed masses"
  7. jwelectric

    jwelectric Electrical Contractor/Instructor

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    Location:
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    On the back of a three way switch is the word common

    On a 240/208 to 24 volt transformer is a conductor that is labeled common

    Then in nails we have what is called common such as a 16 penny common

    I once wired some apartments and they were called The Commons

    And finally, I am commonly called arrogant.
  8. Cookie

    Cookie .

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    I didn't call you arrogant. I am not rude.
    I was laughing about my son with That Guy. It was a funny, warm, memory and one of the definitions of " common", unwashed masses. LOL. It is something between my son who at the time, was usually an unwashed mass, LOL, AND 5 years old!
    It had nothing to do with you at all.


    http://www.google.com/search?hl=en&q=define%3A+common&aq=f&oq
    Last edited: Dec 6, 2008
  9. Cookie

    Cookie .

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    I agree with your post by the way That Guy.
  10. Thatguy

    Thatguy Homeowner

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  11. jwelectric

    jwelectric Electrical Contractor/Instructor

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    Location:
    North Carolina
    Mrs. Cookie
    Didn't mean to imply that you had but my wife on the other hand does quite often.

    It pains my heart to hear of your situation as my wife has been diagnosed with Pulmonary Fibrosis and has a limited time left with us so I do understand just what hope is about.

    Due to other health problems she will not be recommended for any type of implant. She is a very strong woman that has taught me more about life than most will ever get to understand, Thank you Lord for my wife.

    I hope you the best and please know that you will be in our prayers daily.
  12. Cookie

    Cookie .

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    Well, you did it again! I don't like Mrs. Cookie. :D I sound like a restaurant.
    (Sadly, I am no longer a Mrs, I am a widow.)

    I am truly sorry to hear about your wife, I do wish you both the best, and I hope things turn around. Prayer to me is the very strongest power there is on earth. You both will be in my prayers. I know how hard it must be for you, too. It is not easy living with someone who is sick. We get cranky. :D But, then men are just born cranky. :)
    Last edited: Dec 6, 2008
  13. Cookie

    Cookie .

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    I think I like your wife. :D Tell her I said so, lol.
  14. jar546

    jar546 In the Trades

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    There you go. There is the answer you are looking for when you are talking about residential electrical wiring.
  15. hj

    hj Moderator & Master Plumber Staff Member

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    Location:
    Cave Creek, Arizona
    Common

    As a practical matter, "common" can be whatever you want it to be, since any shared wire would be "common" to the items it served.
  16. jar546

    jar546 In the Trades

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    Location:
    USA
    How about adding a common neutral to the mix?

    The NEC does not define a common wire.
  17. jwelectric

    jwelectric Electrical Contractor/Instructor

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    Location:
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    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.â€

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

    Speedy Petey Licensed Electrical Contractor

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    Location:
    NY State, USA
    Jerry needs to lay off the 120v AC wiring and let you handle it! :rolleyes:
  19. Chris75

    Chris75 Electrician

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    Location:
    Litchfield, CT
    Seems pretty "common" to me when someone does not know what they are doing. :D
  20. Johnzane

    Johnzane New Member

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    6
    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:
    http://www.naturalhandyman.com/iip/infelectrical/inf3way.html

    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!

    -Johnzane
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