In most states, the building owner can legally install electrical switches, although contracting the work through a certified electrician can provide a margin of added safety that the average do-it-yourself enthusiast cannot. A homeowner might install the switch to control a large electric furnace directly wired into household current. The owner might locate the switch at the entrance of the room so he can turn on the unit when he switches on the lights.
Identify the required circuit by opening the fuse box and locating the 240V fuses. A 240V fuse usually appears as two regular 120V fuses with the handles connected together. It may also appear as a double-wide fuse with a single handle. There may be more than one 240V fuse, so be sure you locate the correct circuit. Turn off the fuses one at a time. Check the appliance you wish to control with the switch. When you turn off the right fuse, the appliance will no longer work. Turn off the circuit and remove the identified fuse so no one else can accidentally turn it back on. The electricity to this circuit must remain off.
Expose the wires from the appliance to be switched. This may require that you remove the appliance to reach the wires. Locate the green and black wires. You may also find a second black wire or a red wire along with a white wire, depending on the requirements of the appliance.
Install a new electrical box to house the new switch unless the current box has room to house the switch and wiring. Install the box according to the manufacturer's directions. You will probably want to mount this near the original electrical box.
Disconnect the wires from the appliance. Tie a string to the wires and pull them through the wall until they can be placed into the new electrical box. Tie the string to the new wires (usually 12-4, but match the original wiring) and pull them through so they may be connected to the appliance. Leave 9-inches of wire extending out of the new electrical box for ease of work.
Strip and connect the wires according to the manufacturer's instructions for the double pole switch. Strip the white wires in each box three-quarters of an inch and twist them clockwise and all firmly together. Cap the connection with a red wire nut and tightened down firmly. Test the connection by pulling on the wires. They should not move with hand pressure. (Don't use pliers or other tools to test.)
Connect the green wires in the same fashion. Connect a bare wire to the box under the green screw and twist it together with the green wires. (Use a green wire nut, if available.) Connect the remaining wires according to the directions on the switch. Directions may vary according to the switch.
Return to the fuse panel to test the circuit. Reinstall the breaker and turn on the power. Return to the appliance if the breaker doesn't throw. Turn on the appliance. If the breaker trips, you have cross-wired something. Ensure that all wires are connected color to color and that the switch connections comply with the manufacturer's instructions.
Reinstall the appliance. Install the faceplate over the newly installed switch.