Shared neutral

Discussion in 'Electrical Forum discussion & Blog' started by beekerc, Oct 27, 2008.

  1. beekerc

    beekerc IT Consultant / Network Engineer

    Messages:
    94
    Location:
    Seattle
    definitely a question for the pro's here.

    In the process of moving some circuits from the main panel breaks to the generator transfer switch breakers, i discovered a 14/3 wire in the panel. the black wire was connected to one breaker. the red wire was connected to another breaker. white and ground were connected to the bus.

    1) is this code legal? it looks like original construction wire so i would assume that it was inspected back in 1968, so if it's not compliant for new construction it's probably grandfathered

    unless....

    2) is it safe? I've already labeled the breakers for "common neutral" so i'll know to shut down both breakers if i want to work on either circuit. beyond that, i really can't think of any other risk.

    am i missing anything that i need to be concerned about?
  2. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    Perfectly normal, legal and safe.
    You did good by labeling them so you remember.
  3. maintenanceguy

    maintenanceguy In the Trades

    Messages:
    107
    Location:
    Pennsylvania
    Today, they'd be required to be on a 2 pole breaker instead of two single pole breakers.

    And today, two separate buss bars are required, one for the neutrals and one for the grounds. In a main distribution panel, the neutral and grounding bus bars are bonded to the panel. In all sub panels, only the grounding bus bar is bonded.

    But two circuits are allowed to share a neutral. It's called a "multi-wire circuit".
  4. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    This is ONLY under the 2008 NEC with no amendments.
    Not nearly all areas are under 2008 yet.
  5. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    It's safe as long as the two breakers are adjacent.

    FYI: I did a dual outlet run using 12-3wg and then ended up having to buy a 20 amp 240 volt GFI breaker. I could not use GFI outlets at the start of the string since it shared the neutral.
  6. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    This is not at all true.
    At the first box you split the circuit to two 2-wire circuits and have a GFI receptacle at each.
    I do this all the time in kitchens.
  7. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    But in this case the neutral was fed to a string of outlet pairs. I would have had to run two 12-2 wires between each 2 gang box to keep the two neutral wires separate for the rest of the string.

    Combining the neutral after two GFI outlets will cause them to trip at times due to unbalances in the neutral current.

    I am just pointing this out in case the original poster decides to upgrade it to GFI in the future.
  8. Chris75

    Chris75 Electrician

    Messages:
    608
    Location:
    Litchfield, CT
    Who wires like the way you did? :D
  9. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    OH. I see. I misunderstood. I thought you meant you just used 12/3 for the home run to the first box.

    I agree with Chris. This is a very unorthodox way of wiring receptacles.
    And just to let you know, you would have had to use a two-pole breaker even if you did not need GFI protection. You have two circuits on the same device yoke, that requires a handle tied breaker.
  10. jar546

    jar546 In the Trades

    Messages:
    432
    Location:
    USA
    Put them on a double pole breaker no matter what code edition in enforced in your area. it is just a safer and professional practice.
  11. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    I am not looking to get into a pissing match, but this is purely personal opinion.

    Sure, it is "safer" for an untrained lay person. On the other hand, anyone with the knowledge or experience to justifiably be in an electrical panel would know immediately.

    Professional practice? Tell that to all the commercial and industrial guys who do not use two and three pole breakers for MWBC's, and for good reason. MWBC's are the standard wiring method in those settings.
    I have installed hundreds, maybe even thousands of MWBC's. Rarely will I use a multi-pole breaker if it is not required. And I certainly DO NOT consider it "unprofessional".
  12. jar546

    jar546 In the Trades

    Messages:
    432
    Location:
    USA
    In the residential setting, I have witnessed more problems from people tapping into multi-wire circuits, contractors screwing them up and homeowners moving the breakers around changing the voltage potential of the circuit.

    In a professional industrial or commercial setting where there are no DIYrs, then of course it is the norm.

    It is of my opinion that it is a safer and more professional installation in any case. However, my opinion and what the code requires or does not require are two different things. Everyday I approve things that I don't like but they meet the code so I do my job.

    You are entitled to your opinion just like I am.
  13. Chris75

    Chris75 Electrician

    Messages:
    608
    Location:
    Litchfield, CT
    Maybe they should just not touch it?

    Just as many so called maintence guys as there are DIYer's.


    Do you really think you can protect the unqualified?
  14. Rowdy

    Rowdy New Member

    Messages:
    8
    Location:
    Tennessee
    Hi all you DIY,ers

    Here is the problem with a shared neutral: Virtually all current that exits a transformer will return to the transformer. There are only two ways that I know of that this can occur.

    Transformers have multiple taps, single phase transformers used for residential power have three taps, L1 and L2 are your normal hots providing current to your service panel and distributes that power through your branch circuits. Once that current is used to power a load it must return to the transformer in order to complete the circuit.

    The third is a center tap is used to drain the current from a load that uses only one side of the transformer, either L1 or L2, or both L1 and L2 if there is an unbalanced load, this center tap is grounded at the transformer and again at the service entrance.

    If you have a load that requires 240 volts you are using both L1 and L2 they are opposite each other so when one is pushing current the other is draining current, they offset the current on the opposite leg so you don't need to be connected to the center tap to return current to the transformer, the same thing occurs with multi-phase transformers.

    Here is the problem with sharing a neutral if you are using two breakers that share only L1 or L2. The center tap is draining current from both circuits and since they are on the same tap from the transformer if you are using 15 amps on both, you are returning 30 amps on the grounded conductor, 15 + 15 = 30amps. Since there is no overcurrent protection on the grounded conductor you are creating a hazard.

    Your panel is designed so that you can use a two pole breaker and be assured that you are using both legs, L1 and L2 of the transformer, do it.
    Last edited: Nov 21, 2008
  15. beekerc

    beekerc IT Consultant / Network Engineer

    Messages:
    94
    Location:
    Seattle
    I think i follow you. now for the dumb question - is a two pole breaker the same as the double breaker that's used for my stove, dryer and heat-pump? because those are 240V. okay, so i'm asking this because i didn't go to electricians school, but when you're dealing with some these types of items, i notice that the gauge of the wire much bigger than if they were separate.
    example. a 240v/30A circuit, is effectively a pair of 120v/15A circuits and breakers. but while 14 gauge wire is sufficient to support a single 15A circuit, all of the wires servicing teh 240v/30A circuit are 10 gauge. (i presume that should all the current some how get directed down one wire, it won't overheat/melt/short/etc). so how would putting a two pole breaker on a pair of 120v circuits using a shared neutral not require upgrading the gauge of the hot wire?
  16. BluesMan

    BluesMan New Member

    Messages:
    18
    yes...a two pole breaker, or double pole breaker is used on the range, electric dryer, and heat pump.

    A double pole 30 amp breaker is just that...two 30 amp breakers. Each leg of that circuit is capable of carrying 30 amps, and the wire must be sized accordingly.
  17. JLAG

    JLAG New Member

    Messages:
    1
    That breaker should be changed to a DP RIGHT AWAY or the circuit should be disconnected and wire nutted.

    If something happens to you, and some joker comes in and moves one of those breakers without checking to see that the neutral is shared on those circuits you have a SERIOUS risk for the people in that structure. Yes, double current on an unprotected neurtal.

    I have seen melt downs from this. People move breakers to get a space in a panel and not check this because the wires are at the back of the panel and are difficult to see.

    I have even seen two SP breakers where someone jamed an allen wrench through the holes in the toggles to make them trip as a DP. Crazy world!

    Just my two cents.
  18. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,262
    Location:
    New England
    When you are using a double pole breaker to provide 240 volts, what goes out one leg, goes in the other so you've got more potential and more current, essentially add the two together, and it has to go through those two hot wires (no neutral); therefore, the wire needs to be sized accordingly.

    But, if you are using a shared neutral, you have half the voltage (unless the neutral comes loose), and what goes out one leg, returns via the neutral, so you only have 120vac potential, thus lower current (i.e., one half of the double). Since with a double, you are assured of having the two on different legs of the transformer, the current on the neutral would cancel if the load is the same on each leg of the hot, but at worst, can't be more than one leg, which is the single breaker of the pair. So, the current could be up to the single breaker's limit, or zero, if both sides were evenly loaded. If it wasn't a double, but wired with adjacent breakers, if one was moved so it was on the same leg of the transformer, the the current in the neutral would add; i.e., try to carry the current of both breakers, rather than one or nulled out - you'd likely start a fire as the wire heated up.
  19. apparentgenius

    apparentgenius New Member

    Messages:
    21
    Location:
    Oregon
    Well, this thread is good timing for me. I have a receptacle box in my house that has 2 switches that control living room and kitchen lights on 2 different circuits. I recently changed the switches (wife wanted white instead of tan). The neutrals on these 2 circuits are wire nutted together in the receptacle box. In the panel, these are each on seperate 15a breakers. Is this also a problem?

    Thanks,
    glen
  20. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    Read the bold parts! I am certainly for doing things the right way, but I am also NOT living my life, and doing my day to day work, to protect stupid people from themselves.
    All we are doing in this country is "lawing" ourselves into a corner by trying to avoid frivolous lawsuits and protect unqualified and stupid people.
    If you remember, Darwin had a theory about that.

    If you are not familiar with electrical panels and do not know what a MWBC is STAY the HELL out of the panel!!

    I know this is opinion on both sides, but ONCE AGAIN, see post #11.
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