One Light, Two Switches
When I moved into my "new" house last year, I quickly realized that something about the switches for the light above the stairs wasn't right. Once the upstairs switch was "off", the downstairs switch no longer turned it on.
Although it isn't hard to wire up two switches to control one light, cleaning up after someone else who did it wrong can turn into quite a puzzle.
The first step is understanding exactly how these simple, convenient circuits work. The diagram below shows this. This is not the only way to wire it, just the simplest. If you want to see a few of the stranger variations, check out this link. Note that each one does the same thing, it just splices the wires in different places.
To begin unraveling the puzzle you will need a multimeter, screwdriver, and maybe a pair of pliers.
First, flip the breaker for the circuit you're working on.
Pull the covers off both switches and pull unscrew the switches from the box.
Before touching anything, use the multimeter to check for voltage on every wire; obviously, it should be zero.
Next, make sure that both your switches are actually "3-way" switches and not just "normal" single pole switches. The most obvious clue is that 3-way switches do not have ON and OFF written on them and will have three posts on the sides, plus a green ground post. To confirm that they're working the way they should, check for continuity between the hot pole and one of the other two. The hot post should be marked as such, and will also be a different color. Then flip the switch; now there will be continuity between hot and the other pole.
Now, onto the hardest part; figuring out which wire is which, so you can hook them up correctly.
If you're lucky, each wire will be a different color, will follow NEC standard color conventions, and wires of the same color will always be spliced together.
Or, your boxes could look more like mine, with a white wire, a yellow wire, and 4 black wires at one end and two black wires and a red at the other. Um...
If that's the case in your house, its time to make that multimeter pay for itself. The easiest wire to identify is common (usually white). Use your multimeter to check for continuity between one of the wires and ground (the bare copper wire in newer houses, or the metal conduit/box if you have older style wiring).
To identify the two wires that bridge the switches, pick a switch box and number each wire. Then tape or twist them, one at a time to ground at that switch. Then at the other switch check for continuity between each wire and ground. When you get continuity, number the wire at that end.
Unless you have a bunch of extra wires, the only one left will be the hot wire. If you do have extra wires, they are probably just passing through the box and can safely be ignored as long as there are no exposed ends.
References:
http://users.wfu.edu/matthews/courses/p230/switches/3way/variations.html
http://www.allaboutcircuits.com/vol_5/chpt_2/2.html
Although it isn't hard to wire up two switches to control one light, cleaning up after someone else who did it wrong can turn into quite a puzzle.
The first step is understanding exactly how these simple, convenient circuits work. The diagram below shows this. This is not the only way to wire it, just the simplest. If you want to see a few of the stranger variations, check out this link. Note that each one does the same thing, it just splices the wires in different places.
To begin unraveling the puzzle you will need a multimeter, screwdriver, and maybe a pair of pliers.
First, flip the breaker for the circuit you're working on.
Pull the covers off both switches and pull unscrew the switches from the box.
Before touching anything, use the multimeter to check for voltage on every wire; obviously, it should be zero.
Next, make sure that both your switches are actually "3-way" switches and not just "normal" single pole switches. The most obvious clue is that 3-way switches do not have ON and OFF written on them and will have three posts on the sides, plus a green ground post. To confirm that they're working the way they should, check for continuity between the hot pole and one of the other two. The hot post should be marked as such, and will also be a different color. Then flip the switch; now there will be continuity between hot and the other pole.
Now, onto the hardest part; figuring out which wire is which, so you can hook them up correctly.
If you're lucky, each wire will be a different color, will follow NEC standard color conventions, and wires of the same color will always be spliced together.
| Function | label | Color, IEC | Color, old IEC |
|---|---|---|---|
| Protective earth | PE | green-yellow | green-yellow |
| Neutral | N | blue | blue |
| Line, single phase | L | brown | brown or black |
| Line, 3-phase | L1 | brown | brown or black |
| Line, 3-phase | L2 | black | brown or black |
| Line, 3-phase | L3 | grey | brown or black |
Or, your boxes could look more like mine, with a white wire, a yellow wire, and 4 black wires at one end and two black wires and a red at the other. Um...
If that's the case in your house, its time to make that multimeter pay for itself. The easiest wire to identify is common (usually white). Use your multimeter to check for continuity between one of the wires and ground (the bare copper wire in newer houses, or the metal conduit/box if you have older style wiring).
To identify the two wires that bridge the switches, pick a switch box and number each wire. Then tape or twist them, one at a time to ground at that switch. Then at the other switch check for continuity between each wire and ground. When you get continuity, number the wire at that end.
Unless you have a bunch of extra wires, the only one left will be the hot wire. If you do have extra wires, they are probably just passing through the box and can safely be ignored as long as there are no exposed ends.
References:
http://users.wfu.edu/matthews/courses/p230/switches/3way/variations.html
http://www.allaboutcircuits.com/vol_5/chpt_2/2.html
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