How can switches across the room from each other control the same light in my house? Are they in the same circuit? Is there a limit to the number of switches?
In: 10
They are in the same circuit. There is a limit, but the physical/electrical limit is huge. The practical limit is much smaller. There are multiple ways they can control the lights
The most common way is using a 3-way switch. It lets two switches control the same light (or set of lights). They’re all linked on the same wiring.
3-way switches don’t actually have an “off” position where there is no electricity coming out of the switch. Instead they switch back and forth between 2 different outputs on the switch. When they’re connected correctly, when you change the position of one switch, it changes the path of the electricity. 3 way switches require an extra wire (normal wire has 3 wires: black, white and ground – you need a 4th (red) wire for 3 way connecting)
Of course, you can now replace a light with a smart bulb and control it with any number of switches, phones or Alexa, but that’s a different thing.
A normal, two way, switch has two wires: the input, and the output. When the switch is on, they are connected, and when its off, they are disconnected.
You can also make a three way switch which has three wires: an input and two outputs. The switch swaps which output is connected to the input. With this, you can have two switches connected to the same device(s), with the double wire going between the switches and the “input” of one switch being the true output.
You can add more switches by making them four way switches. With these, you have two inputs and two outputs, and the switch swaps which input is connected to which output. Each of these four way switches is put in between the three way switches, and if you have an even number in differing positions, the circuit fully connects.
As a note, you can always make a four way switch into a three way switch by not connecting an input. You can also make a three way switching into a two way switch by not connecting an output. The more paths the switch has, the more complex and thus the more expensive it is.
TLDR: All of the switches will be on the same circuit, and on the same side of the device. There is no limit* to the number of switches you can have controlling the same device(s), however it just adds more cost, and there is rarely a benefit in having more than two switches controlling a device.
You can also use smart switches, and have them talk to each other outside the electric flow as an alternative solution. This is more expensive, newer, and adds more points of failure, however – its only really useful if you cannot add the extra wires or want to be able to control it remotely anyways.
* Well, eventually the resistance in all the wiring will be too high and no power will flow, but that number is so insanely high as to be irrelevant.
It’s called a three way switch. It routes the voltage on a path where either switch can make or break the circuit. A 4 way switch can be used between any two 3 way switches to add another switch. Any combination of 4 way switches and two 3 way switches can control one bank of lights. In any home it will be on one circuit.
There are other setups where multiple switches control one bank of lights that break all the rules above, but for basic setups in modest homes and modest commercial setups what’s above is what you’ll see.
Say there’s a walking path that passes through a tunnel that carves through a hill. The tunnel has two separate lanes that run parallel to one another, but aren’t connected. At each end of the tunnel is a sliding door that always blocks one of the tunnel entrances, but not the other.
Say you start out with one of the lanes completely shut, at both ends. Since each door can only block one lane at a time, and both of them are currently blocking the same lane, it means the other lane must be open. People can walk all the way through, so traffic flows happily through the tunnel.
Now, slide one of the doors over. You now have opened up the previously closed channel at one end, but at the same time, shut the channel that used to be open. Now no one can get through either channel, because both of them are blocked on at least one side. Traffic flow through the tunnel comes to a screeching halt. This will happen no matter which door you choose to move.
Pick another door and slide it. One of two things will happen: either you pick the same door you moved the first time and move it back to where it was when we began, or you pick the other door, and move it over to the same lane you moved the previous door to. Both situations get you the same result: you now have one fully blocked channel and one fully open channel, meaning traffic can flow once more.
With this system, you can always toggle the tunnel between open and closed just by sliding one of the doors. And it never matters which door you slide–all that matters is every time you slide a door, the tunnel toggles to the opposite state.
If you substitute tunnels and traffic flow for wires and electricity flow, and swap the doors for switches, you have the exact mechanism that lets you control a light with two switches.
It’s sometimes called a hotel switch, and it looks kinda like this. Remember that the circuit has to make a loop, but I’ll draw just one direction that has all the switches.
/============. .———
power =====O’ X .O======= light ON
———-` `==========/
^ SWITCH ^ SWITCH ^ SWITCH
So here you see the power going through the lines (the double equals line), and three switches. The left and right switches just switch between the top and the bottom line, while the middle switch can switch between “connecting straight” and “connecting crossed”.
So if you would switch the first one from “top” to “bottom”, the light goes off
———. .==========
power =====O. X .O——— light OFF
‘==========. ———/
^ SWITCH ^ SWITCH ^ SWITCH
Similar with the middle one when switched to “straight connect”
/===========. .===========
power =====O’ – .O——– light OFF
———— ————/
^ SWITCH ^ SWITCH ^ SWITCH
You can add as many of the “cross switches” as you want, having many switches for a single light.
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