I think you are missing the point. If the garage were a separate
building supplied from the same service it would be limited to one
circuit to supply it with exceptions for emergency circuits, different
types of power and so forth. The basic principle that applies to
services, feeders, and branch circuits that supply a separate structure
is that each building is allowed only one circuit to supply any one type
of power. If the voltage characteristics of the two supplies is the
same it would have to meet one of the exceptions to be permissible.
I have never seen a duplex 220v. receptacle. The NEMA 20A 220v
receptacle has one horizontal slot, and one slot that is either
horizontal or vertical. It is designed to accept either a NEMA 15A or
20A 220v. plug, but to make insertion of any 110v. plug impossible.
You don't need to put the wires in conduit unless you prefer it, or they
are subject to mechanical damage. Be sure to pull a 4-wire connection,
even though the NEMA 20A receptacle doesn't use it. You may need it
sometime in the future. In conduit, using THWN, that is not a big deal,
but if you use NM cable, buy 12/3 or 10/3 w/g. For 20A, 12/3 would be
fine, but 10/3 will let you bump the circuit to 30A in the future, if
you need more power.
I use 240v. power for any heavy motor. My air compressor, in
particular, makes good use of 240v. power. I also have a buzz box
welder, though I don't actually weld inside. I haul the leads out into
the driveway for safety. The welder requires a 50A 240v. circuit.
They make them, although the 20A duplex 240V devices are expensive and
not very common. But you can probably get a 15A 240V duplex outlet (has
2 horizontal slots plus a ground) at Home Depot or Ace Hardware for less
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