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wiring up a 3 way switch

This sounds foolish but... the switches at the top and bottom of my stairs were in working order until someone(whom shall remain nameless-but he is a know it all) thought he would do me a favor and put in the new switches the old one were removed and then "tada" the issue....neither I or my "helper" can get the lights to operate properly I didn't see how it was wired but it's old cloth wrapped wire.. for both boxes there is (from what I can tell)a black wire...a red wire...and a white wire? The light is at the end of the run. I can't tell where the live wires come from because my little home owner electrical tester doesn't seem to find any combo of wires that give me reading, when I wire up a switch I can get a reading but the switches don't work the proper way meaning one switch needs to be in a certain postion then the other will work for turning off and on but not both switches. Thanks guys!

Re: wiring up a 3 way switch

It seems there must be 6,392 ways to wire up 3-way switches, and only one of them works. Actually, when you are replacing 3-way switches in existing wiring, there are 36 different ways, and only four of them will work.

TURN OFF THE POWER TO THIS CIRCUIT. Verify that the circuit is indeed off. A non-contact voltage tester is great for this purpose. Eye protection is recommended whenever you are doing electrical work.

Remove the switches, leaving the wires unconnected. Plug in a 3-prong extension cord into a GROUNDED outlet. It must be grounded, not one where someone's put a 3-prong outlet into a two-wire circuit. For safety, place a piece of electrical tape over the "slits" on the other end of the cord, leaving the round ground hole uncovered.

Set your multitester to an "Ohms" setting. Verify that the battery is good by touching the probes together. The needle should move all the way to the right. With the probes touching, turn the "ohms adjust" knob so the needle points to zero -- all the way on the right.

At one switch location, measure the resistance between the ground hole of the extension cord and each wire. You'll make three measurements. One of these will likely show a lower resistance (the needle will move). Make a note of which wire shows the lower resistance.

Repeat the above procedure at the OTHER switch location. If one of the locations doesn't show a reading, don't worry.

Remove the light bulb(s) from the fixture(s) and repeat the resistance test on the wire(s) which previously showed a reading. The wire that shows no reading where it did previously is the wire leading to the light fixture. Make a note of this. Now, at this location, connect the other two wires together. (You are done with the extension cord.)

Go to the other location. Measure the resistance between each pair of wires (three measurements). One of these measurements will show ZERO resistance (the needle will move all the way to the right). The other wire is the line providing the power source.

Now comes the fun part. On the three-way switches, you'll notice that one of the screws is a different color, usually blackish. On the back of the switch, it will be labeled "common". Connect this screw to the wire leading to the light fixture. Connect the other two wires to the remaining screws; it doesn't matter which goes where.

At the other switch location, connect the wire providing the power to the common screw, and the other two wires to the other two screws. Install the switches into the boxes, replace the cover plates, replace the light bulb, and turn on the power. Test your handiwork.

Hint: the two wires that are connected to the brass-colored screws are commonly called "travelers" and simply run between the switches.

A SAFETY NOTE: You should put the cover plates on before turning on the power. In the event there is arcing in one of the boxes, the cover plate will prevent molten metal from being ejected, which could be a fire hazard.

For those reading this who have four-way switches, the three-way switches will be at each "end" of the circuit; the 4-ways will be in the middle. At the 4-way switch locations take a close look at the wires in the box; two will be bundled in one cable; the other two in another cable. Connect the pair from one cable to the black screws; connect the pair from the other cable to the brass screws.

Re: wiring up a 3 way switch

Although there are multiple wiring paths that can be involved there is only one way to wire 3-way switches. Each switch has 3 terminal screws 1 on one end and two on the other. The two screw end is where the travelers are connected that go between the two switches. The one screw end has the power connected on one switch and the load (light) on the other switch. With only a 3 conductor wire in each boxes and if it was wired properly to begin with, in both boxes the black wire should go on the single screw and the white and red on the other two screws (travelers). It makes no difference which side you put th white or red.

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