Three Way Smart Switch Wiring

Introduction

Before getting starting wiring a smart switch, be sure to familiarize yourself with the articles on the introduction to wiring and basic (non-smart) switch wiring. It would also be beneficial if you have already wired a single smart switch. If you have not, review the article on wiring a smart switch for a good overview.

Smart three way switch wiring
Three of the switches in a five-way switch in our stairs.

Three-Way Switch Basics

We all have three-way switches somewhere in our homes. These are switches where one light can be controlled by two switches. Replacing an existing three-way switch with a smart switch may seem complicated. But the process is actually pretty simple and logical and makes use of existing wires in most junction boxes.

The Wires In a Regular Three-Way Switch

First, let’s start with a regular (non-smart) three-way switch. The main difference between a basic single switch and a three-way switch is that the three-way switch has an extra red wire (traveler). This extra wire provides another connection between the switches so that either switch is capable of sending power to the light.

How a Smart Three-Way Switch Works

In a smart three-way switch, only one of the switches sends power to the light (the primary switch). The other switch (add-on switch) communicates with the primary switch to tell the primary switch to turn on or off. In fact, the add-on switch does not even have terminals for the black wires. This may seem strange at first, but it should make sense. In a smart home, only one switch needs to be controllable by the smart network. The other add-on switch is just there for us humans to be able to manually toggle the switch.

How to Wire a Smart Three-Way Switch

With this basic understanding of how the smart three-way switch works, wiring a smart-three way switch should make more sense. The GE smart three-way switch comes with detailed instructions and a wiring diagram that is accurate. But it suffers a bit from too much information. So I am going to simplify the process and walk through the step-by-step wiring requirements for each switch. I will also cover why each is step is needed. Figure 3 below shows the wiring diagram for a three-way smart switch.

Smart three way switch wiring diagram

Try not to be overwhelmed by the diagram. Sticking with the theme of this post, it is not as hard as you think! There are only a few connections to make in each junction box, and you are already familiar with most of them if your have already done a single smart switch. I will walk you through the step-by-step instructions for each switch.

Wiring the Primary Switch

The primary switch in the left Junction Box in Figure 3 powers the light and has the smart controller in it. It does not matter which switch you choose as the primary switch. But the primary switch will have the blue indicator light on it, so that might be a deciding factor for placement. Wiring the primary switch starts off exactly like the single smart switch, with one additional step for the traveler wire. Here are the step-by-step instructions for wiring the primary switch:

  1. Power off.  Always be sure to turn the power off at the breaker before starting any wiring work. 
  2. Connect the black wires (line and load) to the terminals on the left side of the switch. The black wire that goes to the light connects to the top terminal (load), and the black wire from the circuit box connects to the bottom terminal (line). These are the same black-wire connections that you have on your old switch. So just transfer them over in-kind.
  3. Connect the white wire (neutral). For the white wire, it is necessary to splice off the existing white wire inside the box (extra wire conveniently comes with the smart switches). Connect the spliced white wire to the bottom right terminal of the primary switch (neutral). This wire is necessary to provide power to the switch itself.
  4. Connect the red wire (traveler) from your existing three-way switch to the upper right terminal of the primary switch. This provides a connection between the primary and add-on switches.
  5. Connect the ground wire to the ground terminal.
Three way smart switch wiring
Wiring on one-side of a three-way smart switch. This switch has two red traveler wires connected, because it is in the middle of a chain of a five-way switch.

Wiring the Add-On Switch

The add-on switch in the right Junction Box in Figure 3 is a little different. It starts off with a couple of familiar steps with the white wire and traveler, but has one new step with the black wires (step 3). Here are the step-by-step instruction for wiring the add-on switch:

  1. Connect the white wire (neutral). For the white wire, it is necessary to splice off the existing white wire inside the box (extra wire conveniently comes with the smart switches). Connect the spliced white wire to the bottom left terminal of the add-on switch (neutral). This wire is necessary to provide power to the switch itself.
  2. Connect the red wire (traveler) to the upper left terminal of the add-on switch. This provides a connection between the primary and add-on switches.
  3. Twist the black wires together. Twist the black line and load lines together that connected to your old switch. This might seem a little strange at first. It was for me. But if you study the diagram in Figure 3, you will see that what you are actually doing by twisting these wires together is completing the circuit and delivering power to the primary switch. Recall that only the primary switch needs to break the black line in a smart three-way switch. So by twisting the black wires together, you are creating that single black line going to the primary switch.
  4. Connect the ground wire to the ground terminal.
Smart three way switch wiring
Notice how this add-on switch has no terminals for the black line and load wires that connected to your old switch. The black wires don’t connect to the add-on switch. They get twisted together to complete the circuit and send power to the primary switch.

Conclusions

That’s it. A little more complicated than a single switch, but very logical with only a few connections to make in each switch, and most of them are just transferred in kind from your existing switches.

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