In your house or apartment, you probably have at least one
light that is controlled by two separate switches. Whenever
you flip either one of the switches, the light changes its
state -- if it is on it turns off, and if it is off it turns
on.
If you have ever wondered how this arrangement works and
how each switch knows what the other switch is doing, then
read on. In this edition of HowStuffWorks,
we will solve the mystery of three-way switches!
Normal Lights Let's start by looking at how
a normal light is wired so that you can understand basic
residential wiring for a light switch. The figure below
shows the simplest possible configuration:
In this diagram, the black wire is "hot." That is, it
carries the 120-volt AC current. The white wire is neutral.
(For more information on household AC current and grounding,
see How Power
Distribution Grids Work.) You can see in the figure that
the current runs through the switch. The switch simply opens
(off) or closes (on) the connection between the two terminals
on the switch. When the switch is on, current flows along the
black wire through the switch to the light, and then returns
to ground through the white wire to complete the circuit.
The electrician who wires the
house normally uses Romex to run power from the
fuse box to the switches and outlets in the house. A piece of
Romex is shown here:
Romex
Romex consists of an outer plastic sheath (white in this
picture) with three wires inside. The black and white wires
are insulated, while a bare, third wire acts as the grounding
wire for the circuit. Most normal household applications use
12- or 14-gauge Romex.
Three-Way Lights To create lights that
operate with two separate switches, the electrician uses two
special pieces of equipment in the circuit:
Special switches known as three-way switches.
Special Romex wire that has an extra red insulated wire
along with the black and white wires within the sheath.
A normal switch has two terminals that are either
connected or disconnected. A three-way switch has three
terminals, and the switch connects the first terminal to
either the second or the third terminal, as shown here:
In a normal switch, the two terminals are
either connected (as shown) or disconnected. When they
are connected, the switch is "on." In a three-way
switch, the top terminal connects to one or the other of
the bottom two terminals. (Note: The actual terminal
configuration on a three-way switch depends on the
manufacturer.)
You use two three-way switches to control the light, and
wire it up as shown below:
In this diagram, a normal piece of Romex comes from the
fuse panel to the first three-way switch. The black "hot" wire
enters the switch on the left. Three-way Romex (which includes
red, black and white wires) runs from the switch on the left
to the switch on the right, with the white wire carrying
ground and the red and black wires carrying the output from
terminals in the left switch. Normal Romex runs from the right
switch to the light.
Assume that, with the switch up, terminals 1 and 3 are
connected as shown above. If the switch is down, then
terminals 1 and 2 are connected. You can see (if you trace
through the wires and the switches) that in the previous
figure, the light is off. You can also see that if you flip
either switch, the light will turn on. And if you flip
either switch again, the light will turn off. It is
actually a very simple arrangement once you see it all exposed
like this!
There are several other ways to wire three-way switches to
a light. For example, the power from the fuse box could come
in at the light fixture and there could be two switches in
series running from there. Or power can enter at the fixture
and then two switches can be arranged in parallel from the
light.
If you are trying to understand how a set of switches are
wired in your house, using an ohmmeter or a
continuity detector is the only way to reverse-engineer
what the electrician has done (make sure you turn the power
off at the fuse panel before doing anything with electrical
wiring). The great thing is that, if you know the basic
idea behind three-way switches and three-way wiring, it's
really easy to figure it out. And if you simply want to
understand what's going on to satisfy your own curiosity,
well, now you know!
For more information on three-way switches and related
topics, check out the links on the next page.
