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Sorry for the long preamble. I'm doing a remodel which involves adding a new guest room which shares a wall with the kitchen. I remodeled the kitchen a few years ago, so I have a good handle on the wiring there (like seven circuits -- two for small appliances, lighting, dishwasher, disposal, microwave, and one more circuit for miscellany like the fridge, range hood, gas range outlet, the light in the adjacent laundry). I ran all new wires for everything, except for the miscellaneous circuit, I used a 12/2 cable from the 80s which looked perfectly good. Everything has been fine for 5 years.

Now I decide to use the circuit also for the outlets and lights in my new guest room. Everything is wired up, and everything works great. But then in preparing for the inspection I went and bought an AFCI since they will require it for that circuit. AFCI trips. Crap.

Start debugging. Disconnect all the new wiring -- still trips. Unplug fridge -- still trips. Unplug range -- still trips. Disconnect range hood -- still trips. Disconnect all the wires at the first outlet. As far as I know, it is a straight shot for the panel to this outlet. So maybe there really is arcing in the main line from the panel. But it does not trip. OK, I'm feeling pretty good at this point because I can add the downstream wiring one at a time. Reconnect the range hood (without the hood on) -- doesn't trip. Reconnect the laundry lights and new wirings -- doesn't trip! That is all the downstream wiring!

Then I switched on the laundry lights -- trips -- just the lights, maybe 50W. Turn lights off, plug in fridge -- trips.

Basically, I'm forced to conclude that something about the wiring is causing the AFCI to trip -- BUT ONLY when any current is being drawn -- but it doesn't matter by what!

So now the question: Is this really how an AFCI will trip or how an arc will happen? Only when there is any current being drawn? I was assuming that if the wiring is really susceptible to arcing, it would happen just from hot being poorly insulated from ground or neutral. Why does current need to be drawn?? Is this just about how AFCIs work?

What is the explanation of what is wrong with my wiring and/or how to debug it now?

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Your AFCI has some LEDs that indicate the trip source. Could you describe them to us? AFCI breakers can trip for more than one reason. Defective AFCI is an outside chance in this case: try swapping it to a different circuit. –  Bryce Jan 12 at 7:55
    
@Bryce: Already tried two. I'm pretty sure there are no LEDs because I was doing this experimentation last night and didn't see anything. –  ThePopMachine Jan 12 at 16:49
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A higher end AFCI will tell you if it tripped for overcurrent, arcs to ground, or in circuit arcs. –  Bryce Jan 13 at 6:33
    
See diy.stackexchange.com/a/36456/5960 –  Bryce Jan 14 at 5:45

2 Answers 2

up vote 1 down vote accepted

For everyone's info, and because it could help someone else, here is the actual answer in my case.

While I had connected the white wire coming out of the AFCI to the ground bar, I hadn't connected the neutral from the circuit to the AFCI. In my case it was because I didn't realize the extra screw terminal was actually there and the documentation wasn't helpful.

Bottom line: If you install a combination AFCI and connect the hot but not the neutral, it will not pop until there is a load. After the fact, that actually makes sense. This is like a ground fault from the AFCI's point of view because it can't see the current going back through its neutral port.

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Combination arc-fault breakers typically offer ground-fault protection of equipment, which is usually around 30 mA protection. It's possible that you have a ground-fault on the circuit, and that's what's causing the trip.

As Bryce mentions, some AFCI breakers actually tell you what caused the trip (if you know how to read them). Find the documentation on the breaker, and read through it to see if yours has this feature.

Side Notes:

Though I don't think it's what's causing the tripping, You might want to pull new circuits for the guest room, as you may find that you'll overload the existing circuit fairly easily. For example, what if the fridge is on, a TV is on, and a guest decides to dry their hair?

You may also want to consider separating the lighting and receptacle circuits, so a tripped receptacle circuit doesn't put you in the dark.

Depending on your jurisdiction, they may not allow you to extend the kitchen circuit beyond the kitchen. Check with your local government, but typically kitchen circuits (other than lighting) cannot supply any other rooms (with exceptions for breakfast nooks and dining rooms I think).

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Does something about the evidence suggest a ground fault, or are you just suggesting another potential cause? –  ThePopMachine Jan 12 at 16:50
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It's very difficult to determine the cause of the trip without actually being on site, so I'm just suggesting other things to look at. You'll also want to insure that all connections are solidly connected, and that you're not bonding neutrals from separate circuits, or bonding neutral to ground anywhere. –  Tester101 Jan 13 at 11:33
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Worth saying: did you hook the neutral to the AFCI, and the right neutral? An AFCI has one more terminal compared to the breaker you replaced. –  Bryce Jan 14 at 5:44
    
@Bryce: In the end this was exactly it! Adding an answer –  ThePopMachine Feb 11 at 15:34

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