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I had a huge roll of 12/3 Romex that I was trying to maximize the use of, when I recently ran the rough electrical for a home addition project. To save time and $$ on cable, I ran some of the home runs with the 12/3, sharing the neutral between two circuits. Well, as it turns out the new code requires GFCI protection just about everywhere, so the shared neutrals aren't going to work. So rather than remove the 12/3 cable, I would like to just abandon the red conductor, and keep using the 12/3 like it was 12/2. That way I only need to run another 12/2 home run.

So my question to the professional electricians is what is the most professional way of abandoning a conductor like that, as the 12/3 enters the sub-panel? Do I just snip it off right below the cable clamp, and then again at the other end inside the junction box, and not worry about it? Will that be fine? Or should I keep the conductor inside the panel, and cap it off with a wire nut, or label it somehow? I'm an owner-builder, so I just don't want the inspector to look at my work and think it's amateur, or raise any questions. So what is the best practice for this situation?

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    Can you instead leave the MWBC in place and use GFCI receptacles as the first device on the circuit? Depending on the amount of wire you need to re-run, it might be cheaper/easier.
    – Chris O
    Feb 17, 2023 at 18:02
  • Yeah, that might be the way to go. Thanks. Feb 17, 2023 at 18:09
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    Never chop off wires, unless they are damaged - can’t tell you the number of people I’ve cursed for doing that.
    – red_menace
    Feb 17, 2023 at 18:15
  • @red_menace, then what would I do with the wire? Leave it loose in the sub-panel? That seems kind of dodgy. I know that I will never use this conductor for any future purpose. Feb 17, 2023 at 18:21
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    Just cap it off - wire nuts, tape, etc. Just because you don’t plan on it now doesn’t mean that it can’t be used later.
    – red_menace
    Feb 17, 2023 at 18:29

3 Answers 3

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The shared-neutral thing is called an MWBC. Multi-Wire Branch Circuit.

MWBCs work fine with 2-pole breakers

... in fact they are semi-required! With MWBCs you MUST tie the handles of the two breakers! You cannot have independent throw, or guess what happens: a future maintainer plugs in a radio, cuts breakers until the radio goes silent, and turns off half of the MWBC. Maintainer gets to work taking wires apart, and gets nailed by current on the other half of the MWBC. That's why MWBCs need handle-ties.

In a GFCI/AFCI context, you simply use a 2-pole breaker.

Wait. What? 2-pole AFCIs? Yeah. Really. They exist in every brand. Siemens too.

GE has a special trick. Some GE AFCIs do not require circuit neutral, and that means they are compatible with MWBCs. In fact they are made for that purpose. They still need handle-ties, though.

Siemens has another trick: they make tandem AFCIs (2 AFCIs 1 space) which also do not need circuit neutral (there'd be no room on the breaker for the lug, anyway). This may not fit your needs, but the inner handles on two tandems can be handle-tied to serve a 240V load or MWBC. In fact you can stack several of them, using the inner handles between each breaker for each of a stack of MWBCs. (then the very top and bottom handle are singles). So if you have four MWBCs you have 5 breakers, with 1A not being used for an MWBC, 1B-2A being MWBC 1, 2B-3A being MWBC 2, and so on.

For GFCI, the above "no neutral" technology trick will never work with GFCIs, so you use GFCI receptacles. However if you are using a full-width 2-pole AFCI that may be offered in AFCI+GFCI.

If you did abandon 1 wire in cable, cap it off- never cut

In your application, simply leave the red wire the same length as the other wires, and put an appropriate size wire nut on it and push it into the back of the box. Someday, a future someone may find a use for it - finding things like this in a box when you need it, is pure gold. By the way, you are already doing this on your switch loops, per the "neutral to switch loop" requirement in 404.2(D). Black for always-hot, red for switched-hot and white capped off for future smart-switch use. That became code in 2011.

Honestly most novices should throw their wire cutters in the trash. It's like giving a budding auto mechanic a hammer as their first tool. When you own a wire cutter, all the world looks like a wire that needs cutting! Wires should never be cut. Working in cable you only ever cut the whole cable because you've reached your junction box.

  • Wires in boxes should not be cut for snug length - 6" is mandatory and extra is golden, so you have room to recover when a wire end becomes too chewed to put in a wire nut again.
  • Wires in panels should not be cut snug either - wires should be long enough to allow hot and neutral to reach every space in the panel (to allow easy rearrangement, and support future possible GFCI).

So byebye wire cutter - I don't even have one in my toolbox, I use my multi-tool for cutting cables (or THHN wires, which is 99% of my work).

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MWBCs are compatible with GFCI outlets. Remember that the GFCI only compares incoming current to outgoing current. They don't care if an other GFCI is on a MWBC. I have those in my kitchen and they've always worked fine. Never any nuisance trips.

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  • I probably should have stated that these circuits require dual breakers, AFCI/GFCI, and not just GFCI. I don't know if that makes a difference about the shared neutral. But at any rate, the breakers I have say that they can't be used with a shared neutral. Feb 17, 2023 at 18:19
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    Don't use GFCI breakers, @5000fingers, use AFCI breakers as required, then GFCI receptacles with further receptacles wired off the load side. That will take care of protecting anything downstream.
    – FreeMan
    Feb 17, 2023 at 18:32
  • Ok, so then AFCI breakers can be used with shared neutrals, and it's only the dual function breakers that can't be? Feb 17, 2023 at 18:33
  • @5000fingers I think (someone may confirm) that it's okay as long as the GFCI isn't on the shared part of the neutral. After it branches off, it should work. Feb 17, 2023 at 18:36
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    See Harper's answer for complete details on that, @5000fingers
    – FreeMan
    Feb 17, 2023 at 19:33
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First of all, don't chop the third wire too short. Leave around 6" inside the junction box, coiled up and capped with a wire nut. In the panel, leave as much length as you used for the other wires, coiled up and capped with a wire nut.

There are a few different scenarios here beyond "cap the red and call it a day":

  • AFCI-breaker only - Check the specs on the breakers. If they don't use neutral at all then you can wire up to separate single AFCI breakers and you're all set, but you will need to handle-tie the breakers. If they do use neutral then you'll need to use a double-breaker, which may be more expensive than two singles.
  • GFCI only - Don't use a GFCI breaker. Use a standard double-breaker. Run the /3 cable to a good splitting point - e.g., your kitchen or workshop. Then split the white and connect one white with black to one GFCI/receptacle and the other white with red to another GFCI/receptacle. From that point forward you treat them as if they are two different circuits.
  • AFCI + GFCI required - combine the above - AFCI breaker + GFCI/receptacles after the split. Or depending on pricing you can use a combination AFCI/GFCI breaker to do everything in the panel.
  • If you have a 240V only device that only needs a 15A or 20A circuit but doesn't need neutral, you can use this cable. But in that case you must use black and red for the hots, even though with a /2 cable you would use white as one of the hots.
  • If you have a 240V/120V device (i.e., needs neutral) that only needs a 15A or 20A circuit, you can use this cable. There aren't all that many of these, but there are some ovens and other things that require all three wires but don't actually need all that much current, keeping in mind that 16A continuous (max for a 20A) at 240V is equivalent to 32A at 120V - which would require a single 40A breaker, and you never see that. So there are some devices where this will be useful as-is, though more common are things that need 10/3 or 8/3.
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    Would you believe Eaton makes all the way up to a CH170? I have no idea what you would use it for. I'm sure there's some strange scenario out there where this breaker makes sense. Come to think of it, maybe in a 3-phase scenario where you actually use it to pick off a single phase of 277v? Though it's not rated to that voltage. So I've got nothing.
    – KMJ
    Feb 18, 2023 at 8:42

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