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FIGURE A: Here are 120V (left) and 240 volt (right), 20-amp receptacles.
A single cable provides 120-volt and 240V service.
Every article I’ve seen describing the wiring of a shop for both 120 volts and 240V uses separate cables for each voltage, which is a significant waste of time, effort and cable. Here I describe how a single cable can be used to provide both 120V and 240V service to a single two-gang wall box.
For many home woodworkers this wiring method is an easier, less expensive way to add 240V service to an existing 120V shop.
FIGURE B: Here is an example of the 120V/240V wiring for a two-gang box. Leave at least a foot of cable in the box to allow for the accordion folding so the receptacles can be easily connected to the wires and then pushed into the wall box.
I describe only the connection scheme, and appropriate plugs and receptacles here. You should consult one of the many home wiring books carried at home centers for basic wiring instructions.