I have a double pole 20A breaker feeding a 120V MWBC with 12/3 with ground, romex that goes up three levels to two separate 120V duplex GFCI receptacles with common neutral.

I want to have both 120V and 240V small appliances. Wanting to add a 240V 6 amp coffee maker to two small 120V appliances. This is not a kitchen, but indoors, off of a top deck. The MWBC feeds just the two 120V duplex GFCI's, nothing else.

What is best practice to do this?

I could run existing MWBC wires to a small panel, then install one double pole 15A 240V GFCI breaker and two single pole 15A 120V GFCI breakers, and new receptacles. Possible also to use the existing 120V GFCI receptacles and regular single pole breakers fed off this panel too, in this situation.

Or could leave the current two GFCI receptacles alone and take 12g wire pig-tailed from the ends of the MWBC to a 'spa' panel, change out the breaker in the 'spa' panel for a 15A or 20A 240V double pole GFCI breaker and then connect a 240V receptacle.

Or I could attach an inline 240V GFCI to the hot leads of the MWBC in a junction box.

Or I could use a 2000W step up/down transformer plugged into one of the 120V GFCI receptacles. It just seems redundant to convert the voltage when I already have it there, but a plug-in transformer may be the most code compliant and cheapest, way.

I can not run another wire from my main panel. I cannot replace the 20A double pole breaker in the main panel, with a GFCI breaker, as it is actually a quad breaker.

Any other ideas? Other than code won't allow any of it? Wife really wants the coffee maker so it is going to get done somehow.

  • What make/model is your quadruplex breaker? Commented Nov 6, 2018 at 0:11
  • Square D HOM DP--4075, one two pole and two one pole. handle tie across the two inner breakers. The two inner breakers with handle tie, feed the 12/3 with ground that goes to top deck
    – mcysr
    Commented Oct 15, 2019 at 21:06
  • Breaker appears to be called a Quad Tandem Circuit Breaker
    – mcysr
    Commented Oct 15, 2019 at 21:37
  • At the risk of asking the obvious, this is a Square D HOM panel, right? There are many other panels which HOM will "snap in to", however, the clip doesn't grab properly and it will arc and burn up your bus, at best making that slot dead. There are only certain cases where you can cross brands, that's where a company engineers their breaker specifically for a competitor's panel and get UL to classify it as safe. Square D absolutely refuses to do that. Commented Oct 19, 2019 at 17:00
  • Yes, it is a Square D home panel. This is the breaker. 20A. It is clearly common trip on the two center breakers which are connected to the circuit.. 12awg, 3 wire w/ground romex. Sharing a neutral. 2 hots from the two center breakers. Installed by licensed contractor and inspected by the city. I want to run a 220V coffee maker and one or two 120V appliances from this circuit. Is it legal and reasonably safe to do so? Total appliance amperage under 10 amps. Clearly there will be line to line and line to neutral loads.
    – mcysr
    Commented Oct 19, 2019 at 17:14

2 Answers 2


You have a MWBC that is currently wired for 120V-only. For those, two breakers with a "handle-tie" will suffice.

120V and 240V loads can mix on a MWBC if the breaker is 2-pole with common trip (handle-ties do not provide this functionality; it's an internal link mechanism inside the breaker). This is needed to keep a 240V load from re-energizing a leg that tripped.

They even make receptacles for that.

enter image description here

Note how there is no tab to break away on the visible side, since one must be neutral and the other L2. However you can see the tab peeking out from the other side, which could both be L1 of the same circuit.. Because this is legal on MWBCs.

As far as GFCI protection: It's an easy job for a 2-pole GFCI breaker. Replace the current non-GFCI 2-pole breaker in the service panel with a 2-pole GFCI breaker. Liberate the two GFCI receptacles and reuse them somewhere useful.

If you don't have space in your panel, then it's time to fit a subpanel somewhere. Since you're ready to do that, do that... just not out at the patio where it'll be a one-trick pony. Next to the main service is better. After all, you have this "out of spaces" problem all over your house. So fit it somewhere that makes sense for all your future loads.

I know adding a subpanel is a big job, "best" kicked on down the road for awhile longer... But the fact is you'll be right back here again when the wife wants an on-demand water heater or a plug-in hybrid. In this day and age, with so many amazing electrical gadgets, a full panel is no place to be.

  • One potential fly -- if he has a non-common-trip quadruplex breaker, he can't do this, because the rules about common trip for mixed 120/240V loads on a MWBC apply circuit-wide. Commented Nov 6, 2018 at 0:11
  • 1
    OP here. Quad Breaker is a Square D HOM DP--4075, one two pole and two one pole. handle tie across the two inner breakers.
    – mcysr
    Commented Oct 15, 2019 at 21:05
  • 1
    @mcysr -- can you post a photo of the breaker in question? Commented Oct 15, 2019 at 22:31
  • 1
    @ThreePhaseEel I note that straight 120V-only MWBCs do not require common trip. I also note that 240V-only loads do not require common trip. So I want to confirm, you are saying when an MWBC supports both, such as one wired to the above-photographed plug, it does need common trip? Commented Oct 16, 2019 at 0:21
  • 1
    @Harper -- exactly, it requires common trip support. Current production HOMT triplexes and quadplexes are common trip wherever they are common shutoff, but that's not true in some other lines (BR, mostly) Commented Oct 16, 2019 at 2:08

I would install a small "spa" panel then 2 breakers for each outlet and the double pole for the coffee maker. Because you are already feeding 12 AWG protected by 20 amps this would be legal as a sub. Make sure your neutrals are isolated from ground in the new panel, and install the panels prior to or at the split on the MWBC with there own breakers. For a clean install, you could continue with the MWBC from the spa panel but it needs to be a double pole if you do. I would take the easiest / least expensive path, but either will work.

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