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I have two separate 12/2 Romex lines I ran in my house to a dual 20-Amp outlet (top outlet on one line, bottom outlet on the other). I used this to power a small server rack. I am changing my setup to go to 220v and power a PDU with an L6-30R, and run everything off of the PDU.

Is there a way I can utilize my existing wiring to do so safely, and if so; exactly how do you recommend I do this?

I was thinking I could get a double pole 220v breaker and run the hot wire from each of the two existing Romexes to the outlet. This seems safe / reasonable, but the whole idea seems weird and I want to make sure there isn’t anything I am missing.

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  • Why can't you get a L6-20R and a PDU to match at least? Jul 12, 2022 at 1:01

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So you want to run two hots(different phases) and a ground to a 30 amp socket. You can get your double pole breaker and connect a black wire and a white wire to it and to your outlet. Mark the white wire with some black tape to note that it's hot. Use the bare copper from the same NM cable to the ground on the socket. The breaker has to be a 20 Amp breaker. You can't use the other NM cable to "beef up" the circuit.

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  • This makes perfect sense, and I'm not sure why I didn't think of it this way. I bought some 10/3 for the 30 amp outlet, but hadn't had the time to run the outlet yet when I started thinking about the old wires. Utilizing one ROMEX to carry the load makes a LOT more sense. BTW, am I correct in understanding I technically could eek out as much as a 25 Amp breaker safely on 12/2 ROMEX?
    – Doug
    Jul 12, 2022 at 0:14
  • I'm finding conflicting information. Some websites state that 12/2 Romex NM-B only handles 20 Amps, some are stating 25 Amps. I guess I'll stick with 20 Amps to be safe.
    – Doug
    Jul 12, 2022 at 0:21
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    @Doug Yes, there are tables that state higher amperage for cable and wires based on new and improved insulation but the NEC doesn't allow #12 to be breakered at more than 20 amps because there's so much of the older stuff out there.
    – JACK
    Jul 12, 2022 at 0:39
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NEC 300.3 is clear that you can't combine multiple cables to feed 1 circuit.

But you don't even need to. Listen to yourself: your complaint is that you want to run "neutral-less" 240V loads. Well, you don't need neutral for that!

You can pick ONE cable, and re-task the neutral in that cable to be the other hot. You do this by marking both ends of the white wire with black tape. Vieola, it's now a hot wire! And now you can serve a 20A/240V circuit with that same 12/2 that you were using for 120V. Install a NEMA 6-20 receptacle and you're all set.

And then you can pick THE OTHER cable, and re-task ITS neutral to be hot. You can mark it with red tape, and also mark the black wire with red tape. Now you have a second 240V circuit also with 20A.

That's 40A and you were only looking for 30A.

Hold on.

You are going to have to blow that box out to 2-gang. You can't do the "2 circuits 1 receptacle" trick with 240V circuits, because you are required to handle-tie 2 circuits on 1 yoke. You can't handle-tie two 240V circuits, so you must use 2 yokes.

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