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I have 12-3 electrical wire running from the main panel on one end of my ranch-style house to feed both the basement lights and the living room lights/outlets upstairs (opposite ends from the main panel). The red wire is connected to a breaker and used as the hot for the circuit while the black wire is not connected to a breaker in the main panel, it is simply capped off. If I were to connect the black wire to a separate circuit breaker (opposite phase, of course), could I safely branch off at the first receptacle/light in the circuit and use the black hot for the living room and keep the red hot for the basement lights and just "share" the neutrals? I want the areas of the house to be on separate circuits, and thought I may be able to avoid a new run of cable the full length of the house.

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    Can you live with them tripping together (one overloads, knocks the other out)? – Harper - Reinstate Monica Nov 21 '19 at 17:14
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Yes. This is the last person planning ahead. They made sure you would be able to do this. This would create a multi-wire branch circuit and you must follow the rules for MWBCs.

  • There must be 240V across the hots, so neutral carries differential current not the sum of both currents.
  • The breakers must have common maintenance shutoff. It will be impossible to shut off one and not the other; one hand action does both.

    • This will almost certainly have a side-effect of common trip, meaning if one side overloads, it knocks out the other side. In fact, the most normal (idiot-proof) way to wire these is to simply use a 2-pole breaker, because they are readily available everywhere, instead of UL-listed handle-ties for those breakers, which are a specialty, hard-to-find item.
    • This must not be a duplex/twin/tandem breaker, which does not have 240V across it!
  • Neutrals must be pigtailed. You may be accustomed to using the two screws on each side of a receptacle for a splice to extend the circuit. Can't do that on a neutral, because removing the receptacle etc. would then create a "lost neutral" situation downstream! The appliances on the two half-legs would be in series with each other at 240V, and one leg could rise to as high as 240V.

    • If you are past the point where the halves have split for good, then you don't need to worry about this requirement.
  • AFCIs normally go in the service panel, to protect wires. Here, consult your panel's manufacturer (or the maker of UL-classified breakers for your panel) for how to do 2-pole AFCI breakers in their system. Some simply offer 2-pole AFCI; others have you handle-tie two singles.

  • There are only 2 possible places to put GFCI protection: As a 2-pole GFCI breaker at the service panel, or as a device serving only one leg of the GFCI. You can't share the neutral past a GFCI.
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  • Wow, what a fantastic and detailed response. Thank you for taking the time. I've learned a lot from these forums, but just joined today. When you discuss AFCIs and GFCIs - both of these are "optional" right? I just looked up the price of a double AFCI for my CH panel and they run $125. – clwhoops44 Nov 21 '19 at 18:04
  • I don't see anything that indicates GFCI protection is required in the areas you noted. AFCI protection is now required in areas noted. Normally modifying a circuit requires your modifications meet current code. I have found wide and evolving interpretation among electrical inspectors determining exactly what triggers installing AFCI's. – NoSparksPlease Nov 21 '19 at 19:05
  • @NoSparksPlease Interesting. GFGI (or RCD) is pretty much universally required in the UK, but I'd never heard of AFCIs until I came on here. I was going to say "consult your local code, but install GFGI anyway (and buy a fire alarm instead of an AFCI)" – Martin Bonner supports Monica Nov 21 '19 at 19:13
  • @MartinBonner in the UK they put an RCD (GFCI) on the whole house, but at a lesser detection threshold of 30ma instead of 6ma. The US goal is life safety but 6ma is too sensitive for a whole house. So it must be per-circuit. – Harper - Reinstate Monica Nov 21 '19 at 20:34
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    @clwhoops44 Depends. They are mandatory in new work and remodels. But old installations are grandfathered because you can't very well update your whole house everytime Code changes (every 3 years). The pinch point will be whether your AHJ considers this work to be a remodel. AHJ = people who issue the permits for the work. As for pricing, don't go by Internet pricing, which is ridiculous on electrical parts. Exotic breakers notwithstanding, 99% of electrical stuff is far too cheap and heavy to economically sell online, so nobody sells it online except for highballers. – Harper - Reinstate Monica Nov 21 '19 at 20:37
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Doing that creates what the NEC describes as a Multiwire Branch Circuit. The circuits must be positioned in the electrical panel so handle ties or a two-pole breaker can disconnect both live conductors simultaneously.

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Yes, you can do that but you have to handle tie the breakers together. You also have to trace the black wire to find the other end. You mentioned it's capped in the main panel, well you need to find the other end and make sure you know what you're connecting to a breaker. Good luck

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  • Thanks, Jack. The black was tied with the red and the other blacks in the light fixture nearest the main panel in the circuit. It’s now separated from the hot bundle and capped off. – clwhoops44 Nov 21 '19 at 17:32
  • I'm curious -- why does NEC require the breaker handles to be tied together? Would a trip not still be safe? – bobwki Nov 21 '19 at 19:13
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    @bobwki Because you are sharing the neutral with two different phases of 120v lines. If the red circuit tripped because of a bad outlet, you would still have live voltage circulating in the neutral because of the black circuit. Tying the handles (with approved ties) of the two breakers causes both circuits to shut down if something goes wrong with one circuit. Hope this helps. – JACK Nov 21 '19 at 19:29
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    @bobwki I think the intent is worker safety. Really I think it's less an issue in residential, but before this became code I found several times in commercial conduit work that misidentification of shared neutrals caused inadvertent energized open neutrals and single phased equipment. – NoSparksPlease Nov 21 '19 at 19:49
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    @bobwki if the breakers weren't tied together and you only flipped one circuit off to service a fixture/receptacle, the hot wire might be dead, but the shared neutral would still be energized from the other circuit. – clwhoops44 Nov 21 '19 at 20:39

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