Three-way light switch circuits are used when you need to control a room light from two different locations. A four-way switch circuit is required when you need to control a light from three or more locations, such as the first landing, the second landing and the third landing of a stairway in a three-story house. Converting from a three-way to a four-way light system is simply a matter of inserting one or more four-way switches between two three-way switches. The hardest part of this project is running the new wire.
Locating the Four-way Switches
Turn off the branch circuit breaker on the service panel supplying power to the light switch circuit on which you will be working. Check the circuit with a noncontact voltage tester to make sure the circuit is safe to work on. Locate and mark the spots where the new four-way switches are to be installed. Use a stud finder to make sure you will not be drilling or cutting into a wall stud.
Mounting the Switches
Mark the locations of the new four-way switches so the top of the boxes will be 47 inches above the finished floor line. You can use the front of the “old work” or “remodeling” boxes as a template to outline the cutouts for the boxes, then remove the cutouts with a portable jigsaw. Now comes the challenging part: running the new wire. Forget about trying to use the old wire, because if it was installed before the walls were finished, it is stapled to the wall studs.
Running the Wire
Running new wire through finished walls and ceilings can be challenging even for the pros because you never know what kind of obstacles you will encounter. The easiest way to run new wire in a room with finished walls is remove the baseboards and run it in a channel cut behind the baseboard, laying it in notches you cut in the wall studs. That way, all you have to do is fish the wire up to and down from the switch boxes. When installing new wire in this manner, make sure you install steel nailing plates over the notches to protect the wire from damage by nails or screws.
Connecting the Switches
Four-way switches are inserted in the traveler wires between the two three-way switches. A four-way switch has two sets of brass-colored terminal screws and a dark-green-colored terminal screw. The traveler wires from the first three-way switch connects to one set of brass-colored terminal screws, and the traveler wires going to the second three-way switch connects to the other set of brass-colored terminal screws. The dark-green screws with the hex heads are for the system ground wires, which usually are bare copper wires in nonmetallic sheathed cable.
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