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

Picture of 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."
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Step 1: The Light Comes "On"

Picture of 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

Picture of Turning the Light
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

Picture of The Light Comes
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

Picture of 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

Picture of 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"

Picture of 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.
Selfbuilder27 days ago

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.

Phil B (author)  Selfbuilder27 days ago
A 4-way switch could be used as a 2-way or on/off switch. But, if by common you mean the white wire or neutral, do not connect it to the switch, or you will get a direct short between the hot and the neutral. An on/off switch is installed in the path of the black or hot wire.
zappenfusen2 years ago
Great explanation but I must throw this one out there. An instructor presented His class with a Garage having 1 recpt. & one keyless (bare bulb socket), and switch for control.
3 conductors connected Garage to House where opposite 3-way switch is located. Garage recpt. is constantly hot, Garage light controlled by 2 switches in House and Garage. Using only existing 3 conductors how does this work? I must add, no it doesn't meet accepted code requirements. If someone can schematic this out, I'll dig out my old notes to verify because I have forgotten how you cheat this in since I took the course 30 years ago. My instructor was an Electrical inspector in his 70s who took great pleasure in tormenting his protege's with problems such as this even when they obviously were against common code basics. In his time though a grounding, non-current carrying conductor was unheard of except in Submarines.

Phil B (author)  zappenfusen2 years ago
hot 3-way.bmp
Phil B,
The problem & violation as I'm sure you're aware of is polarity switch at ceiling outlet. In varied situations both lamp shell and bulb base can be both hot, both neutral, lamp shell neutral, bulb base hot (code), lamp shell hot, bulb base neutral (code violation). I haven't pulled notes showing ckt. with polarity switching receptacle as opposed to light socket, which was example instructor used to emphasize danger of switching and breaking neutral through switches.
Great explanation though. I too required old electrician's drawings & advice to fully understand that VooDoo, WhoDoo of the multiple location switch points and the various legal wiring methods of same. Great Instructable I'm sure helped many actually understand how & why it works. Original multiple switch, Pull Chain Keyless in multi floor apt. house with long chain down staircase!
Phil B (author)  zappenfusen2 years ago
The image in my response below shows a "hot 3-way switch" circuit. I believe it is what Zapp had in mind. I found it on the Internet by searching for "hot 3-way." When I was my father's helper back in the 1960s, he often spoke of these. I have never seen one.
pedalmonkey5 years ago
 Excellent.  Whenever I wire a three-way I always have to go look it up on the box.  
Phil B (author)  pedalmonkey5 years ago
Thanks.  It would be great if the images in this will enable you to bypass looking at the box. 
ReCreate6 years ago
Ha Wow, I finally get how it works! It makes sense now!
Phil B (author)  ReCreate6 years ago
Thanks for your comment. I am glad to have been of help.
ReCreate Phil B6 years ago
Your welcome.:)
espdp26 years ago
Sweet! Thank you so much for explaining this, clear and concise. I feel like I've been "instructed." :-)
Phil B (author)  espdp26 years ago
Thank you. I am glad it was clear for you. Once you know this, you can use it to do things, like correct a three-way switch that is never worked correctly.
MePerson6 years ago
Wow, so obvious, yet I had no idea. Thanks!
Phil B (author)  MePerson6 years ago
Glad to have been of help.
hilbert95416 years ago
thank you whoever did this thing on 4 way switches. i figured a 4 way switch had 4 terminals, but i couldn't figure how it actually "switched" without leaving a completely open circuit. i think you made 4 way switches' concept very easy to understand. thanks again, josh -south carolina (united states)
Phil B (author)  hilbert95416 years ago
You are welcome. I am glad it was easy to grasp. My father did electrical work and farmed in a rural area of Iowa. I was his helper all through junior high to college. I have 57 other Instructables, all of which are something practical. You might be interested in some of those, too.
richms6 years ago
Thats not how they are wired here in NZ. Also if there are 2 switches it gets called a 2 way switch here since there are 2 switches. What they do is wire one of the switches normally, to add 2 way capabilities a piece of 3 conductor cable is taken from there to the other switch location, and 1 is connected to 1, 2 to 2, and C to C - the cable is red/white/blue since they are allowable phase colours (yellow used to be but in harmonisation with Aussie it was decided not to be since its easy to mistake for a green/yellow earth) Anyway, then the power is connected to the 1 terminal of one switch, and the 2 terminal takes the power out to the lamp. This is done since its normal that the power goes in and out of the same flushbox rather then taking a single conductor to the other end. It is sometimes done the way described with a wire joiner taking the cable to the C of the remote switch to the cable out to the light, but thats not how new installs are done.
Phil B (author)  richms6 years ago
I thought about how you wire 2-way switches in New Zealand. Then I went looking for an example of it on the Internet. Is this what you do (2nd or bottom schematic)? http://www.diynot.com/wiki/electrics:lighting:two_way_lighting
richms Phil B6 years ago
Yeah, thats the one (second in the top image) - sometimes it will go thru intermediate switches which just connect to the wires between the 1 and 2 terminals
Phil B (author)  richms6 years ago
Phil B (author)  richms6 years ago
I am not sure why we call them 3-way and 4-way switches in the USA, but we sure do. My graphic is more of a schematic than a plan for routing wires. Certainly we most often use three strand cables between the switches. What connects to what depends on the layout of the building and where the light, the switches, and the circuit breaker are in relation to one another.
We call them 3-way switches because switches are already 2-way (On/Off)
rimar20006 years ago
Spiff736 years ago
Thanks to Acme School of Stuff, I understood three-way. But, four-way completely baffled me. Thank you so much for clearing it up.
havanacus6 years ago
Thank you, this always bothered me...
Phil B (author)  havanacus6 years ago
I do not think you are/were alone by far. My father did electrical work and I often went with him as his assistant. I have met plenty of people who puzzled over how these things work. It is not that hard, once you grasp the concept. Thanks for your comment.
Scubabubba6 years ago
Thanks Phil! I could never picture how these work in my head, this is excellent.
Phil B (author)  Scubabubba6 years ago
Thank you. If you like being able to understand what may have been mysterious, wait until you amaze your family and friends by making a 3-way switch circuit work that never worked before. You will achieve rock star status. I think, too, the idea of comparing these circuits to a crossover at an Interstate construction zone makes it clear in one's mind.