I'm currently building an addition to our house and in the final phase of wiring - installing the switched, receptacles and fixtures. I am running into an issue and was hoping to get some help troubleshooting this. I have 3 light circuits - one for the garage and exterior lights, one for the lower level, and one for the upper level. These are all on 15 amp SquareD QO CAFI breakers.

I had the garage part already wired and working properly, as well as the upper level. Yesterday, I installed switches in the lower level. There is one pair of 3-way travelers going from a box on the garage circuit to a box on the lower level circuit. I am however not using this set, I am only switching those lights from the garage box currently (the cable - NM-B 14/3 with ground - is just for future use if we decide to not use motion sensor lights).
I had by accident tied the neutral from that set in with the neutrals in the box for the lower level. When I went to turn the garage breaker on, it tripped immediately. I then figured out my mistake and removed the garage-circuit neutral from the lower level bundle. I could now turn on the garage breaker. But when I try to turn on the lower level breaker, it trips right away.

I went through the steps for the QO time saver diagnostics and it trips at 0 seconds, indicating one of the following:

Arcing to ground
Shared neutral
Grounded neutral
Ground fault

My plan now, instead of removing all my switches, I would start in the middle of the run, disconnect a set of load wires, and see if it still trips, indicating which half of the circuit the problem is, and continuing like that, inspecting all my previous work. I do have a DVOM, and a circuit traver available. My questions are:

  • What should I look for specifically? My theory is maybe a bare ground slipped and is touching a hot or neutral.
  • Does the diagnostics indication from the breaker make sense with the previous issue in mind (other breaker tripper when neutrals were connected)?
  • Is there a quicker/easier way to troubleshoot this then my idea?
  • Is my idea the "correct" way to troubleshoot this?
  • What type of cable did you use for the "3-way travelers"? Commented Feb 3, 2020 at 18:05
  • NM-B 14/3 with ground. Currently all conductors not connected in each box (terminated with wire nuts), except the grounds are all connected. Commented Feb 3, 2020 at 18:11
  • Is the lower level bundle wired directly into the main panel?
    – HoneyDo
    Commented Feb 3, 2020 at 18:52
  • What do you mean by directly? Whether there is a subpanel in between? There is a main panel outside (just a shutoff, one main breaker only), then there is only one "subpanel" that has all the breakers for everything, so neutral and ground not bonded on this panel, only outside. There are one or two other switch boxes in line before the box where I had the neutrals inter-twined. Hopefully that answers your question. Commented Feb 3, 2020 at 18:59
  • I ended up opening up all boxes again just to be safe, I did find one black wire that had been knicked by the sheetrockers and seemingly was touching a bare ground. Commented Feb 17, 2020 at 18:57

2 Answers 2


It's possible you have a ground wire nicking a hot or neutral. The easiest way to remove that issue is wrap the outlets and switches with a couple loops of electrical tape so the screws are covered. I do that in metal boxes/EMT, and that doesn't even have ground wires. Also, if the screws are all the way out because they came that way from the factory and you use the backstabs, that creates a larger surface area for that to happen, and the answer is stop using backstabs!

However the more likely threat is your wiring topology.

The most basic rule is currents must be equal in each cable. Now, the rule is relevant to EMF emission (on a balanced cable, all currents cancel each other out, so their EMFs do also). However, it's also relevant to GFCI, AFCI and CAFI, since those are looking for differential current as a sign of a ground or arc fault. But this rule has a very valuable corollary:

Tree topology is safe. Mesh topology is dangerous.

If you consider a tree, it has many branches, but the branches never rejoin. Quite different from a roadway system, which is a mesh - you aren't obliged to go back the way you came. Consider ants exploring a tree all day and going home in the evening. Ants can't jump branches. You could put an ant turnstile anywhere on the tree, and if 130 ants went up past that point, 130 ants come back down. Currents are equal anywhere in the tree.

So as long as your wiring topology is that of a tree, it's hard make a shared neutral mistake.

Now, sometimes your cable topology needs to enter a box from 2 separate directions (e.g. that 3-way case). You can either use 2 boxes, or you can use a box divider (real or mental) to fully separate the cables so it's effectively 2 boxes.

Nowadays with GFCI you have to be really fastidious about this stuff. Before, you could play fast and loose and it would "work" if nobody put a clamp meter around the cable, but that GFCI or CAFI is the clamp meter.

Multi-wire branch circuits need a 2-pole GFCI/CAFI

Perhaps you are using an older trick: the Multi-Wire Branch Circuit or MWBC, also called a "shared neutral" circuit. Two hots on opposite poles share a neutral. Perfectly safe and proper when done correctly, but doesn't play well with single pole GFCI/CAFI. You need to use 2-pole GFCI or CAFI for that.

  • Fully agree even pros nick a wire every now and then. I would be using GFCI in the shop as AFCI’s will be a problem with compressors, welders, corded power tools.
    – Ed Beal
    Commented Feb 3, 2020 at 23:31

Your theory seems to make sense based on the diagnostic. The obvious problem is finding the fault. Appears that the only reason the garage breaker tripped is you had it tied into the lower level circuit via the common which is where the ground fault or arcing is apparently located. Since the lower level is all one circuit your approach of halving the circuit and disconnecting the load at that point shouldn't take too long - just keep repeating this by taking half of the remaining portion of the circuit by disconnecting the load and based on the result go upstream or down stream until you zero in on the problem. If that doesn't work go back and check all your connections in the subpanel.

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