My house was built in the early 60s, I have 4 circuits that are created by using 2 3 wire cables, each hot leg goes to a 120v breaker side by side in the panel. The neutral goes to the neutral bus. I don't see any way the breakers could have ever been tied together. The cables split up somewhere but with all the insulation in the attic I cannot see where the splits are. I'm replacing my entrance panel and have to install afci and gfci breakers so at the breaker I will end up short one neutral wire for each pair of circuits. Can I use a short piece of wire from the neutral bus or split the one neutral or will I have to re wire the circuits so each has its own neutral. I have the electric company coming Wed to shut of the power, I just noticed this wiring today while labeling the wires.

  • See NEC 2011 Article 210.4 Multiwire Branch Circuits. – Tester101 Apr 10 '12 at 17:25
  • @Philip Ngai - GFCI Breakers require a neutral return so they can sense the difference in current between the hot and neutral wire, don't they? That difference is the ground leakage. Most of the ones I've worked with expect the hot and neutral from the protected circuit to connect to the breaker and the pigtail from the breaker then goes to the neutral buss bar. So each circuit would then require it's own neutral. – Fiasco Labs Jun 29 '13 at 17:13
  • Fiasco Labs, you are correct. I deleted my comment. – Philip Ngai Jun 30 '13 at 5:38

I think you are referring to a shared neutral/multi-wire branch circuit. These are commonly found in kitchens. They were and still are to code in many areas, but typically I believe their use is limited to kitchens.

You should run a new cable back to the breaker box. If you were to run an independent cable then it is possible for it to break independently of the rest of the cable, which would leave you with an open neutral which can cause all sorts of problems. Unless your cable branches from another box, you cannot just run a neutral from one box while the hot comes from another circuit. You have to run a new direct wire anyways, so why not just pull a new cable - it's just as much work as pulling a single wire.

If you do opt to keep the multi-wire branch, the breaker should be tied together so that there is not the possibility of one half being off and the other on (a possibly dangerous scenario if someone assumed the power was cut because one of the circuits did not test live). Likewise, if it trips because of a short, there is a good chance the other hot is close by and you want the second breaker to trip too.

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  • Thanks, it looks like I will have to postpone installing the entrance panel to give me time to straighten up the wiring. On one 15 amp leg they put hall lights, porch lights, both bathrooms, part of one bedroom and the forced air furnace and I want to split that into 3 circuits so I might as well do it now. Do you know if anyone makes a 240 volt afci breaker or a separate attachment to connect two afci breakers together. – Dave Apr 10 '12 at 0:32
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    Yes, there are 2 pole AFCI's. They make them for straight 240V and also make them for shared neutrals. Always match the brand breakers to the panel manufacturer. – lqlarry Apr 10 '12 at 0:55
  • One more thing to check; normally, "2-phase wire" (3-conductor wire intended for use with 240V) has a thicker neutral wire than wire intended for single-phase use (3-way switches, split switched outlets, fan/light setups). This is to ensure the neutral can handle the combined wattage of the two phases that will be using it. I'm pretty sure normal white-jacket 14/3 Romex for a 2-phase circuit isn't code-compliant, because the 14AWG white wire would be expected to handle roughly double the wattage it would see in its intended use. – KeithS Apr 10 '12 at 16:37
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    I'm not important enough to leave comments, so I'll leave this here. Regarding the comment from @KeithS above... I think by "2-phase wire", he means "spilt-phase". In any event, the worst-case scenario in terms of current loading on the neutral wire occurs when there is zero load on one side and full load on the other. In the situation where there is full load on both sides, the split-phase is 'balanced' and there is zero current on the neutral. – pilotcam Apr 10 '12 at 19:14
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    Now, if by "split-phase" you mean a circuit with two "hots" that is created by reversing the polarity of one of the supply conductors, then yes, on the neutral the return current will fully cancel because the waveforms are exactly opposite, and the neutral can never carry more wattage than the full load of one side. I have never heard of this being done in the U.S. on purpose, and in fact this is why 2-prong outlets must be polarized; two devices running on reverse polarities can induce deadly arcing. – KeithS Apr 10 '12 at 19:58

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