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I have installed a number of receptacles around my woodworking shop, all NEMA 5-20R duplex receptacles with separate feeds to their top and bottom halves, and a shared neutral and ground. These are all behind a single common-trip two-pole 20A breaker (GCFI), using 12-3/wg Romex to "daisy chain" from one duplex receptacle to the next (two hots from alternate legs, neutral, and ground).

I now find myself needing a 20A 240V receptacle in one of these locations. I am not worried about overloading, as no other tools will ever be operating on the circuit when this one is in use.

Can I replace one of these duplex NEMA 5-20R with a single NEMA 6-20R (alternating hots plus ground), and if so, does the neutral-to-neutral to continue the chain count as a "splice" in my box-fill calculations?

I think that the common-trip breaker exempts me from the "only line-to-neutral" requirement (2020 NEC 210.4.C), and cannot seem to find any other restrictions about mixing line-to-line (240V) and line-to-neutral (120V) receptacles.

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What you built is called a multi-wire branch circuit.

You must follow all the rules for a multi-wire branch circuit, including pigtailing neutral and not splicing neutral through a device (e.g. no using the 2 silver screws on a recep).

If it has only 120V loads, it can use breakers which do not have common trip (e.g. handle tied breakers or quadplexes without common trip). Yes, you are allowed to mix both 120V and 240V loads without restriction... however, if you do, the breaker must be common trip.

In fact, they make a special receptacle just for that. It has a NEMA 6-20 socket and a NEMA 5-20 socket in the standard receptacle package. They also make one with a 6-15 and 5-15.

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  • Can you cite the requirement for pigtailing in the 2020 NEC? The LEVITON 8300-R I'm using for my duplex 5-20R's is labelled for feed-through including neutrals.
    – Ry Biesemeyer
    Oct 22, 2020 at 23:46
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    Pretty sure that's still 300.13(b) - device removal cannot interrupt the neutral/grounded conductor. For that matter, elsewhere the grounding conductor is also not to be interrupted by device removal (and that one is not specific to MWBC, while 300.13(b) is.)
    – Ecnerwal
    Oct 23, 2020 at 0:25
  • @RyBiesemeyer As Ecnerwal says... also read this. Nothing changed in NEC 2020 about the pigtailing requirement, which goes back to at least the 00's if not farther. When a device is labeled for feed-through, that's for ordinary 120V single circuits - it's not referring to the MWBC requirement, and labeling can't override that anyway! The issue is that lost neutrals can cause a LOT of mayhem downline. Sorry you have to redo your work, but use us as a resource to get the whole story, next time you learn a trick like that.
    – Harper - Reinstate Monica
    Oct 23, 2020 at 14:26
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The neutrals need to be pigtailed so removing the device won't open the neutral (as do your other neutrals on an MWBC) and that does not change the box fill (wires only in the box don't count - i.e. pigtails.) Wait - 6-20 R, no neutral - the spliced neutrals count as they always did - two wires terminating in the box. The wire nut (or other splice mechanism) does not add anything to the count. If it was a new install and you did not break the neutral there, you'd save one wire count so long as it was less than 12" long

Other than that detail, you are good, AFAIK, since you have the proper breaker protection for this use.

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