Test and Replace a Single-Pole Light Switch
Chose the right light switch
Check maximum ampere and voltage ratings: 15A-120V indicates that the switch can be used for up to 15 amperes of current at 120 volts. 15A-120V AC or AC ONLY stamped elsewhere on the switch indicates that it can only be used in alternating current systems only which is being used in standard residential homes today.
The two brass terminals are normally found on the side of the switch. The most common configuration of side-wired switches has an upper and lower terminal on the same side. One of the terminals receives power from the main power box and the other is connected to the light fixture. When you toggle the switch to ON, it closes the circuit and feeds the light with power.
Some switches have a grounding terminal on the mounting strap at the top or bottom. The terminal is colored green or marked GR. Grounded switches are recommended for use in plastic boxes that do not contain a grounding bar.
Wire specifications appear on the mounting strap and on the back of the switch. Switches rated at 30A and above and with CU/AL markings accept copper, copper-clad aluminum, or aluminum wiring. A 15A or 20A switch with a CO/ALR stamp can also be used with copper, copper-clad aluminum, or aluminum wire. Switches without these markings are for use with copper wiring only.
The prescribed wire size appears on the back of the switch. Most light fixture circuits use 12 to 14 gauge wire. On the back of some switches, there are push-in terminals accompanied by a wire strip gauge that tells you what size of wire can fit in. It’s recommended to use the screw terminal as they provide more secure connections.
Remove the wall plate
Turn off power to the switch by removing the fuse or tripping off the circuit breaker in the main power box. Remove the two screws on the wall plate, lift the plate from the wall, and tape the screws to the plate so you don’t lose them.
Free the switch from the box
Release the switch from the box by loosening the mounting screws at the top and bottom of the mounting strap. Grasp the mounting strap and pull the switch out of the box to expose the wires. Do not touch any wires or terminals until you’ve checked the voltage!
Identify the wiring
Check the number of wires entering the box. When the switch is middle-of-the-run, at least two cables enter the box. Each cable has one black wire, one white wire, and one grounding copper wire (not insulated). The black wires are hot and are connected to the switch terminals. The white wires are neutral and are connected together inside the box with a wire cap. The grounding copper wire should be connected either to the metal box ground terminal or to the switch ground terminal if the box is plastic. When the switch is end-of-the-run, only one cable enters the box. Both the black and the white wires are hot and are connected to the switch terminals. The white wire should be color-coded with black paint or electrical tape.
Test the switch
Use a voltage tester to confirm that the power is off by touching one of the tester’s probes to the grounded metal box or to the ground wire in the plastic box. With the other tester probe, touch the switch terminals one by one. If the tester indicates that there is power in some of the terminals, trip the correct circuit breaker or turn all the power off if you can’t find it.
With the power on, you can test if the switch is faulty. Using the same procedure, check the switch terminals one by one and toggle the switch. One of the terminals should always have power no matter if the switch is in the ON or OFF position. The other one should show power readings when the switch is ON and no reading when the switch is OFF.
If one of the terminals has power all the time and the other shows nothing no matter of the switch position, the switch is faulty and needs to be replaced. With the power off, you can test the switch with a continuity tester. First, make sure that the power to the switch is OFF. It’s recommended to remove the wires from the switch terminals since the tester could give you false readings.
Attach one of the tester’s probes to one of the switch terminals and the other probe to the other terminal. Toggle the switch and the tester will show you if the switch closes the circuit or not.
Working on the connections
Begin by checking the connections at the terminals. If a wire is loose, rehook it around the terminal and tighten the screw. If the connections appear dirty, detach the wires and clean the terminals and the wires with fine sandpaper. Or clip the wires, strip the insulation to the right length, clean the terminals and reattach the wires. If the light still doesn’t work, test the switch and replace it if necessary.
Remove the switch
Loosen the terminal screws and detach the wires. When there’s a single cable in the box, mark the white wire with black paint or electrical tape if not marked already. If there’s a green grounding terminal on the switch, loosen the terminal screw and detach the grounding wire. If the wires are attached to push-in terminals, insert a small screwdriver into the release slot of each terminal and pull out the wires.
Install the new switch
Loosen the terminal screws of the new switch. Hold the new switch so that the toggle points down when it is OFF.
Hook the wires clockwise around the correct terminals: Connect one black wire to each terminal or if one cable is in the box, the black wire to one of the terminals, and the color recoded white wire to the other terminal.
Tighten the connections firmly. If there is a green grounding terminal on the switch, ground it the same way as the old one was. Set the switch back into the box, carefully folding the wires to fit inside the box making sure that the switch sits straight.
Put the wall plate back in place, turn the power on, and check if the switch turns the light ON and OFF.
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