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Thread: Shared neutral

  1. #1
    IT Consultant / Network Engineer beekerc's Avatar
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    Default Shared neutral

    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. #2
    Licensed Electrical Contractor Speedy Petey's Avatar
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    Perfectly normal, legal and safe.
    You did good by labeling them so you remember.

  3. #3
    In the Trades maintenanceguy's Avatar
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    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. #4
    Licensed Electrical Contractor Speedy Petey's Avatar
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    Quote Originally Posted by maintenanceguy View Post
    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 all sub panels, only the grounding bus bar is bonded.
    This is ONLY under the 2008 NEC with no amendments.
    Not nearly all areas are under 2008 yet.

  5. #5
    Computer Programmer Bill Arden's Avatar
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    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.
    Important note Ė I donít know man made laws, just laws of physics
    Disclaimer: I'm a big fan of Darwin awards.

  6. #6
    Licensed Electrical Contractor Speedy Petey's Avatar
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    Quote Originally Posted by Bill Arden View Post
    I could not use GFI outlets at the start of the string since it shared the neutral.
    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. #7
    Computer Programmer Bill Arden's Avatar
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    Quote Originally Posted by Speedy Petey View Post
    FYI: I could not use GFI outlets at the start of the string since it shared the neutral.
    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.
    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.
    Important note Ė I donít know man made laws, just laws of physics
    Disclaimer: I'm a big fan of Darwin awards.

  8. #8
    Electrician Chris75's Avatar
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    Quote Originally Posted by Bill Arden View Post
    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.
    Who wires like the way you did?

  9. #9
    Licensed Electrical Contractor Speedy Petey's Avatar
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    Quote Originally Posted by Bill Arden View Post
    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.
    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. #10

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    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.
    http://www.inspectpa.com/forum/forum.php
    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

  11. #11
    Licensed Electrical Contractor Speedy Petey's Avatar
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    Quote Originally Posted by jar546 View Post
    it is just a safer and professional practice.
    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. #12

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    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.
    http://www.inspectpa.com/forum/forum.php
    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

  13. #13
    Electrician Chris75's Avatar
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    Quote Originally Posted by jar546 View Post
    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.
    Maybe they should just not touch it?

    Quote Originally Posted by jar546 View Post
    In a professional industrial or commercial setting where there are no DIYrs, then of course it is the norm.
    Just as many so called maintence guys as there are DIYer's.


    Quote Originally Posted by jar546 View Post
    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.
    Do you really think you can protect the unqualified?

  14. #14
    DIY Junior Member Rowdy's Avatar
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    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 by Rowdy; 11-21-2008 at 04:13 PM.

  15. #15
    IT Consultant / Network Engineer beekerc's Avatar
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    Quote Originally Posted by Rowdy View Post
    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.
    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?

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