In this application where two 3way switches already exist a 4way switch may be added by simply installing two 2 wire with ground cables of the same wire gauge as the circuit wiring from either of the 3way light switches to the location of the new 4way switch. Hi phil great article helps me understand how 3 and 4 way circuits work.
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Adding a 4 way switch to a 3 way circuit. One 3 way circuit controls a light at the top of the stairs. Connect the returning travellers from the new switch box location red and white and connect them to the in or the out terminals on the new 4 way switch. All the 4 way switches are optional. Here is an example of a light in the middle of the run. I have an older home with a stair way down to a hallway. The 4 way switch will have 2 terminals marked in and 2 marked out.
Connect the black wire of the new 3 wire cable to the common screw of the 3 way switch that you will move to the new location. If you only need two switches leave out both the 4 way switches. All you need to do is find the 4 wire that currently runs from one of your 3 way switches to the other. Is it possible to connect 2 separate 3 way circuits to make a single 4 way circuit. The second controls the light in the hall. You will need to cutbreak this and add the 4 way switch in between.
You can also add as many 4 way switches as you like in place of any one shown in the circuits that follow. Here are another couple of variations of 3 and 4 way switch circuits.
Gallery of Adding A 4 Way Switch To A 3 Way Circuit
