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:enter image description here

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).

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    You should be more concerned that 20A breakers are connected to 14Ga wire. That's a clear code violation, no excuse for it. The neutral appears to be in a MWBC configuration, if the breakers are arranged correctly, so that's not a concern - IF they are arranged correctly. edit to provide pictures, please. Might be "solved" as easily as two 240V 15A breakers to replace the 4 120V 20A breakers. – Ecnerwal Jan 27 at 4:42
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    Yes, can you post photos of your electrical panel, including the labeling on the inside of the door please? – ThreePhaseEel Jan 27 at 4:49
  • Nate, could you clarify is the conduit section between the cables and the panel or the cable between panel and cables? – NoSparksPlease Jan 27 at 16:00
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    Rather than fix this to be a proper MWBC, which would require replacing the cables that are not in conduit, instead you could convert it all to single non-MWBC circuits by adding an additional neutral wire to the conduits, disconnecting where the white wires are merged currently, and keeping your red and black circuits strictly separate. To make it even nicer, you could buy some grey wire for the new neutral (that's the other legal neutral color), and that way it's easy to tell if you're on the black/white circuit or the red/grey circuit. – Nate S. Jan 27 at 22:33
  • @NateS. Will another ground wire need to be added if I add the "grey" neutral the split out each circuit? – Nate Jan 27 at 23:25

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.

  • I believe there would still be a code violation in how the MWBCs are split into two cables and recombined later, since currents within the cables can be unbalanced, and the neutral conductors are paralleled. For example, if there's only a load on phase A, then in one of the two cables, the hot is flowing the full current for that load, but the neutral current would be split roughly equally between the paralleled conductors in each of the two different cables. – Nate S. Jan 27 at 22:29
  • @NateS. -- you're right, I didn't quite catch what was going on in the OP's post – ThreePhaseEel Jan 27 at 23:29
  • I went ahead and re-added the part about downbreakering, since that part is still needed (the cables that are not getting replaced are the 14 AWG ones). Feel free to edit the wording if you like. – Nate S. Jan 27 at 23:55

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