The equipment

Square D Homeline panel and breakers of recent vintage, with plug-on neutral. Most breakers, and all of the problematic ones, are AFCI+GFCI combo.

The symptoms

There are four breakers in the house panel that occasionally — once every two or three days — will trip for no apparent reason. Most of the breakers are fine. It's just these four. They don't all trip at once. It's random which one actually trips, and most of the time only one will trip. The one time more than one tripped, they didn't trip at the same time, but the second tripped less than a minute after the first.

I also have two rooms with electric floor heat (SunTouch) controllers that have a built-in GFI feature. The GFI in these controllers also trips, on roughly the same random interval (but again, not in concert with each other or any other breaker).

I only just recently learned that each Homeline breaker, in spite of not having an indicator, does have a way to report what the fault that tripped it. Found it on a post here, actually. Using the "Time Saver Diagnostics" feature, I have found that the breakers are tripping because of a perceived ground-fault. This is consistent with the floor controller behaviors as well.

I also have swapped the most frequently-tripping breaker with one that is just AFCI, without the GFCI feature. The AFCI-only breaker does not trip. Again, this is consistent with the ground-fault detection being the problem.

The one electrical appliance that seems consistently associated with the tripping is the well pump (submersible, 1 HP, Subdrive controller). When a breaker trips, it seems that it only happens when the water is running. Granted, it's a busy household and the water runs a lot. But I've had a couple of moments in the middle of the night when everything is quiet and a GFI trips (breaker or floor controller) just after I run the water (e.g. flush the toilet).

Note, however, that the breaker for the well pump itself never has tripped. The breakers that do trip are entirely unrelated to the well pump. In many cases, they trip even when there's not even an electrical load on them (one is for a lighting-only circuit and will trip even if the lights are off).

Attempts at "diagnosis" so far

Frankly, not much, at least not in terms of creative problem solving. Though I have certainly spent a lot of time on the problm.

We have been having problems with breakers generally for a couple of years (since we originally moved into the newly-built house). Mostly, the problems were related to AFCI tripping, i.e. trying to operate certain appliances and having the breakers for the circuit the appliance is using tripping.

The electrician who originally did the wiring for the house took a long time to get around to fixing that, but they did eventually show up about six months ago to put in new AFCI+GFCI breakers on the circuits where we'd been having the most trouble.

After that, the tripping for appliances seemed to stop, but this problem with the GFCI tripping appeared. Unfortunately, this new problem was more disruptive than the old one. The electrician never did take the problem seriously, only visited the house a couple more times to work on the problem, and mostly all they've ever tried was swapping old breakers for new.

The one exception to that was that they heard from someone else that sometimes this sort of tripping happens because of issues with the utility power and that installing a surge suppressor in the panel would help. So they tried that and now I have a surge suppressor but it didn't help.

That electrician has refused to do anything more to try to fix the problem unless I start paying for their time, in spite of the fact that the problem only started after work they did here. That electrician is no longer working for me.

Messing with the panel changes the problem

And indeed, seems to have caused the problem in the first place.

As noted above, I only recently discovered I could "ask" the breaker why it tripped. But since then, all of the data from the breakers point to ground-fault tripping, which is different from what was almost certainly arc-fault tripping that was taking place prior to six months ago.

Which means that the ground-fault problem is new, and started immediately after the visit from the electrician to replace the breakers because of the arc-fault issues we'd had before.

Similarly, while we'd had some problems with the floor-heat controllers before we moved in, they had settled down (for whatever reason) by the time we did, and we didn't have a single spurious trip from either controller for the first 18 months in the house. It was only after that visit from the electrician six months ago that the floor controllers started tripping again.

In addition, we have gone back and forth a bit between seeing tripping occurring multiple times a day, to seeing the tripping occurring only every few days. Every time this pattern changes, it happens only after someone has been changing things in the panel (i.e. swapping breakers). This has been true whether the person swapping the breakers was the electrician or myself (I volunteered to do some of the diagnostic-swapping, so that the electrician wouldn't have to drive out to the house so often).

Changing the breaker doesn't change the problem

Even though the tripping occurs only for breakers in specific panel slots, it doesn't seem to matter which breaker is in which slot. I can swap a breaker from a problematic slot and a not-problematic slot, and the problem stays with the slot, not the breaker.

I have tried to ask a few other people about this already

  • Square D. I tried using their online form to contact them, and got no reply. I have not yet tried to call them.
  • Our well drilling contractor. He's extremely competent and experienced, but had never heard of a problem like this. He's going to check with the pump manufacturer in case they have, but I'm not optimistic. This seems to me like the well pump is mainly just an innocent bystander that happens to trigger a problem that exists in the wiring.
  • Another electrical contractor company. After hearing my sob story, they declined to work on the problem, telling me I should let the original contractor continue to investigate (which I'm definitely not going to do). I guess I scared them off. :(

What's going on? What should I or another electrician be looking for?

I feel like by now I have a lot of data, which should provide a lot of clues, and yet that original electrician refused to do anything except just keep swapping parts. Because of the correlation with the work that had been done, I asked them whether it could be a problem with the panel itself or possibly even with the work, but they said it was impossible.

I can't shake the feeling that this correlation shouldn't be ignored. But I also have no idea what the correlation could mean. I feel like the solution is probably somewhere in the panel itself, but I can't be sure of that, and even if I could I have no idea what to look for.

Given all of the information I know, what is causing this problem? If you know for certain, that's great, but any reasonable speculation that can assist me in a final diagnosis is welcome. Alternatively, additional diagnostic steps that I haven't taken yet would be welcome as well.


2 Answers 2


Your hands are tied

You have a useful diagnostic indication that the GFCI breakers trip in certain breaker spaces but not in others. However you can't develop that diagnostic path further, because a) your 40-space panel is completely full, b) all the wires have been cut short so they can only reach the breakers they are going to now, and c) you do not possess the necessary wire/nuts to make pigtails.

There's also a certain sense to not mess with it if you seek to pursue a warranty claim. So to do diagnosis of your own may be a one-way street; once you touch it, you own it.

The upshot is that you should not expect a resolution of your problem until something breaks the logjam: either get the builder to honor the warranty work, or pursue legal action, or file a home warranty claim, or abandon the above and develop the ability to effectively troubleshoot this by yourself.

Subpanel time

Having a full panel is a ridiculous situation that gives you no maneuvering room to do much of anything.

Since you have an energy monitor, investigate whether circuits can be combined, because they never experience heavy loads at the same time.

However I would generally expect to want another panel simply for the breathing room. I like to finish a job with half the spaces free, so I would consider another 40-space. Feed it from a 125A breaker, the largest 2-pole breaker generally available. Which panel the breakers are in does not affect the house's load calculation.

I tend to assume GF trips are ground faults. Here's the science of detecting them.

Having seen this for years, I have noticed people tend to be in denial: "Surely MY stuff doesn't have ground faults! Ground faults only happen to other people." That is a huge impediment to effective troubleshooting. So, fairly early in the troubleshooting, I would want to test the circuit for actual ground faults.

What makes that hard is materials that behave like VBOs (Voltage Breakover devices) -- they insulate at low voltage, but conduct at high voltage. For these, a ~2 volt pilot voltage of a simple ohmmeter is not enough; you really need a megaohmmeter (slang term "Megger") which operates at a much higher pilot voltage.

This can be particularly pernicious if you have a VBO insulation failure that leaks above normal line voltage; it only leaks during voltage spikes, but once the leakage begins it could persist long enough to trip a GFCI.

It's best to "megger test" at 150-200% of service voltage (which is 120V to ground even if it is 240V power, since split-phase 240V is two legs that are +120V and -120V from ground.) So 250V is a fine test voltage. I would not test at megger's default of 1000V, because your insulation is only rated to 300-600V.

But the high test voltages of a megaohmmeter can damage electronics. (of course if the electronics is "world voltage" 100-240V, it won't mind 240V to neutral). So you have to work around electronics when testing - i.e. pull them out of the circuit and megger the stuff on both sides of them.

Neutral is also supposed to be isolated from ground. And indeed, GFCIs will detect neutral-ground leakage. That often blind-sides people, who tend to take the neutral for granted.

However, since neutral's voltage is quite close to ground's, "weak" N-G leaks are difficult to detect. To test for these weak leaks, you can simply swap hot and neutral on the GFCI. That will mean neutral is 120V to ground, and any N-G leaks will be much more detectable. You shouldn't run that way because it makes other failures more dangerous, but for testing it's fine.

If you lift hot and neutral off the breaker, those wires are supposed to be entirely isolated from ground. They should test out as infinity ohms to ground, even if you tie hot and neutral together and test them as one.

In fact, that gives you a "cheat" to work around the risk of damaging electronics. If you tie a circuit's hot and neutral together for testing, then the voltage that's supposed to be across the electronics is 0 volts, which will not harm electronics obviously. So if the megger has one probe on hot+neutral and the other probe on safety ground, that shouldn't harm the electronics.

Effective testing

Of course you can give the circuit an initial try with a plain voltmeter. It'll probably read infinity ohms to ground... but if it doesn't, that gives you a positive detection to work with.

As it happens, you already own a "megger" that operates at 120V test voltage and detects at 5 ma leakage. It is called a GFCI. That test voltage is weak tea, but it has the merit that you already own the test equipment :)

So for any given circuit, you can gather its hot+neutral, wire nut them together to a pigtail, and attach that to a megger. (or for a 120V test, the hot leg of a trusted GFCI).

Effective diagnosis

Once you detect something, you need to narrow down the source of the problem. Here, you "divide and conquer": break the circuit at some point, and test again; now you know which side of the circuit the problem is on. Break that side of the circuit in half and repeat. Soon you narrow it down to only one possibility.

It's vital to understand that hot and neutral are of equal importance. So when separating parts of the circuit, both hot and neutral must be separated.

What about safety ground? That MUST remain connected and complete at all times! It is needed to reliably detect ground faults. One can disconnect ground and see ground faults disappear, but that is a false indication, and often blind-sides people who don't know better.

  • Wow, that's a lot to digest. But thank you for the extensive guidance...I will look into applying the suggestions, either myself or have an electrician I hire follow these steps. You wrote "Since you have an energy monitor" -- I don't, actually...if you can tell me what I might have written that makes it sound like I do, then I can edit my question to make that more clear. On other points above: a) a warranty claim is already out of the question; the responsible electrician has refused further warranty support at this point. So I'm looking at tracking down the problem myself ... May 26, 2021 at 22:45
  • ... and/or hiring someone to do that, and then taking it to court after the fact to recover costs, if it turns out to be something that was their fault in the first place (which it very well might not be...it seems premature to assume it is, however likely that is). b) there is already a subpanel, but unfortunately the electrician installed a small one for that (12 spaces), and it's full too. They used up the last two spare spaces in the main panel with the surge suppressor...since that didn't fix the problem I guess I could take that out and use those slots. :) c) nothing in ... May 26, 2021 at 22:45
  • ... your post suggests a theory for why this problem only started after the old "trippy" breakers had been replaced with new ones. There's nothing wrong with that per se, but if you have any theories on that question, I'm all ears. d) I agree with you about the "assume GF trips are ground faults" in general, just having trouble reconciling that with the fact that the trips happen on so many different circuits, including those with their own local GFI protection (floor-heat controllers). That said, given where I stand in terms of solving the problem, you're 100% right that it makes ... May 26, 2021 at 22:45
  • ... sense to investigate for actual ground-faults...if we find a problem, then we can fix it, and if we don't, then at least we've ruled that out. May 26, 2021 at 22:45
  • @Peter I wouldn't do any more theorycrafting, there's a gross overabundance of that and nowhere near enough collecting of hard data. Start testing! May 27, 2021 at 2:09

Are these breakers together? I've heard that Homeline plug-on-neutral breakers are more susceptible to thermal overload. What is probably feeding that is how the new Homeline panels do things.

An AFCI or CAFCI needs a neutral connection so it can monitor the full circuit. For older panels, that means you connect the neutral to the breaker and then run the pigtail to the neutral bar elsewhere in the box. Homeline (and a lot of modern panels as well) features a distributed neutral so they can put the neutral within reach of the breaker. The proprietary breakers then have a clip that grips the neutral bar.

In this picture, the neutral bar is that inverted metal "U". Note how it extends down both sides, behind where the breaker sits. What's nice about this (I just put one in myself) is that the bar has holes that are next to each slot so you can put the neutral next to its hot (no more fishing neutrals all over the box). They even conveniently split the ground bar as well.

Homeline panel

What I suspect might be happening (I used regular breakers so I can't test) is that the neutral bar being there consolidates the heat as well. If your neutral on that side gets warm it could affect the breaker as well, causing a nuisance trip.

One thing to try might be buying a regular breaker and replacing the CAFCI (the only caveat here is Square D regular breakers may be hard to find due to Covid backlog). They're not expensive and you can return it if need be. If the regular breaker does not trip, that could help explain the problem.

  • "I've heard that Homeline plug-on-neutral breakers are more susceptible to thermal overload" -- would the Homeline breaker still report a thermal overload as a ground-fault when using the "Time Saver Diagnostics" to check the cause of the fault? My understanding is that it's supposed to distinguish the two (the documentation says that would be the case). These breakers are reporting ground-fault (trips immediately on reset), not thermal overload (trips after a five second delay). May 26, 2021 at 15:55
  • "If the regular breaker does not trip, that could help explain the problem." -- the trip reason being reported is a ground-fault. And as I noted in my question, if I replace a breaker that normally trips with one that lacks the GFCI feature (whether it's AFCI-only or a plain vanilla breaker without AFCI or GFCI) the problem doesn't happen on that breaker any longer. But that doesn't differentially distinguish between a false-positive ground-fault problem, and an overly-sensitive-to-thermal-overload problem. I'd expect the "Time Saver Diagnostics" to distinguish though. Can't I trust that? May 26, 2021 at 15:58
  • On that point I'm not sure. One thing I do know is that the AFCI/CAFCI breakers are fiddly things that don't always perform the way you expect. I've got an AFCI that's moody about my wife's hair dryer and I couldn't tell you why
    – Machavity
    May 26, 2021 at 16:00
  • Yeah, they do seem fiddly at that. Unfortunately, if I put a regular breaker in place of an AFCI+GFCI breaker, and the tripping doesn't happen, that doesn't diagnose anything. I mean, it causes the problem to go away, sure. But that could just as easily be because I took the GFCI out of the equation as anything else. (Heck, likely even, given that the breaker is telling me it's detecting a ground-fault and not a thermal overload). And frankly, while I can obviously live without GFCI on the lighting circuit, on others (like the bedroom/bathroom circuit) it's not optional for me. :( May 26, 2021 at 16:29
  • "I've got an AFCI that's moody about my wife's hair dryer and I couldn't tell you why" -- yeah, that sounds like a typical AFCI thing. Conventional (i.e. not brushless) electric motor creates lots of sparks, which commonly causes false-positive AFCI tripping. FWIW, those false-positive trips we did see go away when the breakers were replaced with newer versions. The manufacturer is constantly updating to address false-positives they learn about in the field, so newer breakers usually work better than older. May 26, 2021 at 16:31

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