How to Have Two Switches Controlling One Light Bulb

This article focuses on the types of switches and connection needed to have two switches operate one light bulb. It also explains how to wire two switches and a light fixture in your house.

In any electrical circuit in your home, the wire carrying 110 volts is known as the hot wire. The hot wire that is connected to the fuse in your fuse box is usually a black wire.

If you want two switches to control one light bulb, you cannot use the ordinary switch typically used in an arrangement with one switch controlling a light bulb. So let's look at the different type of switches.

The article covers two types of switches. The single pole single throw switch shown in figure one is used when you want one switch controlling a light bulb. The pole is the common point on the switch. The switch has two positions. It either connects to the contact labeled A providing an electrical connection to that contact or it does not connect thus making the switch an open circuit. This switch is often referred to as a SPST switch. SPST stands for single pole single throw.

The other type of switch shown in figure one is a single pole double throw switch. With the switch set one position, the pole is connected to contact A. With the switch turned to the other position, the pole is connected to contact B. There is no position making the switch an open circuit. This switch is often referred to as an SPDT switch where SPDT stands for single pole double throw. This switch is used when you want two switches controlling one light bulb.

For this explanation we will refer to the switches via their designations in figure two: S1 and S2. The letter S followed by a number is the standard designation for a switch.

Figure two shows two SPDT switches connected to one light bulb.You can view figure two on page two of this article. The switch S1 is in a position to connect it's pole to contact A and the switch S2 is in a position to connect it's pole to contact B. The light bulb is off because there is no electrical connection between S1 contact A and S2 contact B.

The user changes the position of switch S1.

Now switch S1 has it's pole is connected to contact B. Note that the switch S2 still has it's pole connected to it's contact B. Hence we have an electrical connection from the hot wire and the light bulb as shown in Figure 2 and the light bulb is on.

The user changes the position of switch S2.

Now switch S2 has it's pole connect to contact A. Switch S1 has it's pole connected to contact B. The light bulb is off because there is no electrical connection between the two switches.

The user changes the position of switch S1.

Now switch S1 is has it's pole connected to contact A. Switch S2 also has it's input connected to contact A. The light bulb is on.

That concludes the explanation of how to connect two switches to one light bulb. Now let's talk a little bit about how to connect this up in your house. Of course you must follow the National Electrical Code published during the current year. You could find an abridged copy of that code at your local book store.

I strongly advise the reader to have a licensed electrician install and wire the switches and light fixture. The following paragraphs explain the wiring involved.

You will have a two wire cable from the fuse box to the first switch S1. We will reference this cable as cable A. The hot wire connects to the pole of switch S1. The other wire is common. Since common is connected to ground in your house, the diagram showed it as ground.

You will have a three wire cable to connect between the two switches S1 and S2. We will reference this cable as cable B. The first wire will connect contact A of switch S1 to contact A of switch S2. The second wire will connect contact B of switch S1 to contact B of switch S2. The third wire is connected to the common wire (from cable A) in the utility box holding the first switch S1. That common wire came from your fuse box in the two wire cable.

You will have a second two wire cable that we will reference as cable C. One wire will connect the pole of switch S2 to the hot black wire of the light fixture. The other wire will connect the common wire of cable B to the common white wire of the light fixture.

That concludes this article. I hope it was helpful.

References:
I have a Bachelor of Science in Electrical Engineering
I majored in interior house wiring in a vocational technical high school.

Miller's Guide to Home Wiring
ISBN 0-07-144551-X