We have all had the misfortune of having the electricity go out in our home when a switch needs to be changed out. In order to replace the switch properly, you must first identify the switching situations you might encounter. Below are 2 common types:
Single-pole switch: The most common switch is of the type called single pole; it is the only switch controlling the branch circuit it serves. Older single-pole switches have only two terminals: one connected to a hot wire coming in and the other connected to an outgoing wire that is hot when the switch is on. It doesn't matter which wire is connected to which terminal, because all the switch does, in effect, is touch the wires together when it is on and pull them apart when it is off.
Some newer switches have grounding terminals. These are hooked up the same way, but they also have a green terminal attached to the yoke for the ground wire. If, when you open the box of a single-pole switch, you find only one cable coming into it, this is a switch loop, a run of wire serving only the switch. One of the wires going to the switch will be black.
If the wiring is in shielded cable, the other wire going to the switch will be white, and it should have a bit of black tape or paint on it to identify it as a hot wire. However, if the wiring was done with separate conductors run in conduit, the second wire could be any color. It doesn't matter; just hook one wire to each brass terminal. Other wires may be connected in the box, probably with wire nuts. Leave those alone unless you'll be grounding the switch as described next.
Three-way switches: Most houses have at least one set of three-way switches. You'll usually find them at the top and bottom of staircases or at each end of a long hallway. These are switches that let you turn a circuit on or off from either switch, regardless of whether the other switch is on or off.
The first step in replacing a three-way switch is finding out which one is faulty. With old push-button switches you can sometimes tell by just taking the switches out of the boxes. The mechanical contact mechanisms are exposed, and you'll probably find a broken contact arm. For other switches, you'll need a continuity tester, an inexpensive battery-operated device with an alligator clip on one end and a probe and light on the other.
Turn off the power at the service box and remove the switch. You'll find two brass terminals and a dark terminal marked "common." Put the tester's alligator clip on the common terminal, and touch the probe to either of the brass terminals. The tester light should come on when the switch is in one position - either up or down. Leave the switch in the position that activated the light, and move the probe to the other brass terminal.
The light should not come on until you flip the switch to the other position. The wires coming into a three-way switch could be various colors. Like a single-pole switch, all the wires are hot. But unlike a single pole, it matters which wire is connected to which terminal. Your best bet is to draw a diagram of the switch showing which wires go where before you disconnect them.
When you buy a switch, make sure you get one that can carry enough amperage. Most circuits in your house will be 15 amps, but check the marking on the fuse or circuit breaker before you buy a switch.
Below are the 7 steps on how to replace a switch:
Step 1: Turn off the juice. This is the most important step. At your electrical service box, turn off the circuit breaker or pull the fuse serving the switch. Check to make sure the switch can no longer provide power to the receptacle or fixture it serves. If, for any reason, you are uncertain that you've turned off the right circuit, turn off all power to the house at the main breaker or fuse.
Step 2: Remove the switch plate and pull the switch. Take off the switch plate by removing the two switch plate screws. Next remove the two screws, at the top and bottom of the switch, that hold it in the wall box, and pull the bad switch out of the box, being careful not to touch the terminals.
Step 3: Test the circuit. To make sure the circuit is off, use a neon voltage tester. This consists of two probes attached to insulated wire, with a small light between. Touch one probe to the side of the electrical box and the other to each switch terminal in succession. If the light comes on at any time, the switch is still hot. Play it safe and turn off all power to the house before going further.
Step 4: Make a diagram if you need to, and when you are sure the power is off, disconnect the wires. Some newer switches and receptacles are not connected by terminals but are back-wired into terminal holes that use internal spring clips instead of screws to hold the wire.
You probably won't encounter this since you are probably replacing an old switch, but if you do, there's a little release slot under each hole. Stick the end of your screwdriver in the slot to release the wire. If you haven't bought your new switch yet, you can put electrical tape on the ends of the wires, stick them back in the box, replace the switch plate, and turn the power back on until you are ready to replace the switch.
Step 5: Install the new switch. Again, make sure the power is off. Connect the wires to the terminals, making sure the wire loops curl clockwise, the same direction the screw turns when you tighten it.
Step 6: Replace the ground wire. As mentioned, the new switch may have a green grounding terminal screw on the yoke. If the switch you are replacing had a green or bare wire attached to it, attach that wire to this screw. If the old switch didn't have this green screw but there is a green or bare wire running through the box, attach a short piece of 14-gauge green or bare wire to the screw and then use a wire nut to splice the other end of this wire into the green or bare wire running through the box.
Step 7: Reinstall the switch and plate. Push the switch back into the box, install the two screws that hold the switch in the box, and then install the switch plate.
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