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.
Best Answer
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.