Disconnect switches are useful to have in case of flooding or fire, when it would become dangerous for your home to have live AC power. Having a disconnect switch outside the home also provides a safe way to turn off your home's power source. Adding a disconnect switch is inexpensive and doesn't require much time or equipment.
Things You'll Need
Multimeter or Voltage indicator light
Turn off the breaker, or power, to the circuit where you will be installing the disconnect switch. Use a multimeter or voltage indicator light to ensure that there isn't any live voltage in the circuit.
Mount the disconnect switch as close as you can to the wire that feeds into the circuit you want to interrupt.
Wiring your disconnect switch will vary slightly from one type of switch to another. Depending on whether you have a 110V or 220V circuit, you will have either 3 or 4 wires. One will ground (a bare copper or green wire), one will be neutral (a white wire), and one or two will be hot wires (black or a combination of black and red). Also, depending on the type of disconnect switch you have, you may or may not have to cut the neutral wire. Refer to the disconnect switch's packaging for these details.
Attach the ground wire according to your disconnect switch's instructions for safety. This may require using additional electrical wire, depending on your switch. The neutral wire may or may not be cut depending on the switch style.
Cut the hot wire(s). If your switch designates a connector for the supply side, hook the wire coming from the circuit breaker or power meter here. The other wire, which leads to the appliance or the rest of the circuit, will be connected to your disconnect switch on the side designated as the load. If the disconnect switch doesn't designate power and load, you can connect either wire to either side.
Make sure the circuit has no power before you cut any of the wires. Ensure that you follow all electrical safety procedures to avoid injury.