weressl
Esteemed Member
I think you need to reread the OP. It said there was 12/3 BX feeding to a junction box. The neutral terminals of the back-stabbed receptacle served as the common point for several multi-wire branch circuits.
I did read it and nowhere in the OP was it clear that the circuit TO the junction box is served by TWO different circuit breakers on TWO different legs eg. L1 AND L2. 12/3 RX wire is unknown to me and you're generously allow for a typo. As I pointed out the three wires could be serving Phase, Neutral and Ground. There were TWO circuits leaving from that point on, and the OP called those MWBC but unless there were two different legs involved in the feeder to the JB then that description is inaccurate.
The description was almost as incomplete as one would read a newspaper articles description of an overstated case because of the lack of understanding on the journos part, not an accurate description by a professional.
Sorry for for pointing out that accuracy is an essential part of communication and unambiguity is not unlike cleanliness.:happysad: