Adding two new 20a AFCI circuits to a basement project. I tied the circuits to the breaker panel and all outlets test out ok with a tester.

The second I put any load on either circuit, however, BOTH breakers trip.

I ran out of time to troubleshoot so left them turned off. My only guess is that I may have accidentally reversed the neutral wires on the AFCI breakers introducing a bridge between the two circuits.

If that's not it, what else could be causing both circuits to trip on load?

I'll be back tonight for more troubleshooting.

  • Hello, and welcome to Home Improvement. What does your tester show if you turn off one breaker but not the other (try both ways)? And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Aug 26 '19 at 14:41
  • When the breakers trip, is it an overcurrent trip (flip handle to reset) or an AFCI trip (press RESET buttons to reset)? – manassehkatz-Moving 2 Codidact Aug 26 '19 at 15:27
  • Are these new breakers just AFCI, or are they combination AFCI/GFCI? – brhans Aug 26 '19 at 15:36
  • I have to flip the handle to reset although I don't think they have a separate reset button (there is a test button, but it trips the handle). The new breakers are AFCI only. I've done very little troubleshooting as I ran out of time. The property is an hour away so I'm looking for ideas of things to check when I return. Thanks in advance for any and all suggestions. – PurplePJ Aug 26 '19 at 15:50
  • 1
    Square D, QOC40US Series S01 Type 1 Enclosure and adding Q0120CAFIC 20 Amp breakers. Troubleshooting delayed until Wednesday evening so no further info from last night. – PurplePJ Aug 27 '19 at 16:49

Remove the hot and neutral of one circuit (e.g. Remove that breaker's pigtail and snap the breaker out and let it lie loose).

Then, troubleshoot the surviving circuit in the normal fashion. A wide variety of "hook up 2 new circuits at once" mistakes will instantly become apparent.

In the future, you must have very good discipline about the relationship between neutral and hot. Every circuit must be painstakingly matched- of course, that's super easy where cable is involved, because it's either 1 circuit per cable or /2/2 cable where red's neutral has a red stripe. What gets you is individual wires in conduit, and you are required to mark circuits somehow. This is why I own 11 colors of wire and 10 colors of tape!

  • Very colorful junction boxes. – JACK Aug 26 '19 at 19:07

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