I have an AFCI outlet in my foyer, and the circuit continues through that outlet and out to the porch for an overhead light above the front door, and then past that to the front of the house for an overhead lamp above the steps leading up to the porch. The two outside lights are on a switch inside the foyer, and the wiring runs through the porch roof. It's 'inside' in the sense that it's not exposed to the elements, but it's 'outdoors' in the sense that it's not inside the walls of my house, the construction is old, and there is some suspicion that the porch roof may have moisture in it.

When the circuit has load (i.e. one or both of the lights are installed), flipping the switch and supplying power will immediately trip the outlet. When there is no load (no lights are installed and the wires are not connected), flipping the switch does nothing.

It was suggested that the two possibilities are:

  1. The AFCI outlet is not rated for this use, and a small amount of moisture may be causing the trip
  2. The circuit has a short in it somewhere between the outlet and the first porch light. I've attached an image of how the porch light is wired. As you can see, the neutral on one of the wires does have a tiny nick, but it doesn't seem to go through to the copper.

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What are people's thoughts about the most likely explanation?

  • 1
    What kind of lights are these (incandescent, etc.)? Also, are you sure the neutral's insulation has not been cut all the way through? When I zoom in that cut looks pretty deep.
    – tnknepp
    Nov 16 '20 at 21:05
  • Did it ever work properly? A miswired circuit can also cause this. Nov 18 '20 at 1:34
  • This is probably pretty petty in light of the other answers here; but, shouldn't there be a wirenut on those ground wires? It looks like they are just loosely twisted together.
    – spirito43
    Jan 8 at 1:59

The above are all excellent answers, but none was my actual problem. Because the wiring in this part of the house had a lot of stuff going on; it was installed with a floating neutral. I'm not sure if that's the correct term, but that's what it was called. Basically the neutral wire did not run back through the AFCI outlet, and instead when to the box via some other connection. This bypassed the AFCI outlet and resulted in it thinking there was a big fault and tripping immediately.


Is this an AFCI outlet or a GFCI outlet? GFCI is more common for outlets. AFCI is normally (but not exclusively) at the breaker. GFCI can be either at the breaker or at an outlet. Either way, the device protects all devices following it (if wired correctly).

My hunch here is that this is a GFCI outlet. While moisture, in theory, could lead to tripping an AFCI device, that is not the normal design. But that is the normal design for GFCI: a little moisture in the wrong place lets some current leak to ground and the GFCI detects that and trips to keep someone from getting zapped.

In most cases, GFCI isnot required for lighting, even if there are receptacles on the same circuit that require GFCI protection. That being said, when you know there is a problem, it makes sense to fix it rather than simply bypass the GFCI for the lighting part of the circuit. Since the boxes look pretty clean, that leaves three likely culprits:

  • Moisture getting into a light fixture. This can certainly happen - there are holes in the box in the picture. If you test with a basic fixture and the problem does not happen initially, but does happen after you put everything together and let it sit for a day (accumulating drips), then that sounds like a moisture problem. If you have a moisture problem, I would (a) see if you can replace (or properly close the holes in) the box so that moisture can't get in.
  • Faulty fixture. If you install the fixture on a dry day and the GFCI still trips as soon as you turn on the switch, you may have a bad fixture, the type of problem GFCI is actually designed to catch. That can be due to insulation breakdown in the fixture, a pinched wire or a number of other problems. Replacing the fixture is the solution.
  • Crossed Neutrals. This is a tougher problem to figure out. This should never happen, but it does. A typical case is if you have a bunch of switches in one box that includes multiple circuits (breakers). If the neutrals aren't kept matched to their hot & switched hot wires, things will likely work fine until you install GFCI. And then they won't work at all.
  • I am certain it is an AFCI outlet - but it looks like it's a combo outlet. The house was rewired almost entirely and I chose (correctly? incorrectly?) to put these outlets as the first stop in all the circuits (excepting the bathroom, where there's the GFCI outlets.) Specifically it's an AGTR2-W Nov 16 '20 at 21:16
  • 2
    One possible clue: Do you know if it is AFCI or GFCI that is tripping? From the instructions: GFCI Trip - if the AFCI/GFCI trips either from a ground fault or pressing of the TEST button the indicator will turn OFF. AFCI Trip - if the AFCI/GFCI trips as a result of detecting a potential arcing fault the indicator light will turn OFF similar to when it trips due to a ground fault, but will also display two quick flashes of RED every five seconds. My hunch here is GFCI but knowing for sure would help with diagnosis. Nov 16 '20 at 22:14
  • 2
    I am pretty sure it is a GFCI trip then, I did not see any red flashes, but I didn't know to look for them so it's possible I got unlucky and just never looked at during them. Nov 16 '20 at 22:23
  • 1
    @GoneFishing -- can you reproduce the trips? Nov 17 '20 at 1:24
  • 1
    @GoneFishing -- can you do the diagonstic check manassehkatz described then? Nov 17 '20 at 4:25

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