Two new circuits - both trip simultaneously on load

Question

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.

Answer 1

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!