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My goal is to have 20A receptacles in a particular spot in my garage dedicated to power tools (such as saws) whose motors can be configured for either 240V or 120V, with a switch as a handy second level of safety (like when changing saw blades but it's inconvenient to unplug the machine). I'd prefer for the 240V and 120V receptacles to share a box.

Would it be correct/safe to install a 20 amp two pole GFCI with load neutral, then use 12/3 to power NEMA 6-20 and NEMA 5-20 receptacles sharing a box?

I would add a quality 20 amp double pole switch (or 30 amp... apparently some are derated for motor loads) in a convenient place in the garage.

Then I think it would go like this in the box housing the receptacles: black and red to the NEMA 6 receptacles; on the NEMA 5 receptacles I'd break the hot tabs, wire black to half of them, red to the other half, and pigtail the neutrals.

I understand the NEMA 5 receptacles will make a MWBC since they share a neutral. I'm not sure how the definition of MWBC overlaps with the NEMA 6 receptacles being in the mix. It also seems strange to mix 240V and 120V in the same box, but I'm totally new to all this (including 240V tools) and have seen other answers say that's okay.

If this is an okay method then I'll run the numbers to see if this method has good bang/buck for me versus other setups.

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    Inconvenience is not a reason to not unplug a power tool when doing maintenance on it. If it is truly inconvenient, change your configuration so it IS convenient, especially since you're already talking about re-wiring outlets. How much is your personal safety worth to you?
    – Milwrdfan
    Nov 22, 2021 at 22:49
  • @Milwrdfan Well, I did try resawing on my radial arm saw if that tells you anything ;) You make a good point though. In a normal routine I'd have no way of being sure the switch works, unless I also hardwire a light into it. But then I'd have a lightbulb to tend to. Maybe I'll just put the receptacles within easy reach and replace them if frequent unplugging wears them out.
    – Matt Thomas
    Nov 23, 2021 at 2:51

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Yup, that's legit on a Multi-Wire Branch Circuit

Since you are in a garage, you need GFCI protection on the 120V outlets. Once NEC 2020 comes into effect in your locality, you will need GFCI on the 240V outlets as well. The most efficient way to protect the whole shebang is just as you plan: a 2-pole GFCI breaker.

Most people wire their MWBCs with all 120V outlets on both phases, simply sharing the neutral for wire economy.* However it is completely legitimate to also and simultaneously have 240V outlets tapping both hots (or neutral too, even).

Note that when 120V and 240V loads coexist, the breaker must be a 2-pole "common trip" breaker. But in your case, GFCI requires that anyway.

So yes, your plan is correct. Further, you would be correct to make sure to tap both hot legs for 120V outlets, so that if you have two large 120V loads (table saw and dust collector) you can load the circuit evenly.

Further, you can extend this circuit as far as you want around the garage, with as many outlets as you please. You also extend parts of the circuit, e.g. extend one leg of hot+neutral to a 120V outlet.

They also make receptacles specifically for this use.

enter image description here

note the breakable tab for the shared phase.

For instance, you could put two of these in a 2-gang box, with each 120V receptacle fed by the opposite phase.

Note that the same rule regarding "15A receptacles on 20A circuits" still applies to a 20A MWBC: 15A sockets are allowed as long as there are 2 or more sockets (per phase).

* The "wire economy" reason has fallen out of favor due to GFCI/AFCI complications.

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