Three-Way and Four-Way Switches--How They Work

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Introduction: Three-Way and Four-Way Switches--How They Work

While a three-way switch is very simple to many who visit Instructables.com, it is a mystery to many others. Understanding how the circuit works satisfies curiosity. It can also help to diagnose a three-way switch that does not work because someone wired the circuit incorrectly.

This is the basic circuit for a three-way switch. The gray circle represents a light bulb controlled by the two switches. It is gray because it is "off." The two lines ending near the left side of the drawing go to a power source, like the circuit breaker panel in your house.

The green rectangles represent the switches. Notice that one conductor comes into each switch, but two go out. When the toggle is thrown the pathway inside the switch shifts from one of the out conductors to the other.

Here you can see that electricity can flow along the upper wire through the first switch, but its pathway is broken at the second switch and the light remains "off."

Step 1: The Light Comes "On"

In this graphic someone has entered a room and flipped the switch at the right of the frame. The electricity that flows through the first switch now finds a pathway through the second switch and the light is "on" as represented by the yellow bulb.

Step 2: Turning the Light "Off" From Another Switch

Let us assume this is a large room. The person who entered and turned the light "on" with the switch at the right of the drawing decides to leave the room near the switch at the left of the graphic. He or she flips the switch to its other position. This makes a break in the circuit again, and the light is now "off."

In my mind I like to think of three-way switches as like a construction zone on a four lane highway. If one set of lanes is closed, a crossover in the median moves traffic over to one of the lanes normally used for traffic going in the other direction. If there were no second crossover, traffic would stop. But, the second crossover brings traffic back to the original lanes and traffic continues to flow.

Step 3: The Light Comes "On" Again

Someone enters the same room near the switch at the right of the graphic. He or she flips that switch. Now there is again a pathway for the electricity. This time it flows over the second of the two wires running between the switches.

When you encounter what was supposed to be a three-way circuit, and you can turn it "on" at one of the switches, but not at the other switch, unless the first switch is already "on," the problem is usually that one of the wires going into the switch is on a terminal for one of the two wires going out of the switch.

Not all three-way switches are the same, either. All of them have two screws on one side of the switch and one screw on the other side. But, the screws for the wires running between the switches may be on the same side of the switch, or they may be on opposite sides of the switch at the same end of the switch. You cannot make assumptions. It is not uncommon to find that one switch in a three-way circuit uses one arrangement, but the other is from a different manufacturer and uses a different pattern for the screws.

If you have a continuity tester, you can shut "off" the circuit breaker and test the switch to determine which screw connects to what.

I helped a friend at his vacation home. He had a three-way switch at the top and bottom of a stairway. Since the time the home was built thirty years ago he has had to turn the switch at the bottom of the stairway "on" first. Then he could turn the light "off" or "on" from the top of the stairway where the bedrooms are. If the switch at the bottom of the stairway was "off," he could not turn the light "on" from the top of the stairway. The problem was what I mentioned above. The wire going into one of the switches was on a screw terminal for one of the wires going between the switches. In this case, I did not have any meters or testers, but had to reason out the problem. I got it solved in about ten minutes. He is happy now because his three-way circuit works as it should.

See the second image. This is an update. I discovered an exception to the rule that three-way circuit problems are due to two wires transposed into each other's place. The photo shows a dimmer switch that can be used as a single-pole switch or as a three-way switch. These switches fail in time due to overheating. When they do, the switch may work when the toggle is up or down, but not in both positions. This can make you think two wires are in each other's place, but the switch simply needs to be replaced. A continuity tester does not work on one of these switches. If you find one on a three-way circuit that is not working, just replace the switch and your problems will likely be solved.

Step 4: Four-Way Switches

Suppose you have a very large room with more than two exits. You want to control the lights from any of the exits. You need a four-way switch for each additional exit.

The graphic shows the pathways through two four-way switches. One represents the pathway when the toggle is up and one represents the pathway when the toggle is down. Think again about median crossovers on an Interstate highway. A four-way switch has the effect of reconnecting the two wires running between two thee-way switches. Four-way switches accomplish the same thing as if someone ran to one of the three-way switches and flipped it.

Step 5: A Four-Way Switch in the Circuit

Here you see a four-way switch added to our circuit. It is installed between two three-way switches. In this graphic it provides a pathway for the electricity and the light is "on."

Step 6: Turning the Light "Off"

In this graphic the four-way switch has been moved to its other toggle position. There is a pathway for the electricity through the four-way switch, but that pathway ends at one of the three-way switches. Trace your finger over the wires as shown on the screen and you can see this more clearly.

Any number of four-way switches may be added between two three-way switches. Flipping any of the switches in the circuit will turn the light "on" if it is "off" and vice versa.

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    38 Comments

    thanks for this. I was just talking to the ex's dad yesterday about this. however his wiring diagram is different... so now I am confused haha.

    I think i play with this on a 12v board using leds before setting this up live (home automation .. relay and 3 way switch depending on if i wana turn on with relay or manually)

    Below a man from New Zealand commented that 3-way switches are wired differently there. Yet, after I found an example of a circuit used there, basic key features are the same. And, my father did electrical work. He sometimes spoke about a "hot 3-way." You can search and find diagrams of how they were done, but, they are now illegal because current feeds back through another circuit and can give shocks when you think everything has been turned off, but is not really off.

    Hi Phil, great article helps me understand how 3 and 4 way circuits work. Is it possible to connect 2 separate 3 way circuits to make a single 4 way circuit. I have an older home with a stair way down to a hallway. One 3 way circuit controls a light at the top of the stairs. The second controls the light in the hall. There are 2 switches at the bottom of the stairs that control each of the circuits. Can I put a single 4 way switch at the bottom of the stairs and be able to turn all the lights from all switches?

    I am not fully qualified to say. It seems like it might be possible. In days of old people sometimes got creative with 3-way switch circuits and improvised variations that are now illegal because they are dangerous. You can read about the so-called "hot theee-way." I would want to be certain I know what each wire in your walls does and where it goes. Be sure to cap off any unused wires.

    Levitron, Lutron, and everyone else who make switches should pay you, print this, and jam it in the package with their 3-way and 4-way switches.

    Thank you; this is outstanding because it made something that was previously black magic into something that's actually pretty simple. :-)

    Thank you for looking and for commenting. I am glad it is useful to you. I understand the black magic part. When I worked for my father as his go-fer, I did not have trouble understanding 3-way switches. My father would try to explain 4-way switches to me, and I just did not get it.

    I hope one day you will be able to troubleshoot someone's improperly wired 3-way switch and make it work. That person will look on you as a god. Everyone needs that now and then.

    Dear Phil following my earlier post re:(4 pole switches).
    I have just watched your youtube post on 4 way switches..
    I must say what a god-send explanation on 'intermediate switches' ; (which my son has been tuitioned to call them,?)..
    A priceless explanation.
    "Just by your use of calling feed wires, 'travellers'. I can now see the wood from the trees.!!
    THANKS EVER SO MUCH Phil B...
    Now i can explain it to my son, no problem..
    (Fantastic, simple, short & Sooo easily explained.).
    WELL DONE & Cheers again Phil...
    Stay Safe dear instructor friend.

    I am glad your problem has been solved. I do not have a video at YouTube, but I am glad to know there is a good video there on these switches.

    I recently did an Instructable on how I added a reverse to the 12 volt child's toy electric vehicle for my grandson. I used a double pole, double throw switch to reverse the polarity to the DC motor.

    Thank you for looking and for commenting.

    My son has just started an Electrical Installation course..?
    I am a 'long-time ago' qualified electronic engineer..? lol.
    The son has been learning to wire up switches to lights, resistance etc..
    But a 2- way switch ( 3 wiring contacts 1 contact takes the conductor flow & the other two contacts, where your choice of switchin from one conductor pathway to another).It was called a single pole-double throw, in my day..
    He has been asking about a 4 contacts switch they call an 'intermediate switch'..
    I always called it a double pole- double throw: & it had a few various wiring possibilities..Dont quote me but i think he said ' it can only be switched on or of.'.His homework diagram on said; "Intermediate switch," seems to have 2 seperate contact paths, each pole to pole, ALWAYS ON, (1 for say Neutral wire, the other Live; in A.C wiring; both running in parallel.)..
    It therefore looks like if you clicked the switch, "it would just short-circuit both conductor paths.?? VERY DANGEROUS & IS OBVIOUSLY WRONG;
    Especially if 1 is Neutral running & the other Live..As his course is more Electrician than electronics.
    I wonder if someone could explain what an 'intermediate switch' is please & how it works.? Any help appreciated.
    Apologies for my paragraph sized explanation..Soz & Thanks.!!

    I just want to check that I can use an intermediate (4 way) switch as a 2 way switch by using one of the top connections as the common. Comments please.