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  1. #1
    Join Date
    May 2008
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    Default Shared Neutral question

    I think I know the answer to this but I'm looking for confirmation or correction from people here who I can trust.

    I'm getting ready to add a gable fan in my attic and found something suspicious. There is a grounded 12/3 Romex run in the general area. I've traced this back to two 15A breakers in the panel. One circuit (with the black wire) is in use. The second circuit (with the red wire) is capped off and the breaker has always been off. Presumably it has been this way since the previous owners lived here, since myself and an air conditioning contractor are the only ones who have been inside my panel in the couple of years I've lived here.

    The suspicious part is that the breakers are two tandem breakers sharing the same spot in the panel. So this means they are sharing the same pole coming from the power company... correct? I assume the intention was to someday use the red wire for something else and share the neutral and ground.

    My belief is that this would be a very dangerous idea. Am I right?

    Now, if I were to move the red wire to a new breaker, making sure it was on the opposite pole, could I use the red wire for my new circuit and still share the neutral and ground?

    Finally, if that answer is YES... do both breakers need to be the same amperage rating? And is there anything I need to do to mark or indicate this is the case?

    Thanks!

  2. #2
    Join Date
    Jun 2007
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    Default Re: Shared Neutral question

    You are talking about wiring 220 with neutral using 2 120v breakers then spliting it into two seperate 120v circuits with only one neutral. First this would not be to code and could be dangerous because of feedback between the two circuits. Second, if it were allowed, the toatal breaker value would have to limited to 20 amps on 12 ga. wire which would mean you would need to use 2 10amp breakers of a 15 amp and a 5 amp breaker. That's because the neutral wire carries the same current as the hot wire or wires. This type of hook-up is a good way to get someone killed. Please do not do it.
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  3. #3
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    May 2008
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    Default Re: Shared Neutral question

    Thanks for your input.
    I understand the potential hazard well enough... which is why I'm not entering into this lightly.
    I guess I'm just asking if there is a way to do it safely.

    I've done a little further reading and several sources indicate that if the hot wires are on different legs/phases/poles (choose your favorite) then the loads on the neutral wire cancel each other out, having a "differential" effect as opposed to a cumulative effect. Which I take to mean that on 12 gauge wire I could have 2 20 Amp breakers with a shared neutral, as long as they are on different legs.

    So that leaves the problem of still having a load on the neutral when only one breaker is switched off. One source I read suggests using a double pole breaker which would force the hot wires to be on seperate legs and could also be tied together to force both breakers to be switched on and off together. I suppose the panel legend could also be marked as "Shared Neutral" or something like that.

    So, that's a nice theory and all, but I guess at this point I'd appreciate experienced opinions on this idea or other options.

    I know it's very easy to say "... best to run a new line..."
    Certainly the safest thing would probably be to do a whole new run. However if I can safely avoid running a new line from the basement to my very hot and very cramped attic... I'll take it.

    Thanks again.

  4. #4
    Join Date
    Jan 2008
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    Lakeland ,MN
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    356

    Default Re: Shared Neutral question

    Quote Originally Posted by JLMCDANIEL View Post
    You are talking about wiring 220 with neutral using 2 120v breakers then spliting it into two seperate 120v circuits with only one neutral. First this would not be to code and could be dangerous because of feedback between the two circuits. Second, if it were allowed, the toatal breaker value would have to limited to 20 amps on 12 ga. wire which would mean you would need to use 2 10amp breakers of a 15 amp and a 5 amp breaker. That's because the neutral wire carries the same current as the hot wire or wires. This type of hook-up is a good way to get someone killed. Please do not do it.
    Jack
    I am sorry I have to disagree. This is a common practice in residential and commercial projects. But there are a few things you have to do to make this safe and code compliant.
    The black and red wire must be on a two pole breaker not a twin breaker that only uses one space in the panel. The two pole breaker has one handle that turns both circuits off at the same time. It also feeds one circuit from phase A and one circuit from Phase B. Second the maximum breaker size on a #12 copper wire is 20 amps. So you can use a 15 amp breaker if you desire.If it alluminum 15 amps is the maximum size breaker
    The neutral current only sees the load from each circuit at different times as they are 90 degrees out of phase.
    To make it simpler AC current goes from 0 volts to 120 volts to 0 volts to -120 volts then back to 0 volts in one cycle and this happens 60 times a second. Which is 60 Hz power we use?
    When phase A is 120 volts phase B is zero volts. So the neutral current on phase B is zero when Phase A is at its maximum. When Phase A is at 120 volts then phase B is 0 volts.

    What I am trying to explain is the current on each phase flow at different times and the neutral only sees the current of one phase at a time.
    Although there are factors when a shared neutral is not a good idea to use but that would be in a commercial application that has inductive loads that cause harmonics.
    Last edited by Ravens53; 06-18-2008 at 09:42 AM.
    Harry

  5. #5
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    Default Re: Shared Neutral question

    Oops, sorry, you are correct about the current flow if the breakers are on separate poles. I was a little tired and was thinking of the two situations separate poles and the same poles. My comment about total value of the breakers would be if they are on the same poles. The danger with separate poles is if there is a loose or broken connection on the neutral going back to the panel. Then the loads would be in series across the 240 volts. If the loads of the two circuits where not balanced you could end up with a voltage potential on the neutral wire of anywhere from a few volts to over 200 volts. This could be a dangerous situation for anyone working on the circuit and be a possible fire hazard.
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  6. #6
    Join Date
    Jun 2008
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    16

    Default Re: Shared Neutral question

    Quote Originally Posted by xyxoxy View Post
    Thanks for your input.
    I understand the potential hazard well enough... which is why I'm not entering into this lightly.
    I guess I'm just asking if there is a way to do it safely.

    I've done a little further reading and several sources indicate that if the hot wires are on different legs/phases/poles (choose your favorite) then the loads on the neutral wire cancel each other out, having a "differential" effect as opposed to a cumulative effect. Which I take to mean that on 12 gauge wire I could have 2 20 Amp breakers with a shared neutral, as long as they are on different legs.

    So that leaves the problem of still having a load on the neutral when only one breaker is switched off. One source I read suggests using a double pole breaker which would force the hot wires to be on seperate legs and could also be tied together to force both breakers to be switched on and off together. I suppose the panel legend could also be marked as "Shared Neutral" or something like that.

    So, that's a nice theory and all, but I guess at this point I'd appreciate experienced opinions on this idea or other options.

    I know it's very easy to say "... best to run a new line..."
    Certainly the safest thing would probably be to do a whole new run. However if I can safely avoid running a new line from the basement to my very hot and very cramped attic... I'll take it.

    Thanks again.
    I can see your reluctance to run a new line if it not necessary!
    If I understand your description correctly, you will be fine with using the red wire for a second circuit as long as you place it on a breaker using the opposite phase as the black wire (like you described).

    Tying the breakers together sounds like a good idea to keep from being "surprised" in the future. Also, I would label each light switch / receptacle plate / j-box that uses these lines with a note indicating the shared neutral and the breaker #'s. Write the info on tape and put the tape on the back side of the cover plate.

    Although I didn't use shared neutrals, I did that when rewiring our house many years ago. It has come in handy refreshing my memory which receptacle or switch a supply came from and where it goes from that spot. I also have the drawings, but it is easier for me to look on the back of a cover plate than to dig out the drawings.

  7. #7
    Join Date
    Aug 2007
    Location
    Denver, CO
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    666

    Default Re: Shared Neutral question

    Are we sure it needs to be a two pole breaker if a shared neutral is used? Or, does it just need a handle tie for two single pole breakers next to each other on different phases?

  8. #8
    Join Date
    Aug 2007
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    Default Re: Shared Neutral question

    xyxoxy .... it's not all that uncommon to have a 3 conductor cable run into an attic with one conductor labeled "spare" or "future use" for a second circuit sharing the neutral.

    similar to that .... up here..... for kitchen counter plugs 14/3 is run to a duplex receptacle with the hot jumper tab cut turning the duplex into 2 separate simplex receptacles.
    So the black would be on the lower and the red would be on the upper receptacles with a double pole breaker at the panel ... this is basically 2 separate circuits sharing the same neutral. ( this kitchen plug wiring is for illustration and not allowed in the US NEC )

    However .... it's a good idea to use a double pole breaker to ensure you are on separate legs ... but also for safety. If one circuit trips the breaker this ensures that both conductors are dead in the cable ... so when you are trouble shooting in a junction box there are no live wires from that cable thinking that it should be dead.

    Inside the panel there are 2 bus bars the breakers connect with .... leg 1 / phase A ... leg 2 / phase B.
    The grouping of the breakers will alternate between legs / phase ... for example .... breakers 1&3 ,2&4 will be on leg 1 / phase A .... breakers 5&7 , 6&8 will be on leg 2 / phase B.

    A double pole breaker is meant to be attached to both leg 1 / phase A and to leg 2 / phase B.

    Using the example above .... the double pole breaker would attach to position 3 and 5 .... meaning the black wire would be on leg 1 / phase A ..... the red wire would be on leg 2 /phase B.

    Even though there are two separate handles on a double pole breaker ... inside they are physically joined together so as to disengage the other if one was to trip.


    Hope this makes sense and helps.
    Last edited by canuk; 06-18-2008 at 04:42 PM. Reason: correction on numbers
    "" an ounce of perception -- a pound of obscure "
    - Rush

  9. #9
    Join Date
    Aug 2007
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    Default Re: Shared Neutral question

    Geez ... with my slow 2 finger typing I see many folks had jumped in the meantime ... sorry for duplicating things.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  10. #10
    Join Date
    Jun 2007
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    Default Re: Shared Neutral question

    Sorry, I should have qualified my answer as refereing to two seperate breakers.
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

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