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I've isolated the problem, but unfortunately I'm not hip to all the terminology to ask the appropriate question and decipher the answer. I am going to break it way down to (try) to avoid confusion

Long story short the same 15A AFCI (arc-fault) breaker was tripping when I turn on fan in bedroom #1 or #2. I pulled the panel cover off to see that there are two hot wires wired into one 15A breaker. One red and one black, then the white neutral on the backside.

And there is one 15A AFCI breaker that is completely empty even though one was labeled for each bedroom.

Bedroom 1 and 2 share the same cable (series of wires), its a 4 wire cable consisting of red, black, white, and copper.

So both the red and black wire from the same cable (series of wires) is run into the same breaker, powering two rooms with one breaker.

I put one of these wires, we will say the red one, into the empty 15A breaker. The only problem is since they share the same cable I don't have a second neutral to wire into the back of the newly occupied breaker so everytime I turn the fan on in the room now powered by the red wire the breaker trips.

My question is can I wire nut two new wires into the neutral wire and run one each to the back side neutral nuts in the breakers?

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    What make/model is your panel? Aug 24 at 23:05
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    Or for that matter, can you post photos of your panel, including the breaker that's tripping and the labeling on the inside of the door, please? Aug 24 at 23:29
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You have a Multi-Wire Branch Circuit or MWBC.

I put one of these wires, we will say the red one, into the empty 15A breaker. The only problem is since they share the same cable I don't have a second neutral to wire into the back of the newly occupied breaker so everytime I turn the fan on in the room now powered by the red wire the breaker trips.

Yup, and that's to be expected. That's why AFCI and GFCI breakers don't work with MWBCs, with rare exception.

My question is can I wire nut two new wires into the neutral wire and run one each to the back side neutral nuts in the breakers?

Nope, that will not work. That will only trip both breakers anytime any load connects.

So both the red and black wire from the same cable (series of wires) is run into the same breaker, powering two rooms with one breaker.

Because that's the only way you can make it work. However there is nothing wrong with this confguration; it doesn't violate any rules.

There are no rules anywhere that say 1 circuit must power 1 room. In fact that's a waste of materials, forcing you to encircle each room and every wall has 2 cables passing right by each other. Silly. Cheaper and more sensible to run 1 circuit down each wall, and serve rooms on both sides of the wall to the extent allowed by Code. This also gives each room access to 2-4 circuits - handy if you're tripping breakers due to heaters, home office, etc.

I gather this AFCI issue was not known to the installer when they chose to install an MWBC? That's a very typical amateur move. It raises questions of what else they may have "screwed up".

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  • Both "legs" are on the same phase - is it still classed as an MWBC?
    – user253751
    Aug 25 at 16:38
  • @user253751 Both legs of an MWBC on the same phase will overload neutral, since neutral is returning the sum of currents from the 2 hots (e.g. 15A+15A = 30A on the #14 neutral WHOOPS!) The only exception is the situation OP found, where they are pigtailed onto the same breaker screw. That is fine since the sum of the hots won't exceed 15A, therefore neither will the neutral. Aug 25 at 18:48
  • However, is it still pedantically classed as an MWBC?
    – user253751
    Aug 25 at 20:18
  • @user253751 i would say accurately classed. Multiple (hot) wires in a branch circuit is the literal definition of a MWBC. Aug 25 at 22:57
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You can do that, but there's no particular reason to think that it will work correctly, since the neutral current does not know which breaker it should go back to. That certainly would be expected to fail with two separate GFCIs, I don't know how much AFCI's look at differential currents.

Use a two-pole 15A breaker (assuming you have 14Ga wire somewhere in the circuit if it's 15A now - if it's all 12G you can use a 20A) and you can expect it to work.

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This is called a multiwire branch circuit, after the 2008 code change most locations require them to have a handle tie. Multiwire branch circuits have really gone out of fashion because the common neutral causes problems with GFCI’s and AFCI’s, the fan motor on a circuit that is dedicated may still have problems depending on the actual type of motor / speed controll. But yes what you have is still code compliant today with a Handel tie.

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    Handel tie? Are we Bach to that again? Aug 25 at 0:08
  • Other than a double pole that is the only way since 2008 @manassehkatz since the op was not aware of the circuit type.
    – Ed Beal
    Aug 25 at 3:28
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    I agree with handle-tie or double-pole (regular either way, for AFCI has to be double-pole) - my comment was about the spelling of Handel Aug 25 at 3:36

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