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Here's the first part of the story. Summing up, existing wiring was put on AFCI breakers. Some outlets had hot/neutral reversed. Electrician corrected this.

Now, the front porch light, not part of work that electrician performed, trips two AFCI breakers! The lights go off in the two of the bedrooms and the hallway outside the bedrooms. The two bedrooms are where the outlets had hot/neutral reversed. I didn't test the all receptacles to see if they have power. Each room had one receptacle that didn't have power.

What could be happening? How do I go about debugging this wiring? What are the steps?

Update Found this Siemens guide for troubleshooting. I have the Siemens QA115AFCP single-pole 15-amp Combination AFCI Breaker.

Update The house was built in 1939. There are metal junction boxes in various places (I've seen them in the attic). There is metal conduit that runs between junction boxes. I don't believe they are grounded.

Update 1/15/17 I tested all of the outlets in the house. Here's more info. All of the outlets in the living room (and one outlet in the adjacent room that shares a wall with the living room) trips the two AFCI breakers when I plug in a light and switch it on! (Is it basically one offending wire that feeds all of those outlets?)

Update 1/16 The electrician checked it out and said that the wiring consisted of two hot lines and a shared neutral. He's rewriting two of the rooms that we're tripping to have their own home run.

Update 1/17 The electrician fixed the situation and ran a new home run with an AFCI breaker to the two bedrooms. He also discovered that the forced air heating unit was wired with a shared neutral so he also ran a new home run to that (with a normal breaker). The living room with outlets that tripped an AFCI breaker, they wired to a normal breaker.

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    How many trip LEDs are lit on the tripping AFCIs? (Turn them OFF and back ON to get them to display the trip code.) – ThreePhaseEel Jan 12 '17 at 3:46
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    Also, is it an intermittent trip, or does it happen every time the load is turned on? (I'm presuming the breaker isn't tripping unloaded here.) – ThreePhaseEel Jan 12 '17 at 3:59
  • Same light trips both breakers every time. It may even be an offending switch panel. There are two switches on that panel. I'm not at the house right now, but I think both switches trip the same two breakers. – milesmeow Jan 12 '17 at 4:15
  • Both Arc Fault and GND are amber. – milesmeow Jan 12 '17 at 4:19
  • Just found this reference for my breakers (QA115AFCP). downloads.siemens.com/download-center/… The two lights says they are Arc Faults to Ground. – milesmeow Jan 12 '17 at 4:24
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You don't need to decrypt breaker indicators. The fact that two breakers trip is all you need to know.

Why would one thing trip 2 breakers? These two circuits have their neutrals crossed, tied together, or interacting in some way. Perhaps one device which takes its hot from circuit 1 is taking its neutral from circuit 2.

Hots and neutrals are supposed to be monogamous -- they're only supposed to partner with each other, ever. That's always been in the code. But until recently, crossing neutrals "worked" - it didn't cause any detectable problems.

What happened "recently" was adoption of GFCI and AFCI. Those absolutely require hots and neutrals be monogamous. If current comes back on the neutral in an unexpected way, that makes it trip.

So you have to go through each circuit, look closely at every box, and find the place where a neutral doesn't match its hot. It's a tedious job, and I've done it. If it helps, get gray tape and put it on the neutral from one of the circuits. Gray is an allowable color for marking neutrals.

  • What are matching neutrals and hot? Basically the paired up ones would have an outer insulator holding them together? – milesmeow Jan 12 '17 at 9:34
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    Yes, generally hots and partner neutrals must travel in the same sheath or raceway, switch loops beeing an odd case. This was already a code requirement for other reasons. The point is all current going down one hot must come back on the matching neutral, so the GFCI/AFCI can work properly. – Harper Jan 12 '17 at 13:46
  • The neutral can be shared between 2 hot wires. This is called a multi wire branch circuit and it is very common. The conduit should be grounded and may be used for grounding purposes. AFCI breakers do have problems with dimmer circuits, modern flouresent ballast and variable speed motor controls. Verification of all outlets and lighting wiring on these 2 circuits would be the first step. – Ed Beal Jan 14 '17 at 16:34
  • I just added more details. See Update 1/14/17. I discovered that all receptacles in one room trips both AFCI breakers when I plug in a light and switch it on! – milesmeow Jan 16 '17 at 5:04

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