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Situation: A 1950s house with some equally old wiring, and some remaining EMT wiring that is being put onto a new QO main svc panel (QO142M200PCAFVP). This is a plug-on-neutral panel. I believe per NEC2017, one is supposed to upgrade all 20AMP or smaller breakers to either GFCI or AFCI (I’m replacing with a combo GFCI/AFCI) when upgrading to a new panel.

One AC/BX cable in-particular (purple circle) which has two 14ga hots and one 14ga (shared) neutral is giving me pause. Back in the day someone went and wired separate circuits onto the separate hots. Each hot serves 3 non-grounded regular 15amp outlets, 2 switched 15amp outlets that were used for lamps, and one has an actual hallway light switch.

I originally put each hot to a separate 15amp GFCI/AFCI breaker (blue oval) and tried to throw the one neutral onto the neutral bar (white circle). This doesn’t seem to work (circuits on those two hots are dead). I believe this is because there is no continuity between the hots and the neutral, as with the new GFCI/AFCI plug-on-neutral breakers, the neutral is supposed to go on the breaker. Does that sound right? I can’t exactly put the one neutral onto one of the GFCI/AFCI breakers, that just doesn’t seem right.

Should I use non-GFCI/AFCI breakers (which was the case before and the two circuits worked)? If so, I would hope that the inspector would allow for this exception to requiring the GFCI/AFCI breakers, as I’m not changing anything on the circuits, just connecting them to a new panel.
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    I think you need a double pole breaker for this MWBC to work right. – JACK Aug 6 '20 at 22:13
  • Can you get us a closeup of the inside top of your panel please? AC and MC are not the same.... – ThreePhaseEel Aug 6 '20 at 23:14
  • You are correct, they are AC cables (thin bonding strip, no ground wire, but the jacket can be used as ground). Updated main thread. The cable in question is the 4th AC cable form the left here: diy.stackexchange.com/questions/200444/… – sil80 Aug 7 '20 at 0:01
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    Do any of the receptacles or loads on these circuits require GFCI protection? – ThreePhaseEel Aug 7 '20 at 0:11
  • They are all old receptacles, so technically, if I don't touch them - no. I was hoping to do the best I can and if possible, protect the circuit, but per the below answer, there is no such thing as a QO double gfci/afci breaker. – sil80 Aug 7 '20 at 11:42
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Three separate, but related, issues here:

MWBC

An MWBC must have at a minimum a handle tie for common maintenance shutoff. But the usual thing is to just use a double-breaker. Either one accomplishes two things:

  • Makes sure that if you turn off one half of the circuit for maintenance, the other half is off too. Otherwise you could end up with some dangerous situations on the shared neutral.
  • Makes sure that the two circuits are on different poles (i.e., 240V between them). Otherwise you could easily end up with an overloaded neutral.

GFCI

GFCI can be provided effectively at either the breaker panel or at the first device in each chain that needs to be protected. Two combination GFCI/receptacle devices often costs less than a GFCI/double-breaker and has the advantage of putting the GFCI reset button closer to point of use. On the other hand, a GFCI/double-breaker has the big advantage of simplicity. Once you get the breaker installed correctly, everything else is "normal" - no concern about LINE vs. LOAD.

AFCI

Unlike GFCI, AFCI is most useful either as a breaker (in this case, a double-breaker) or as a deadfront installed close to the panel. That is because AFCI protects against problems with receptacles, fixtures and other devices and also protects against wiring problems, so putting AFCI close to the start of the wiring run makes the most sense.

Note that code does not require GFCI or AFCI "everywhere". But assuming that you are required (or want to do so for any reason), the typical installation would be a GFCI/AFCI double-breaker. However, as far as I can tell, but one of the people who actually really knows this stuff can tell us for sure, Square D does not make QO double-pole AFCI + GFCI breakers. Which means that either you have to give up one of the functions, or get the AFCI protection in a double-breaker and push the GFCI protection to either a deadfront or replace the first receptacle in each of the two halves of the MWBC with a GFCI/receptacle device.

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    The closest thing I can find rated for 15A is this: se.com/us/en/product/QO215CAFIC That would take care of my AFCI needs and like you said, I could push the GFCI on individual receptacles, correct? – sil80 Aug 7 '20 at 14:02
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    @sil80 Correct. – manassehkatz-Moving 2 Codidact Aug 7 '20 at 14:21
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    Thank you so much for a great explanation! – sil80 Aug 7 '20 at 14:30
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In addition to the great answer above, I have found some good detail outlining the why behind the how: https://www.ahouseonarock.com/midlothian-home-inspector/what-is-an-mwbc/

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