I just did the same with 10 ga wire but installed the single phase breaker and dead-headed the other hot. When I move the 220 machine into place its just a matter of replacing the outlet and breaker. Depends on how picky your inspector wants to be. In my area this is within code.

networkcrasher wrote:This is probably a code question, but I'll toss it out there. can anyone suggest a good way to pull the appropriate wires that I'll need for the outlet? If I could fine a 4/1 cable, that'd be perfect - 2 hots for the 220, 1 hot for 110, 1 neutral, then the /1 for ground. Don't think such a thing exists, though.

You'd be amazed. Depending on the length of the pull, and the numbers of corners without access, I'd go with five spools of wire on one spool caddy (google it, it's pretty easy to mock one up for a one-off gig) and pull three colours for the hots, one white for neutral for the 110V, and one green for the ground for the 220V, all 12AWG. I'd pull an extra green for the ground for the 110V, but I like a dedicated ground like that.

I had thought about that, too, but don't the wires need to be encased on some level? I know the individual wires have a sheath, but don't all the wires need to be sheathed together?

Also, do I need to run 2 neutrals, or does it matter? I know the neutrals are all common at the panel, anyway, but don't know about having more than one per box if there's 2 separate circuits in said box.

Ehm, either pull through existing conduit, or run new conduit. If done tastefully, external conduit can be useful and unobtrusive, if not attractive.

You need a neutral for each 110V circuit, but since the 220V uses only the hot wires from the transformer, and not the center-tapped neutral, none is required for the 220V circuit. All of your appliances should have a safety ground, however.
The neutrals *are* connected together at the panel, but you're considering the ampacity of each wire. You can use the same wire to carry two neutrals from the same side of the transformer (now you're starting to figure out why they use black *and* red for 'hot', yeah?) back to the panel, but tit must be rated to carry the load from *both* circuits. It's usually easier to pull two 12AWG THHN wires than one 8AWG.