CNC Dan
Veteran Member
Two 110/120v receptacles in the same box would normally be on the same circuit, but two boxes in the same room, even along the same wall, are frequently wired on separate circuits so that the room will still have power if one breaker trips. Even then, though, you are right that they must be on different legs at the service panel in order to have 220/240v. Then it will work o.k., and it's easy to make up a pigtail to do it. Joining two 120v outlets that are on separate circuits but on the same side of the service panel will give us 120v.
Either way, doubt there are many local fire marshals or electrical inspectors that would be o.k. with it.I've seen it used for very temporary applications (e.g., 220v floor sanders, 220v metal and woodworking machines at shows). There is big safety issue, as the breakers are not ganged together with a bridge bar as in a regular 220v breaker, so one side can trip, while the other side stays hot.
That's the magic. Inside of the box there is something to prevent one input cords prongs from being 'live' when the other cord is plugged in. And something to prevent the load from getting any power unless you have two out of phase 120 circiuts. It has a UL aproval.