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?
Best Answer
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.