Adding a switch to an existing box for a light is easier than you think. You need to ask, are you putting in a double stack or adding a separate switch. Either one is fine. This article is going to assume you have the wire ran already. Read #'s 1-9 of this article if you haven't ran or installed the light or wire yet. Do not do #10 unless your not putting dry wall up. After 9 come back to this article.
Adding a Switch to an Existing Switch Box
If you have any questions please ask in the comment link below, I will answer them to the best of my ability, also if you have future projects that you would like step by step instructions please post in the comment box I will usually answer the same day.
b) Linesmen pliers
c) Wire Nuts
d) phillips head screw driver
f) ground screw
2) Finding the hot wire. Look at the existing switches. If your other switch has two wires turn the switch to the off position, grab your meter and set it to AC. It should be set on a voltage higher than what you are working on. We are working on 120v so probably 200 on your meter. Touch one of your leads from your meter to the metal box, if your box is plastic touch it to the bare wires in the box. Take your other lead and touch it to one of the screws on the switch that has the wire connected to it. We are looking for the live wire, so if that screw has no power touch it to the other screw, that one will be live all the time. Mark the live wire with black tape. If your switch has three screws test the black screw the same way. If you loose power in the screw when you flip the switch check in the box for other wires that have constant 110v. If you do not, you may need to add a wire from a receptacle to your switch to achieve this. You should have white wires in your box so be sure and test. Take your leads from your white wires to the hot, you should have 120v. If you don't have any whites in you box your light will not work without a neutral, so get a white wire from a receptacle and feed it to your switch.
2) Shut the power off to the switch. Go to your panel (your panel has the breakers or the fuses in it) and turn the breakers off one by one. Changing a switch is a one man job but if you want to avoid running back a forth another person standing there and yelling at you when the light shuts off helps. Or shutting all the breakers off and the main just to be sure is what I would recommend. This is electricity SAFETY FIRST!!!!!
3) Connect the Neutral and ground. You should have a white wire with your new wires. Connect it to the other white wires in your box. You should also have a green or bare with your new wires(unless you have pipe, we use the pipe as ground so your good) connect it to the grounds in your box or connect it directly to the box with a ground screw. The box has a threaded hole in the back for this screw.
4) Single pole switch. Skip this step if you have a double stack switch. Remove the live wire from the switch (with the power off.) Cut two 8" lengths of black wire and connect them to the live wire with a wire nut. Your going to do this by making sure you strip the two 8" wires about an 1" or more and holding them all together, and twisting. It should look similar to this IMAGE. Now you have two wires connected to the one live one, so connect one wire back to the old switch and the other to the new switch. Connect the wire coming from the the light(not the white, green, or bare probably black) to the other screw on the new switch.
5) Double stack switch. Connect the live wire to the side of the switch that has the copper bar in between the screws. Connect your old switched wire to one of the screws on the side without the copper bar in between the two screws. Now connect the new wire to the remaining screw on the side without the copper bar in between the two screws.
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