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  1. #1
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    Default How to break up a circuit with a shared neutral?

    We have recently moved into an old house (circa 1950) with much of the original wiring (not K&T). The previous owner(s) had done all sorts of weird and wacky things to the house, and we are slowly discoving them and correcting them. Most of the electrical wiring is original (paper insulated Romex without grounding wire). I have a question related to the recent discussions about shared neutrals.

    I've found out that 80% of the house is wired onto 2 branch circuits with a shared neutral!! What looks like 14/3 wire (without ground) is connected to 2 (adjacent) 15A breakers that are not tied together with a handle cross-bar. This wire runs to a junction box in the basement and powers (with a shared neutral) 2 branch circuits. From there multiple 14/2 wires (without ground) run out to various parts of the house. These 2 circuits supply power to lights and receptacles throughout the house.

    I'm not happy with this situation and want to do something to reduce the potential load on the single neutral wire. This is what I propose to do:

    I plan to split off as many of the end run wires as I can onto newer circuits that do not have a shared neutral. I should be able to split off enough wires such that one of the 2 main circuits now has no load at all, making it redundant. I plan to take the hot wire from this circuit (which now carries no load) and convert it into a ground wire (tagged and labelled) for the other circuit. This will free up one breaker slot for a new, properly wired circuit. The remaining circuit now has a dedicated neutral, a greatly reduced load and is grounded at least as far as the first junction box.

    Is this a reasonable thing to do? Thanks in advance for your opinions!!

  2. #2
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    Default Re: How to break up a circuit with a shared neutral?

    I was always taught a shared neutral with the hots being different phases is never a problem.{The hots feed from each of the legs in the load center, that is**. And those phases cancel each other out.
    I do agree that 80% of all your circuits feeding from just two hots could be an area of concern. Are you tripping breakers?

  3. #3
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    Default Re: How to break up a circuit with a shared neutral?

    Thanks for the answer, Ernie.

    No, we are not tripping breakers (yet). I'm more worried about a potentially increased load on old circuits as more and more stuff gets plugged in. Also, I'd like to change the breaker to an AFCI, and to do that the circuit cannot be on a shared neutral.

    Is what I'm proposing safe to do? Is there something I have not thought of?

  4. #4
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    Default Re: How to break up a circuit with a shared neutral?

    What you are talking about doing would be safe but it would be reducing from 2 15amp circuits to 1 15amp circuit.
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  5. #5
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    Default Re: How to break up a circuit with a shared neutral?

    Thanks for the answer, Jack.

    That is exactly the idea....reduce to one 15A circuit, eliminate the ungrounded shared neutral setup and take the loads on the other circuit and split them off to other circuits that are properly wired, grounded, etc. Thanks for your opinion.

  6. #6
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    Default Re: How to break up a circuit with a shared neutral?

    what you are describing above in your first post is safe and was allowed by the NEC. But then as the code evolved the only change that was made is it requires the breakers side by side with either a two pole breaker or a handle tie. If you want to make a test to see how safe this is load both circuits up and use a amp probe. Clamp the amp probe around one of the hot wires and take a reading. then measure the other circuit. Add them together. The clamp around the neutral wire. You should only see the higher of the two measurements you made on the hot wire. Shared neutrals only become serious problem on commercial inductive and capacitive loads.In all my years in the trade I have never seen and problems in residential installation of shared neutrals. In commercial installations they still use shared neutrals to this day but they use a overside neutral to keep the neutral from overheating.
    Harry

  7. #7
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    Default Re: How to break up a circuit with a shared neutral?

    rick ... while shared neutral circuits are allowed by code and for the most part are safe there can be issues with that type of configuration.
    So .... converting to dedicated runs can be a good thing.

    If you recall in your other thread the 3 wire single / split phase mains power at the breaker panel is basically a shared neutral. It's because of the neutral center tap you have 2 seperate 120 volt circuits.

    Now to scale it down to the circuits within the home.... ...... if the neutral connection at the panelís neutral bar or at a box the cable is run to goes bad ..... the two circuits will be arranged in series with the other and become one 240-volt circuit .

    Since 120 volt lights and appliances are not designed for 240 volts the items served by one circuit will tend to run brighter than usual and those on the other dimmer ........ also damage can occur to sensitive electronic equipment.

    If you notice a regular weird dimming of lights in one part of the home and lights burning brighter than normal in another part .... this is one possible cause. The other possibility is a similar situation occurring to the homeís whole system when the main neutral at the panel is bad.

    Just a thought.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  8. #8
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    Default Re: How to break up a circuit with a shared neutral?

    Quote Originally Posted by rickpantel View Post
    Thanks for the answer, Ernie.

    No, we are not tripping breakers (yet). I'm more worried about a potentially increased load on old circuits as more and more stuff gets plugged in. Also, I'd like to change the breaker to an AFCI, and to do that the circuit cannot be on a shared neutral.

    Is what I'm proposing safe to do? Is there something I have not thought of?
    The shared neutral has caused problems in that very situation with Arc Faults. It was common practice round here to run 12/3 instead of 2 12/2s to save on pulls, etc. Something I was never big on myself, but see enough of it.

  9. #9
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    Default Re: How to break up a circuit with a shared neutral?

    Quote Originally Posted by Ravens53 View Post
    what you are describing above in your first post is safe and was allowed by the NEC. But then as the code evolved the only change that was made is it requires the breakers side by side with either a two pole breaker or a handle tie. If you want to make a test to see how safe this is load both circuits up and use a amp probe. Clamp the amp probe around one of the hot wires and take a reading. then measure the other circuit. Add them together. The clamp around the neutral wire. You should only see the higher of the two measurements you made on the hot wire. Shared neutrals only become serious problem on commercial inductive and capacitive loads.In all my years in the trade I have never seen and problems in residential installation of shared neutrals. In commercial installations they still use shared neutrals to this day but they use a overside neutral to keep the neutral from overheating.
    Two pole breakers or handle ties went into effect for NEC 2008, correct?

  10. #10
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    Lakeland ,MN
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    Default Re: How to break up a circuit with a shared neutral?

    Quote Originally Posted by Ernie_Fergler View Post
    Two pole breakers or handle ties went into effect for NEC 2008, correct?
    In commercial industrial applications it went to effect in 2008. Residential applications it has been effect for a while but I do not know the exact year
    Harry

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