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Would like to use an existing 12/2 line from a now defunct water heater running 230 Volt to this junction box. Would like to change or split this 230 Volt 12/2 into 2 each 12/3 lines to run 2 separate 115 Volt plugs. How do we do that without having to run another new 12/3 line from the panel?

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    12/2 or 12/3 sounds like cable and not separate wires in a conduit. 12/2 is only good for one 115(it is now a standard 120v) circuit, hot and neutral. You need a 12/3 from the panel to make a MWBC circuit, or two 12/2 for two 120v circuits. The hot water heater used the white wire as a second hot to have 240v plus the ground.
    – crip659
    May 16 at 15:45
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    Please read and edit your question so it is clearer what you want to do. You wrote that you want to "split" a 12/2 into "2 each 12/3", without running any new wire. You must mean something else. You obviously can't magically turn 2 wires into 6 without running new wire.
    – jay613
    May 16 at 15:46
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    Trying to guess and interpret .... if what you want is 120V rated for more than 20A, you need to run new cable.
    – jay613
    May 16 at 15:48
  • If you have 2 hots, neutral and ground at the water heater, you're good to go with some additional work at the panel. If you only have 2 hots & ground, then no, you can't do this. We need more details. If you're not sure what you have, turn the breaker off, then open up the outlet & pull it out without removing any wires and take a pic, then edit the pic into your question. Loads of people here will be able to interpret it for you.
    – FreeMan
    May 16 at 16:32
  • As the old saying goes: "You can't get there from here". All you can do without running a new cable from the panel is ONE as in ONE (1) 120 volt circuit. You'd need to re-wire the cable in the panel to a new, single pole breaker and connect the white wire to the neutral bus, removing any code tape in the process. There are dangerous and not code legal ways of doing it, but I'm certainly not going to say what they are....but believe me, they are dangerous. + to @crip659 for his comment. May 16 at 16:33

2 Answers 2

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You don't.

There is no safe way to get two 120V circuits from 12/2 cable, unless you are willing to install a transformer and a panel.

What you can do is get a single 120V/20A circuit out of it, by moving the white wire to the neutral bar and installing any number of NEMA 5-15 or 5-20 receptacles. (you can keep using the same breaker). That will carry a fair amount of load, though not quite as much as you'd hope for.

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    This is, of course, assuming that OP really has a 20A 12/2 and not a 30A 10/2 (not realizing it is 10 AWG) or a 30A 12/2 (which should never be, but...) May 16 at 22:43
  • Emphasis on no legal and SAFE way to do this. Ungrounded circuits were outlawed in the US in, what.. the early 1970's?
    – FreeMan
    May 17 at 12:27
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Your question has some confusing details, but I will try to answer what you are likely asking.

Most likely you have 240v being fed by 12/2 (or 10/2) cable, if done properly the white should be re-identified (marker,paint,taped) and connected in the panel as a hot.

You can't practically get two dedicated 120v circuits from 12/2 (or 10/2) cable because you would need 4 wires, which are two hots fed from opposite legs in the panel, one neutral, and the ground. What you have is two opposing hot legs, a ground, and no neutral conductor.

If your cable is black/white/bare then if you can remove the re-identification from the white, re-terminate the white wire in the panel to the neutral bar, and then could run multiple receptacles on the same single 120v pole of a 15 or 20 amp breaker.

It is a little odd to have 12/2 feeding a water heater, a majority of water heaters are fed by a 30A circuit with 10/2. If this is the case you could (somewhat impractically) re-terminate the white as above, mount a small electrical panel (or fused disconnect) and feed both legs of the panel with the same hot and feed two 120v 15A circuits.

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