I have an old house - somebody (licensed electrician? I can’t be sure) did some electrical updating. What I have is four 14/2 circuits, each on a single pole 20amp breaker (none handle barred together). The source cables all come to a junction box where a pair of cables is spliced to essentially a 12/3 cable (but it’s actually THHN because it exits the junction box via surface mount conduit). This is done twice since there are four source cables. These are all general bedroom lighting/receptacle circuits.
My concern is that the neutral is shared once the 14/2 cables splice to the 12/3.
I’m a DIY’er with limited electrical background. Is this configuration a problem?
The junction box is on a second floor apartment. Panel is in the basement. I would estimate 40-50 foot run. Everything to the right of the junction box is through surface mount conduit (except the 12/2 cable which is a short section through a wall).