**Have you ever found this type of installation in a dwelling? **
In 40+ years of working in and around buildings including farms, a dairy, under a licensed electrician for 3 years and 24+ years inspecting, have never seen that setup. The horizontal must be a 6-15R . In the right (or wrong may be better term) setup, this will cause frequent tripping. Is it still allowed?
PS: The more I think about it…If I wrote codes, I wouldn’t allow it!!
As long as a grounding means existed it’s fine.
I’ve seen those devices but never wired one.
Yes you could get frequent tripping ,but you also can get frequent Tripp on a regular circuit with two high loads plugged in.
Unusual set up but no difference then we get in a panel with various loads fed from Breakers.
You can get the same unbalance in the main breaker.
…Cookie
Joe, good topic
i have installed many of these in apartments just under a window in years gone by.
This will allow either a 120 volt or a 240 volt window air conditioner to be used.
Aha!!! The special use receptacle for warm climates.
By your codes, does this have to be a dedicated circuit?
Yes .
Then as code writer, I’d probably put it back in!!!:shock: :shock: :roll: :roll: 
You can see I come from cold climes…snow flurries and 28*F at noon here. Live on the ocean…may have wanted AC here 1 or 2 days in 15 years.
Why would this have to be a dedicated circuit unless you specifically knew it was for a window AC or some other large load?
I could see this in a shop where you might have a mix of 120 and 240v tools which would be used one at a time in different locations.
BTW this picture comes from the handbook where they are talking about multiwire circuits.
I have exactly this type of 3w +g circuit going to my pool pump and heater although it is hard wired, not cord and plug. The 240 runs the pump and one leg plus neutral of that goes to the gas heater igniter and blower. (as required by the heater instructons)
The only real code requirement is this must be fed with a double pole breaker. (the point of the picture Joe posted)
Then be careful with the loads on this duplex receptacle to prevent nuisance tripping. The total amperage on the phase wire common to both halves of the receptacle is additive rather than subtractive as it is in a split kitchen counter receptacles or other three conductor installations with shared neutral. It would be easy to get over 15 amps in a shop situation such as a 240 volt table saw and a 120 volt dust collector
You certainly have to match the load to the circuit but that is true anywhere. Mike’s example (a window shaker outlet) would be the perfect use for a device like this.
Usually a dust collector would be on a circuit by itself anyway since they can be a true 2hp motor.
I’m sure Joe was just pointing out that this is a legal configuration and you could be using 120v loads on each side and a 240v across the two if they were small enough.
BTW to see this in a real life application most people see frequently, look at ranges and dryers. Both usually have a mix of 120 and 240v loads on the same circuit.
You can do the same with Receptacles any where in the home .
Electric Heater 1,000 watts and some one plugs in a saw.
You have just exceeded the 15 amp Breaker .
Cookie
Thank you for this post, Joe. From an HI perspective, this one is very relivant. I have not seen one of these but if I did I would be posting here looking for help. If I understand right this is similar to the way an electric oven or dryer pulls off one leg for lights and controls.
The only time I’ve seen those receptacles has been for under-window receptacles installed in the 60’s. I suppose they were for either a 110 or 220 window air conditioner? In any event, it’s a moot point nowadays, since nobody will be installing them new. My price book says that I’ve have to charge almost 200 bucks to put one in. I suppose a home inspector could run into one every once in a while, but unless you’ve have a sharp eye, you might not realize that the other half of the duplex is a 240V arrangement. Something to keep in the back of your mind.
Why so much Marc. A Leviton 5842 (the commercial grade, 20a model 120/240 receptacle) is only 7 bucks or so from the online guys. You have some 12/3 and a 2 pole breaker vs 12/2 and a single pole.
If I follow Oregon SOP I would be sure to notice it as I try to stick my tester in it. Help my tester won’t fit!!!:o :o :o :o 
Hey Marc, your PM box is full.
Huh? Are you seriously asking me how an install price is derived? Were you an employee during your working years or a business owner? I think I know the answer to that.
Yeah, I like it better that way.
I priced out contracts for IBM. It was parts+labor+risk with the profit added in parts price and labor price before I got them.
I don’t see the labor in a 120/240 being that much more and the parts are not that much more expensive. I guess the question is, how much would this be vs a regular 120v 20a circuit?