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how to add a switch to an existing circuit

hello... i want to add a second switch to my garage. we have one switch in the garage that turns all the power on and off but i want to add a second one. in our house we have two switches that conttroll one light and i want to do the same for the garage. how do i do that?

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Posted 2012-06-26T18:13:35+0000  by sharpes sharpes

Hello sharpes.  Welcome to the Community!


The two switches in your house that both control one light are called 3-way switches.  They work differently than a regular switch in that there is no "on" or "off" position or labeling on 3-way switches.


3 way switch.PNG

Let's watch a Home Depot video on replacing a 3-way switch to get an idea on how they work.

Click on the picture below:


3 Way Video.PNG


Here is a diagram of how the switch circuit looks:


Switch Diagram.jpg


Ok.  How these switches will exactly be wired into your house system can vary.  I do know that you will be replacing the original single switch with a 3-way switch, and adding a 2nd 3-way switch and the wiring to get to and from it.


I also recommend that you consider purchasing the Home Depot Wiring 1-2-3 book.

You can find it here:


Wiring 123.jpg



I hope this helps,




Posted 2012-06-26T19:47:30+0000  by Chris_HD_CHI

Hello sharpes, let me make this even easier for you.. You need to install what we call in the Electrical field as a BACKFED 3WAY SWITCH.

  If you already have an existing switch that's a great start: half the work is already done


1. Turn off the power and remove the switch. (You will need to use an electrical tester) You should have a POWER SOURCE and a SWITCH LEG connected to the switch. Also a ground which will be either BARE COPPER or GREEN.

Last thing there should be WHITE NEUTRAL wires tied together buried in the back of the box.


2. After removing the switch, turn the power back on and with your tester put one lead on the NUETRAL wire and the other lead on one of the other wires that were connected to the switch. ( You need to find out what wire is the POWER SOURCE.)


3. When you find the POWER SOURCE mark it and  also mark the SWITCH LEG, now push them aside  ( now turn the power back off)


4. You now need to run 14-3 romex from your existing box to wherever your new other switch will be located.


5. Now go back to your original switch box and wire your 3way switch.  You will notice the 3way switch has 4 screws on it: 2 will be the same color and one will be a separate color.  Last one is Green which is ALWAYS your ground.


Take the BLACK and RED wire out of the 14-3 you ran and connect them to the 2 screws that are the same color...( IT DOESNT MATTER WHAT ORDER)

Now take your SWITCH LEG wire you put aside and attach it to the other colored screw on the switch.

Last thing attach the ground wire to the green screw and tie it with the other grounds in the box.


6. You should now only have 2 Wire left sticking out.   Your POWER SOURCE  and the WHITE wire coming from the 14-3. 

 Tiethem together and put your switch in the box and install the plate.


7. Now go over to your new switch..Now this is very simple there is only 4 wires and 4 screws left.

Take your RED and BLACK wires and connect them to the 2 same colored screws on the switch( DOESNT MATTER WHAT ORDER).

Next, connect the WHITE wire to the 1 screw that's by itself.

Last thing is to connect the ground wire to the GREEN grounding screw.


8. Now close up the box and turn the power on and enjoy your new switches.


Hope this helps you.




Posted 2012-06-27T15:15:44+0000  by AAriondo
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