Before You Begin
Check the rating of the dimmer switch to find out how many watts of power it can handle. Then count the number of bulbs that the switch controls and add up the wattage of each bulb. Most can handle about 600 watts of power. That means that the typical dimmer switch can control five or six 100 watt light bulbs. If you have a fluorescent fixture you want to purchase a dimmer switch designed to work with certain types of fluorescent fixtures.
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Step 1 : Turn off the power
Turn off the power to the switch at the circuit or fuse panel.
Step 2 : Remove the switch plate cover and test
Use a screwdriver to unscrew the two screws that hold the cover to the wall. Then remove the screws located at the top and bottom of the switch that hold the switch in the electrical box. Carefully pull the switch out of the box and, before you begin, touch any of the wires to make sure there is no voltage Then test the circuit with a voltage test to make sure the power is turned off.
Step 3 : Remove the wires on the old switch
Use the screwdriver to loosen the screws on the side of the switch. Then remove the wires from the switch. Note which wire is connected to the copper colored screw. This is the hot wire and is usually black or colored.
Step 4 : Attach the wires of the new fixture
Depending on the type of dimmer switch you purchased, the new switch may have terminal screws on the body like the old switch you just removed, or it may have a set of wires coming out of the switch body.
If the switch has wires, use the wire nuts supplied to attach the black wire to the colored wire. Twist the wires together and then screw on the wire nut. Do the same with the white (neutral wire) and the green or bare copper (ground) wire.
If the switch has screw terminals, wrap the black or colored wire around the copper colored screw in a clockwise direction and then tighten with a screwdriver. Connect the white wire to the silver terminal and the green or bare copper wire to the green ground terminal.
Step 5 : Attach the new dimmer
Push the new switch back into the electrical box and screw it in place. Then screw on the switch plate and turn on the power.