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I'd like to add an AFCI to the middle circuit here. But that circuit shares a tandem breaker with the circuit above it, so I don't know if there are any subtleties of this setup that I don't understand. They don't appear to be a multi-wire branch circuit--they have separate neutrals going to the neutral bar. Can I just move the middle circuit to a new slot?

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  • Oh, I know. Red and black both going to the same duplex breaker is a red flag for the most common MWBC mistake. Just to ease the workload of the next guy, you might want to rearrange the aesthetics. – Harper - Reinstate Monica May 19 '16 at 19:25
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    DO NOT arbitrarily rearrange for aesthetics. Do it wrong and you overload the neutral, and not only increase the workload for the next guy, but you compromise the install. Color coordination is preferred, but I'm unaware of it being a code requirement. AFAIK, the CBs are to be linked, and (the three wires are to be) identified where they enter the box, and secured together with a zip-tie or tape. – Mazura May 19 '16 at 19:54
  • I'm seeing something new this viewing. We see 3 hots and 3 neutrals, so that red wire beng red is a mystery, because the sides of the pic are cutoff, we can't get the full story. (Perhaps someone used a piece of red to lengthen a black within the panel.) I agree with @Mazura tho, do not rearrange for aesthetics... If the red and black of /3 NMB are both hots leaving the panel they must use a double pole breaker regardless of whether it's 1-240v circuits or 2-120v. In the latter case the neutral will be overloaded unless the hots are on opposite poles. – Tyson May 19 '16 at 21:45
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You have a panel with 12 spaces and you are using 4. Just move the red wire to an AFCI in a new space in the panel.

I would also move the lower black wire to where the red wire is now, so it becomes visually obvious that both wires are meant to be on the same pole. I'm all for using a rainbow of colors in conduit/THWN, but in residential use with NM cable, red always means something special in some way.

On a tandem (I call them double-stuff) breaker, both circuits are on the same pole. It is very easy for someone to confuse them for a 2-pole breaker, which MWBC's require for safety. So when an electrician sees a panel with a double-stuff with red and black wires, he'll pause and follow the wires just to make sure that mistake wasn't made.

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  • The preferred term is tandem circuit breaker. double and duplex are apt to be confused with '2-pole'. Anecdotally, my dad is apt to think that a 2-pole breaker most likely feeds a 220v load, not a MWBC (the 2-P or "linked" breakers requirement is a rather new addition to code). – Mazura May 19 '16 at 20:02
  • The picture doesn't clearly show the story. We don't know if the red is on the same or opposite poles as it's black. The odds are it's wrong tho. Whether this is a single 220v 20amp circuit or two 110v circuits sharing a neutral it must be on a double pole breaker (not a twin). The 220v is more obvious why, but two 110v circuits sharing the same neutral must also use a double pole breaker to ensure the remain on opposite poles otherwise the neutral will be undersized. – Tyson May 19 '16 at 20:06
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    @Mazura I wish there was one word... duplex and tandem are both widely used and widely confused. I've run into this before and checked. Makes me crazy, which is why I prefer to call them double-stuff. But like "fetch" it probably isn't going to happen. And yeah, I've found out all about the recent tying requirement for MWBC, having been left a factory full of them... We are complying even though we are grandfathered. – Harper - Reinstate Monica May 19 '16 at 20:33
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    Mostly there, except for sourcing the handle ties, which are sold only by yetis. The copper thieves were a huge help, because they forced me to ring out every circuit, so I discovered all the crossed neutrals, a furnace hot using neutrals on several different MWBCs, fan hots taking neutrals from different service panels which were on different phases, you name it. – Harper - Reinstate Monica May 19 '16 at 22:18
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    Oh! You're in conduit. Sorry, I'm used to everything in residential being NM cable (Romex). In conduit, red is not special, use any color you like, long as it's not green or gray. – Harper - Reinstate Monica May 20 '16 at 5:20
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I'd move the breaker with the red wire (the one you're replacing with the GFCI) to the other side of the panel as access to the neutral bar will be easier.

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