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?
Sketch of the wiring:
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).
Best Answer
This wiring is easier to fix than you think
Since the culprit run of wire was done using THHN in conduit instead of a cable such as NM or AC (BX), this is actually quite readily fixable. Simply grab a suitable length of 12AWG grey or red/white striped THHN wire, run it through the conduit (this may require removing the existing wires and repulling them), and move the 2nd circuit's neutral over to the new wire at both ends.
(P.S. you only need one new wire as it's legal to have any number of circuits share a single grounding wire; in fact, if your conduit was made of metal, it would be a legal grounding conductor.)
Replace the 20A breakers with 15A ones
Since your circuits have 14 AWG wiring in them, you need to use 15A breakers. You can only put a 20A breaker on a general receptacle circuit if every piece of wiring on it is 12 AWG or larger.