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My 1993 home in WA state has a MWBC for the upstairs bedroom circuits, currently fed by a 2-pole 15A breaker (HOM215CP). This is probably no longer allowed for new constructions but it is what it is.

I recently modified one of the circuits to add new recessed ceiling lights, and I'm interested in adding AFCI protection on that circuit both for safety (in case of mistakes in the new wiring that might lead to arcing) and bringing that circuit into compliance with current code (unless there's an exception for MWBCs?). In theory, it seems that Schneider Electric makes 2-pole Homeline 15A CAFI breakers (HOM215CAFI), but realistically they don't seem to be stocked anywhere and SE lists them for $600 (more than I'm willing to pay).

I don't think I can go with two 1-pole handle-tied 15A CAFI breakers due to the shared neutral, so what options do I have here? Should I intercept this circuit in the attic (near the panel) and add a deadfront AFCI in a box? Would that even work on a MWBC?

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  • What is the make of your breaker panel? What is the list of approved branch circuit breaker types? I have to ask... some slap HOM breakers in everything because they're 30 cents cheaper than competitors, so the other breakers being HOM doesn't tell us much. Commented Jun 19, 2023 at 18:58
  • It's a homeline 200A 40-80 panel
    – peter
    Commented Jun 19, 2023 at 19:09
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    What SE lists as a price is evidently sometimes box quantities. Since SE doesn't actually sell direct, as far as I know, try an electrical supplier if you are striking out with hardware and/or big box stores.
    – Ecnerwal
    Commented Jun 19, 2023 at 19:18

2 Answers 2

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SE's website price is nonsense. No one pays that.

MWBCs are certainly allowed today; the only reason they're falling out of favor is that AFCIs are challenging. Now, some panel makers have AFCI MWBC solutions for their panels. Schneider is not one of them; HOM is their el cheapo line and they under-support it to protect their QO industrial tier line. If HOM was an all-singing, all dancing line, who would pay twice as much for QO? But they have no answer in QO either.

But since it's a retrofit, instead of AFCI at the breaker you can have AFCI at the first receptacle.

Of course, the fact that it's a MWBC complicates matters. Nobody makes 2-pole AFCI receptacles, so if you're sharing the neutral beyond the first receptacle, that is not going to work.

Here, you are saved by the fact that you're not required to have AFCI at all on the existing parts of the circuit because they're "grandfathered". Therefore you do not need to comply with the retrofit rules at all (which seek to protect the wiring in the walls, that being the top reason they want AFCIs on so much stuff). It's like if you have a 1954 Dodge and you put seat belts on it, the government can't tell you what kind.

Therefore you are at liberty to simply use AFCI receptacles at each receptacle location, don't use the Load terminals at all, and call it good. You can't split a receptacle, so you'll have to put the whole receptacle on one hot or the other.

On your 240V outlets on that MWBC, simply don't put AFCI on them at all. There is no achievable way to do that short of that unobtainium breaker.

On the circuit extension, AFCI may be required to be arranged so it protects the wiring in the walls. In that case, you can extend off the Load terminals of an AFCI receptacle, but not as an MWBC. You will have to pick one hot or the other.

If you want to extend both sides of the MWBC into an addition, then blow out that last existing receptacle box into a 2-gang box, mount two AFCIs there (one on each hot wire), then extend dual /2 cables (or /2/2 cable) with independent neutrals for each hot beyond that point. You will not be able to put any 240V loads on that extension, sorry.

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  • Very helpful, thank you. FWIW I didn't add any receptacles in the extension (only lighting loads), and it's all /2 in that part of the circuit. Should I just add a deadfront AFCI (or AFCI receptacle in the nearest junction box to where I started work? (Note: that box is in the attic)
    – peter
    Commented Jun 19, 2023 at 22:34
  • The NEC requires the protective device to be in a "readily accessible location", defined in the NEC as "capable of being reached quickly for operation, renewal, or inspections without requiring those to whom ready access is requisite to actions such as to use tools, to climb over or remove obstacles, or to resort to portable ladders and so forth." So in the attic would not work. Adding a new unneeded accessible outlet may be your best option, but still doesn't quite satisfy the requirements, so an inspector could just say you just can't extend that existing circuit. Commented Jun 20, 2023 at 15:23
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Realistically, if you can't find stock right now due to (whatever forces - supply chain, states adopting NEC 2020 creating more demand, etc.) you should just continue to live with the grandfathered breaker you have until you can find a CAFCI replacement.

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