Two hot and one neutral

Discussion in 'Electrical Forum discussion & Blog' started by pitonyak, Feb 12, 2010.

  1. pitonyak

    pitonyak New Member

    Messages:
    6
    Location:
    Columbus, Ohio
    I have a 40 year old house.

    I have two hot wires running to the kitchen with only one neutral return. One of the hot wires connects to the garbage disposal. I thought that this was very odd.

    I had intended to install a GFI breaker, but, not with a strange setup such as this.

    Does anyone else think that this is odd?

    Side note: This is not the only line that runs to the kitchen.

    Any thoughts on GFI breakers as opposed to at the plug? In the bathrooms, I installed GFI plugs. I had intended to use breakers for the kitchen.
    Last edited: Feb 12, 2010
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,134
    Location:
    New England
    You can't use a GFI on that unless you rewire it if I understand the situation properly. What I think you have is what is referred to as a shared neutral. It is coming off of a double breaker. A double breaker can also be used for a 220vac circuit. Your house has 220 coming in, and the neutral is tapped on the transformer half-way across, so if you measure from one side or the other to neutral, you get 110. When wired this way, because the two hot legs are on the opposite side of the transformer, the current going back through the neutral from one hot will be positive when the other side is negative, and they cancel out. This allows you to save a little bit on the wire - using 3-wires, you get two 110vac circuits. Since the current cancels out, that neutral will never carry more than the current that is on one leg. Since a GFCI needs a dedicated neutral on each leg, it will be unbalanced most of the time and therefore trip.
  3. Lightwave

    Lightwave New Member

    Messages:
    98
    Location:
    Vancouver, BC
    You probably have a multi-wire branch circuit (MWBC). These are safe provided the two hot wires are on opposite legs. If you have a volt meter and are comfortable testing energized circuits, measure the voltage between the two hot legs. If the meter shows 240 (nominal) between hot legs and 120 between either hot and ground, you have an MWBC. If you see zero volts between the hot legs and 120 (nominal) between both hot legs and ground, then you have a code violation and/or a fire hazard. Depending on the quality of your wiring, the two hot legs may or may not be connected to a tandem breaker.

    MWBCs are more hazardous than most other residential wiring. Aside from thea 240 volt shock hazard when working them live, any failure in the grounded conductor (neutral) can subject downstream fixtures to 240 volts--with potentially explosive results.

    More information on MWBCs:

    http://www.homeinspector.org/resources/journals/Multiwire-Branch-Circuits.pdf

    Single pole GFCI breakers don't work with MWBC wiring. You must use a tandem GFCI breaker (very expensive) If you need breaker-level GFCI protection. Standard GFCI plug outlets will work on an MWBC without any special wiring.
  4. pitonyak

    pitonyak New Member

    Messages:
    6
    Location:
    Columbus, Ohio
    Two breakers are used, and they are on the same side of the box, which means that I expect that there is no voltage difference between the two hot leads. I can verify this (but I will be working the weekend). I posted here because I had intended to install a GFI breaker and it was unclear to me how appalled I should be. I don't really want to run another line for this, but, I am certainly able to do so if that is the safest thing to do.
  5. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego

    Not a good assumption! ALL double pole breakers are physically on one side of the panel, or the other, but the design of the buss bars and the breakers are such that the two breakers pick up different legs. On two single pole breakers, you would have to measure the voltage between the outlet screws ...it will be 240, or zero.
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,815
    Location:
    Cave Creek, Arizona
    Adjacent, or an even number of slots BETWEEN the breakers will usually indicate they are on opposite poles of the supply feed.
  7. Alectrician

    Alectrician DIY Senior Member

    Messages:
    689
    Very common. You will have to go with GFCI receps.

    1) Don't rely on the device to make the in/out neutral connection to the DW/Disp
    2) Be sure to make this neutral wirenutted connection 100% tight.
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