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The circuit breaker for our kitchen lights trips instantly when I turn on ANY light switch on that circuit. It is 6-month-old new construction and everything worked just fine until a week ago. And then this behavior suddenly started without us changing anything.

It is a 15A circuit with five switches:

  • Switch #1: Lutron Caseta wireless dimmer with 6 LED recessed lights (7.6W each)
  • Switch #2: Lutron Caseta wireless dimmer with 8 LED recessed lights (7.6W each)
  • Switch #3: Lutron Caseta wireless switch with 4 LED exterior lights (10W each)
  • Switch #4: regular toggle switch with exterior motion detecting flood light (36W)
  • Switch $5: regular toggle switch for pendant lights but no lights installed yet.

The circuit breaker looks like this: https://www.homedepot.com/p/Square-D-Homeline-15-Amp-Single-Pole-Combination-Arc-Fault-Circuit-Breaker-HOM115CAFIC/202353305?irgwc=1&cm_mmc=afl-ir-27795-483420-&clickid=VB9TeuUQPxyOWTXwUx0Mo34GUki2SIXtR1fETE0

  • With no switches on, the breaker remains on.
  • If I turn on any of the switches 1-3, the breaker instantly trips.
  • If I turn on the switch 4, the breaker trips in about 2 seconds.
  • If I turn on the switch 5 (no lights), the breaker doesn't trip.

The breaker has a white "test" button. The test button trips the breaker ok.

Given the fact that breaker trips when turning on ANY switches with lights installed, can this be due to a faulty breaker? But again, it's brand new and I don't know how likely it is.

Or can it be a faulty switch? But the breaker doesn't trip when no switches are on. Can a faulty switch cause other switches to trip the breaker?

Also, I don't know why the breaker trips after 2 seconds with the motion detect light, while it trips immediately with other lights.

What are some possible explanations for this behavior? Is there anything I could try before hiring an electrician?

UPDATE 1: Per @ThreePhaseEel's suggestion, I tried the Square D breaker's built-in diagnostics (holding down the white test button and turing on the breaker), and it tripped immediately. According to the instruction, it means "Fault to ground (arcing to ground, grounded neutral, shared neutral, ground fault)". What can cause this after 6 month of use?

UPDATE 2: I checked the switch wiring. AFAIK, the wiring looked ok. However, switch #5 seemed faulty because my pen-type voltage detector detected voltage from both black wires even when then switch was off. I changed it to a different switch. Switch #4 was backwired but no wire's exposed.

I tried to untangle wires so that no wires/switches are touching anything, but sadly the breaker continued tripping even after that with turning on any of switches #1-#4 (instantly for #1-#3 and in 2 seconds for #4).

Guess it is time to call an electrician, but I'm attaching the pictures of the switch boxes in case anyone can find anything suspicious.

One thing I forgot to mention above was that both switches #1 and #2 are originally wired as 3-way (switch boxes 1 and 2 below), but they are wired as instructed in pages 3-6 in https://www.casetawireless.com/documents/0301710a_caseta%20advanced%20inst.pdf with remotes.

Switch box 1

Switch box 2

Switch box 3

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  • Which switch did you try last? Also, can you turn the breaker off, then press and hold its TEST button while turning it back on, then time how long it takes from turning it back on to when it trips again and tell us that time? (This is as per the diagnostic procedure found in this flyer) – ThreePhaseEel Apr 25 '20 at 22:54
  • @ThreePhaseEel Thank you for the info on the diagnostics. I tried it and it tripped immediately. According to the instruction, it means "Fault to ground (arcing to ground, grounded neutral, shared neutral, ground fault)". For your first question, the first four switches would trip the breaker immediately. – user2356172 Apr 26 '20 at 0:34
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First, all our answer rely on your claim that your breaker is "just like this one". That one is a CAFCI. If your breaker is somewhat different, then all our advice is no good. The good news is Square D takes button color pretty seriously, and white-button Square D's should all be CAFCI.

Chasing an arc fault

Move through each of the switches, then receptacles, then lamps and check these things.

Ground wires touching screws. Make sure that as the devices are pushed back into the box, you don't have a ground wire getting near a hot or neutral screw.

Backstabs. Arc fault breakers were initially developed for electric blankets. But they discovered they were really good at detecting another wiring fault: Backstabs. Backstabs are when you have #14 wire and you just jab it into the provided hole on the recep or switch. . The mechanism is surprisingly hokey - after all you are getting 4 of these for 50 cents. They regularly cause circuit burnout, and AFCI breakers are proving they cause a lot of arcing before they burn out.

So I recommend working through the switches and receps on that circuit. For each backstab connection, firmly pull and twist the device back and forth while pulling the wire steadily out (no need to waste wire length unless you have loads of it). Then move the backstab wire to the nearest screw. If the screw is already occupied, someone is cleverly using that trick to do a splice, and you should pigtail those 2 wires together with a pigtail going to the switch. I do not recommend getting any fancier than that, or else you need to take a crash course in replacing receptacles - issues like tabs broken off, 3-way terminal colors, all that jazz. It's way more complicated than it ought to be.

Wire nuts. Bad wire-nut jobs are a common cause of arc faults. First, are the wire nuts taped? Exception: A wire nut on a single wire should be taped (otherwise the nut will fall off).

Tape on a wire-nut usually means the installer has learned that wire-nut connections will fall apart if not taped. That is because the installer's bad technique. The same thing that makes them fall apart also makes them arc. So, the moment you see tape on a wire-nut, take it off. Then, give the wire nut a "pull test". Grab the nut proper and pull hard on each wire one at a time. If the wire pulls out, that was a bodge job that needs to be re-done. Make sure the bared section is at least 80% of the length of the wire nut and no more than 95%. It will stick out initially, but get sucked in as you twist.

Wire length is precious - do not cut off the curly bits, just coarsely straighten them. Put the wires back together with ends even. Then put a new, good wire nut like an Ideal brand on it, and crank HARD, and clockwise - same as screws, bolts and nuts. DO NOT pre-twist - if you are tightening hard enough, the wire nut will do that for you, believe you me! If the wire nut is not twisting the wires, you're being way too much of a soft touch. This is one place where you want "gorilla tight, not monkey tight".

Now you do a pull test on it just like that, and it better pass with flying colors. If not, refine your technique.

If you're having trouble getting multiple wires to stay even as you set them up, feel free to tape the wires together, say into 2 groups, about 2" back of the wire end, before putting them into the nut. Then nut it, remove the tape and pull-test.

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  • @harper-reinstate-monica Thank you for the detailed reply. Yes, the breaker has the white test button. Looks like those two flip switches can be backstab wired. I will check the switch wiring and wire nuts and update. BTW, there are no receptacles on this circuit, only lights. I also updated the original post with the breaker diagnostics - "Fault to ground (arcing to ground, grounded neutral, shared neutral, ground fault)" Can any of the problem you mentioned cause "fault to ground"? All the wires in each switch box belongs to the same circuit, so I am not sure if shared neutral is possible. – user2356172 Apr 26 '20 at 0:54
  • @user2356172 Far more likely ground wires touching screws then. Or possibly a plain old GFCI ground fault, and there are lots of ways for lamps and fans to do that. AFCI breakers detect parallel (H-G N-G) arcing by doing (weak) GFCI detection. They basically don't care if it's arcing or not, any H-G or N-G flow trips. – Harper - Reinstate Monica Apr 26 '20 at 3:22
  • @harper-reinstate-monica Thanks. Checked what you mentioned but things look okay to my novice eyes. I added the update (#2) to my original post with my findings and pictures of the switch boxes. – user2356172 Apr 27 '20 at 0:32
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It could be the breaker is doing its job correctly and detecting an Arcing fault. (In new construction it would be common for a nail or screw to have pieced the insulation of the wire. This often won't cause a failure immediately but over time as the house sifts and moves it will eventually cause a fault.)

If the fault is somewhere between the breaker and the first switch there will be no current flow, and therefore no arcing until at least one switch is turned on.

It could also be a faulty breaker. Either way I would recommend contacting an electrician. Finding the fault is probably going to require tracing the wire and breaking the circuit up into smaller pieces until the location is found, then opening walls.

If the home is new construction consult any warranties the builder may have provided. Often this kind of issue is covered under a 1 year guarantee but you have to contact the builder who will use their own crews to repair the issue.

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    Yeah, given that this is an arc to ground, I'd put my money on this being a workmanship defect in a box somewhere, (i.e. a screw being pushed into a ground wire, or a wire pinched against a metal box) – ThreePhaseEel Apr 26 '20 at 0:46
  • @ThreePhaseEel Thanks. I updated the original post with my findings and pictures of the switch boxes. – user2356172 Apr 27 '20 at 0:37
  • @mfarver Thanks. I agree that it is time to call an electrician. – user2356172 Apr 27 '20 at 0:39
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If you are comfortable with and your skill set is such that you can do this, I'd turn off this breaker and another one with the exact rating and switch the wires for the two breakers, turn them both on and see if the problem "moves" to the new circuit.If it does, then you have a bad breaker. If the new configuration still trips the 5-switch circuit, then there's a wiring problem it that circuit. You'll need to switch the black (or red) and the white wire from both breakers, but not the white wire going to the neutral buss.

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  • Thanks for the info. It's the only 15A circuit in that subpanel, though. – user2356172 Apr 26 '20 at 0:36
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    @user2356172 For testing purposes, you could move your 15A breaker to a lightly loaded 20A circuit to see if it trips like it's been doing. Then you'd know if it was the breaker. If it doesn't trip, it's your circuit. I can't in good faith say to put the 20A breaker on your 15A circuit. – JACK Apr 26 '20 at 13:45
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Here is something else to look for. I am a licensed Electrician of twenty years. I encountered something very similar. The backwired type of devices only require less than a 1/2' of insulation to be stripped from the romex. If more than that is stripped the wire will not completely seat in devices. This goes for switches and receptacles. In a new housing complex about 4 weeks after turning on and testing the circuits we started seeing breakers tripping on circuits that were previously working as designed. What I found was the bare ground wire was laying across and touching the exposed copper of the Neutral conductor because too much insulation had been removed during installation. Even thought the "N" conductor and the grounds are landed electrically at the same point in your panel the Neutral imbalance is enough to trip the Arc Fault breakers immediately.

You should call an licensed electrician out to test the system. They should be able to test and find the fault fairly quickly and you will be money ahead. Remember time is money.

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  • Welcome to DIY.SE -- good explanation of the issue, BTW, didn't think of overstripping as a possible cause of this but especially with backwired devices (of either type), that could definitely cause this sort of problem! – ThreePhaseEel Apr 26 '20 at 3:29
  • @michael-bennett Thanks. One switch was backwired but no wire is exposed outside the switch. Will call an electrician. – user2356172 Apr 27 '20 at 0:42
  • @user2356172 -- let us know what they find! – ThreePhaseEel May 5 '20 at 0:19

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