Warning, this may be a bit long ... I'll do my best to be brief but it's a somewhat complicated situation. Primary issue is that the AFCI breaker trips in a very random pattern that I can't figure out; secondary issue is that said breaker seems to have way too big a load and I don't fully understand the wiring.

The basement in my single-family house was finished by the previous owner in what looks like a combination of professional and DIY work. There are a few weird things about the wiring:

  1. Almost every light (20!) and the outlets in the room that was finished as a bedroom are one a single, 15A, AFCI circuit breaker.
  2. This breaker is wired to a 14/3 NM-B (Romex) Cable, with BOTH the red and black wires connected to power at the breaker.
  3. In the basement this cable goes into a junction box, where it is connected to two other cables - the black connects to the black of a 14/2 cable that powers all the lights and outlets in the bedroom; the red connects to the red of a 14/3 cable that powers all the other lights in the basement.
  4. There are multiple switched lighting subcircuits that fan out from each of these lines - 4 in the bedroom, and 6 in the rest of the basement.
  5. As stated above, the AFCI breaker occasionally trips, with no discernable pattern or cause. There can be a lot of lights on and it will trip, or it won't. There can be just a couple of lights on and it will trip, or it won't. There is no pattern to which lighting sub-circuits cause it to trip, or time of day, or time of year, or anything.

Here are my questions:

  1. How can I figure out what is tripping the AFCI? Given the complete lack of a pattern I'm at a total loss here.
  2. From what I've read it sounds like I don't actually need an AFCI and they can have a lot of nuisance trips; can I just swap it out for a regular breaker?
  3. Obviously there is a big load on this one breaker. I'd like to split it out into two circuits and there is a path to this given that there are already two circuits in the basement coming out of this junction box. I've read where I could connect the red and black wires from the 14/3 cable to two separate breakers in the junction box and the have the neutral be shared for the two circuits in the basement. Is this safe and does it meet code?

Thanks in advance for any help - this has been a pain for the last 2.5 years since I moved into the house; I'll really be thrilled if I can get it fixed!

What I'm thinking I could do is replace the AFCI w/a regular breaker, then connect the red wire to the unused breaker you can see in the picture, and connect the red wire to one of the two circuits that connect in the junction box; in this way I'd have two separate half-circuits on two separate breakers so the load wouldn't be so great.

Here's the whole panel

  • This is where the pic of the whole panel would help ... that's 1/2 of a 240V circuit. You can kind of see where the trip bar (hope the name is right) is bridged to the breaker next to it.
    – joecool
    Aug 1, 2019 at 21:00
  • some labels on the new pic of the entire panel would help.
    – Skaperen
    Aug 1, 2019 at 22:06
  • in the junction box, the 14/3 going to the other lights, is its black wire connected to anything or capped?
    – Skaperen
    Aug 1, 2019 at 22:42

2 Answers 2



AFCI generally detects problems with wiring, but can be triggered by certain devices that may or may not be a dangerous situation. The more devices, receptacles, wire connections, etc., the more likely that (a) real problems will crop up that are actually being protected by the AFCI and (b) false "nuisance" trips will happen. But until you know for sure, you should assume the trips are "real".

Do you need AFCI? Probably, but depends on location. Unlike GFCI, which is life-safety based such that out-of-reach (e.g., lighting) is normally exempt), AFCI is fire-safety based so lighting is as in need of protection as anything else, subject to local code requirements.


You can connect two circuits to one breaker. You use a pigtail to connect to the breaker on one end and the two separate circuits on the other end (pigtail plus the other 2 wires in a wire nut). However, that doesn't really gain anything here since your total current is limited by the neutral wire, unless you go to an MWBC.

Multi Wire Branch Circuit - MWBC

What you have described as a solution is a MWBC. Basically you have two hots and one neutral == /3 cable - connecting to two breakers. There are a number of catches to this:

  • The breakers must be on opposite legs. Traditionally that has been easy to do. However, if you were thinking of "replace a full size breaker with a double-stuff pair of 1/2 size breakers", that won't work here because that would be on only one leg. There are, for some panels, quad 1/2 size breakers where one of the pairs can be used this way, but that is very panel dependent. The simple way to look at it is: If installed correctly, red to black will measure 240V, red to white (neutral) 120V, black to white 120V. If you get red to black 0V then everything is on one leg == NO GOOD.
  • The breakers must have a common maintenance shutoff because otherwise neutral current could be quite dangerous when working on 1/2 the circuit when the other 1/2 is off.
  • AFCI and GFCI get messy unless handled as part of the breaker.

Note that an MWBC is not "two separate circuits". It is one multi-wire combined circuit. Effectively two circuits in one, but the savings being 3 wires (plus ground) instead of 2. Neutral "cancels out" which is why it works in general but also why it must be on a properly installed double breaker.

Two truly separate circuits would require 4 wires (plus grounds) - i.e., each circuit needs its own paired hot & neutral. An MWBC shares the neutral in a safe way.

  • Re/whether or not the trips are real, I would think it's unlikely that there would be multiple bad connects across the circuit, but as I stated, I have had it trip on multiple different combinations of lights being on ... and also not trip at all for months with the same lights being used regularly. Is it really possible that there could be multiple bad connections in there somewhere that would be so intermittent? I feel like the failure pattern doesn't really point to bad wiring or a bad device. But, I don't want to be stupid and ignore it if there is a chance it's a real problem. Thoughts?
    – joecool
    Aug 1, 2019 at 17:48
  • I was told by an electrician the only way to determine if there is bad wiring is to check the wiring at every device in the circuit ... something I don't want to pay an electrician to do given how many connects points are on the circuit, and something I'm kind of loath to do myself. Am I just stuck doing this to make sure everything is OK?
    – joecool
    Aug 1, 2019 at 17:49
  • Finally, It sounds like trying to use the existing 14/3 to create two separate circuits off two separate breakers is problematic. Sounds like the best thing is just to pull a new wire then, right?
    – joecool
    Aug 1, 2019 at 17:50
  • It is a bit off a guessing game. FYI, particular lights on/off would make a difference if the problem were in a particular light fixture. If there is ANY current flowing (even just one light on) then a wiring problem anywhere in the circuit (which you described as quite a large & complex circuit) could cause a trip. Checking every single connection - redoing anything that is even a little "bad" - replace backstab connections with screw connections, replace doubled wires with pigtails (which is, by the way, something you could do with the existing breaker), replace any wire nuts that are loose Aug 1, 2019 at 17:53
  • would be a lot of work but hopefully solve the problem. But not a guarantee - it is actually possible you have a nail into a cable inside a wall somewhere - a classic problem that an AFCI will catch and prevent from causing a fire. The only foolproof solution would be new wiring. Aug 1, 2019 at 17:54

Unfortunately that GE AFCI breaker is an "alien breaker" which does not belong in a Siemens panel. It needs to go. Sell it on Craigslist. Whether to go with a 1-pole AFCI or 2-pole AFCI will soon be discussed.

What you had was a multi-wire branch circuit, which is 2 hots sharing a neutral. It's a common technique to get two 120V "quasi-circuits" out of one cable. It requires careful breaker choice and placement with many ways to botch that up. As far as we know, there was nothing wrong with the MWBC.

The last guy converted to AFCI. He didn't do the conversion for chuckles. He did it because he had to. That means you are not at liberty to remove it.

He converted it to a simple circuit, by joining the hots at the panel. There is nothing wrong with doing this, except he can't put 2 wires on a GE breaker, so he should have pigtailed. Trivial flaw. He did that because either he didn't know how to connect an MWBC to an AFCI, or they didn't make 2-pole AFCIs for this panel at the time, or he was too cheap to get one.

I have to say, the "join the MWBC" plan isn't so unreasoanble in the age of LED lighting, where 200 watts of lighting does the work of 1200. Since you haven't been reporting any overcurrent trips, I don't see why you're "complicating things" by chasing a non-problem.

Correctly breakering an MWBC

Obviously if two hots feed the same neutral, the neutral will overload. MWBCs exploit a feature of split/multi-phase power to get the neutral to carry only differential current, but the hots must be on different poles/phases.

In the old days, this could be done with careful, competent placement of the breakers. However nowadays idiots open up panels and move things around. Further, the Electrical Code now absolutely requires "common maintenance shutoff" for the two half-circuits, so at the very least, breaker "handle ties", but those aren't perfect so we recommend a 2-pole breaker.

Regardless, you have to use a 2-pole breaker because a 2-pole AFCI is the only thing that can support a MWBC, given your panel.

You need to use a Siemens 2-pole AFCI, or an Eaton CL (not BR) 2-pole AFCI.

Solving the AFCI trip problem

The root of your problem here is the nuisance trips. These are happening because arcing is occurring somewhere in your wiring.

Unfortunately, your option is to go through all the wiring carefully and look for loose connections - wire nuts improperly done (don't survive a "pull test"), or the #1 source of arc faults, backstab connections.

The GE AFCI is an alien breaker and does not properly fit the panel bus. This could create arcing, damaging the bus stab. In fact, that may be the arcing it is detecting!

  • So you are saying there was probably an MWBC there before, but this was converted to a single AFCI with both sub-circuits tied to the AFCI, which is actually perfectly OK, correct? Re/chasing the wrong problem, how can I tell if the trip was an arc trip or an overcurrent trip? Part of the reason I'd like to split this out into two circuits is it's just kind of a nuisance to have to turn off basically every light in the basement to work on anything. I understand that if I want to split it out I really need to pull another wire. This would also help isolate the arc right?
    – joecool
    Aug 1, 2019 at 18:38
  • Sorry, another follow-up ... while it sounds like connecting two wires to the breaker (through a pigtail) is actually ok, why not just connect one wire (say, black) to hot at the breaker, and then wire the two sub-circuits to the black wire in the junction box? Doesn't this achieve exactly the same result, without the somewhat confusing and unnecessary use of the red wire for power?
    – joecool
    Aug 1, 2019 at 19:25
  • if you cap the red wire on both ends you can abandon it.
    – Skaperen
    Aug 1, 2019 at 22:20
  • arcing can also happen in cheap or worn out switches, going on or going off, especially when going off. AFCI detects arcs as a high frequency current in a short duration. such current can readily be induced into the other wires and even trip GFCI or those AFCI that include ground fault detection. arcing motors like AC/DC universal motors can do it. nearby radio transmitters can do it (i have tripped GFCI from 222 MHz).
    – Skaperen
    Aug 1, 2019 at 22:32
  • @joecool to distinguish from an arc trip to an overcurrent trip, you'll need to read the pamphlet on your breaker. It will have a way of showing that. I think it is reasonable to re-split it into 2 subcircuits and restore it as a MWBC, for that very reason. However, this won't solve your "turn off every light" problem because the MWBC halves must be handle-tied. Yes, you can pull another cable and defeat the MWBC entirely, pulling a single neutral wire is not legal nor safe. . Aug 1, 2019 at 22:34

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