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Rough in wiring of a three way switch (Lesson One)

By Edited Sep 8, 2016 0 0

Safety First!

When a home renovation comes around, there is no doubt  a three-way switch has the potential to cause a stir of questioning to the do-it your self homeowner.  Although some may know an electrician who can do the wiring job for them, it seems saving money might be more attractive than hiring it out. Let's  start by encouraging safety before taking on any type of construction, let alone electrical wiring project. As typically required all around the country within the professional field of the construction industry, safety must always come first! Please thoroughly consider working safely before you start your project. With that being said let's move on.

Understanding the cables and terms.

There are many different scenarios for rough wiring a three-way. For the sake of keeping the task as simple as possible, we will discuss the most relevant and acceptable situation in which the homeowner might likely want to follow. To begin, first let us discuss some of the lingo involved. For starters, rough-in basically means installing the cable system within the exposed wall studs before you secure the drywall in place. Before I go on, let me clear up what the three in three-way means. It is important to note although the term three-way may seem related to three switches or three wires, it is neither. It is actually referring to two light switches controlling a set of one or more lights. The term three includes the light system itself along with both switches in the overall count of three.  Anything more than two switches controlling one or more lights is actually considered a four-way switch system, even if there are five, six or seven switch locations in the overall count. Next and most importantly the feed is where the constant power comes into the switch circuit. This is a 2 conductor cable coming into the box on the left in the image below. In the U.S. this is 110 volts for any residence. Before continuing please be sure you understand how the basic power circuit works in an alternating current (AC) system. This is necessary for terminating the wires properly before a final installation and inspection. Finally the last two important terms are the switched leg and travelers. The switched leg is the 2 conductor cable from the light to one of the switch locations (in this case the switch box on the right in the image below).  The travelers, also known as switch loops, are just two of the conductors within the 3 conductor cable which will run between each plastic nail-on switch box shown. These will connect from switch to switch during the last steps of the installation process.

The Code

Please keep in mind this article does not intend to cover any part of the National Electrical Code (NEC) related to the codes and standards for your project but rather presents the necessary romex cables needed to rough in the three-way switch system. The cable scheme below represents the least confusing as well as the best option for the do-it yourself mindset.  Please remember the homeowner should research the NEC before attempting any part of such a project. Currently the 2011 NEC is an acceptable document to use. Finally, it is also important to consult the local building authority if more codes and standards apply in addition to the NEC for your particular wiring installation. 

Travelers

Romex

Rough-in cable diagram for a three-way switch.
Credit: M.W. 2011
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