AFCI randomly tripping, and related questions

Question

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.

Answer 1

AFCI

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.

Pigtail

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.

Answer 2

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!