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I'm installing a subpanel to bring power to a garage. Answers to similar questions make it clear that a subpanel must isolate ground from neutral, so the bonding screw will be removed at the subpanel. The standard recommendation is to run /3wg cable from the main panel to the subpanel.

But I've got some NM-B 8/2wg cable that I'd like to use. I believe that would allow:

  • Either 120v service (a single pole breaker on the black wire at the main panel, white as neutral)
  • Or else 240v service (a double pole breaker on the black and the red wires - having taped white as red, with no neutral)

But I can't have grounded circuits of both voltages at once without a transformer.

Can I use my 8/2wg and simply run a single #8 insulated copper as a separate neutral conductor beside it to get both 120v and 240v service at the subpanel?

If so, must both the cable and the single wire be within a conduit? (the run is in a dry interior, through framing covered by sheetrock). The total distance isn't particularly long - about 30'.

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  • What type of conduit? If it's a metal conduit, such as EMT, that's a valid ground path and you don't need to add anything.
    – Nate S.
    Commented Jul 28, 2020 at 22:41
  • @NateS. but the 3rd conductor in the OP's 8/2 is not insulated and therefore can't be used as a neutral surely?
    – brhans
    Commented Jul 28, 2020 at 22:49
  • Ah you're right -- I read too quickly and thought they were adding a ground, not a neutral.
    – Nate S.
    Commented Jul 28, 2020 at 22:50
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    Why are you trying to stuff 8/2 W/G NM down a conduit to begin with?!
    – ThreePhaseEel
    Commented Jul 28, 2020 at 22:56
  • Would only use a conduit if doing so made solution code compliant. See answer below.
    – jbbenni
    Commented Jul 29, 2020 at 13:33

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Nope. All conductors must be in the same cable or conduit. NEC 300.3(B).

Further, individual wires are not physically rated to be installed "loose" outside of a conduit or other raceway. The interior conductors inside 8/2 are also not rated to be used without a sheath.

Lots of people find themselves in this situation, but the simple fact is you need to get the right cables or wires for the job.

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    One last ditch clarification: Using conduit to contain both the cable and the wire together would allow all conductors to be in same conduit, and the individual wire would not be loose. This seems to thread the needle, but the answer seemed to preclude it. Final question: Would 8/2wg and an individual neutral be a permitted solution IN A CONDUIT?
    – jbbenni
    Commented Jul 29, 2020 at 13:31
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    @jbbenni Well, that's the $1000 question. Or more likely the $5 question, as I doubt you'll save that much buying conduit instead of the right cable. I would expect that to be a matter of interpretation for the inspector, and so I would ask the inspector. I for one would expect to be red-flagged. On the upside it's easy to correct since it's conduit. However, I would add a red wire not a white one, since there's a white one already there.
    – Harper - Reinstate Monica
    Commented Jul 29, 2020 at 16:57
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    Thanks Harper. I just found a definitive assertion that cable isn’t permitted in conduit in the discussion of this question. diy.stackexchange.com/questions/113276/… I’m satisfied . Thanks. Incidentally, persistence on this isn’t only from my being cheap - which I am. But we newbies learn stuff from gnawing on these seemingly dumb questions.
    – jbbenni
    Commented Jul 29, 2020 at 17:20

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