According to the McGraw-Hill Dictionary of Scientific and Technical Terms, 6th edition, 4-way light switches used in conjunction with 3-way light switches enable lights to be turned on and off from three or more locations. Installing switch circuits is an easy project that almost any handy person can complete even if they have no prior experience with electrical work. The most difficult aspect of wiring a 4-way switch circuit, as it is with any home electrical project, will be routing the cable through finished walls and ceilings while doing minimal damage to the finished structure.
Determine where the switches are to be located.
Use the noncontact voltage tester to make sure there are not any current-carrying wires hidden inside the wall where you will be drilling and cutting to mount the device boxes. The presence of wires means that you will proceed very carefully to not damage those wires.
Before drilling and cutting, use the electronic stud finder to make sure that you aren't drilling and cutting into a stud. Keep in mind that there has to be enough room for the device box mounting wings to turn out and draw up against the inside surface of the wall to secure the box in place.
Route the 12/2 Romex cable from the service panel to the first switch location and from the last switch location to the lighting outlet box. Route the 12/3 Romex cable between the switch boxes.
Insert the cables into each box and remove enough of the cable jacket so that there's a minimum of 15 cm (6 inches) of free conductors available in each box. Carefully insert each box into the wall and ceiling and draw the retaining wing snuggly against the wall, securing the box in place. Strip (3/4 inches) of insulation from the ends of each conductor.
The White, "Neutral Conductor" simply passes through each switch location on its way to the lighting outlet, so splice these wires together first.
Place the stripped conductors together and twist them together in a clockwise direction using the Lineman's pliers. Once twisted together, screw on a wire nut and check to see that no bare copper is exposed. Remove the wire nut and trim back the bare copper splice if needed.
Wrap this splice with electrical tape and fold them into the back of the device boxes as far as they will go.
Using a 15 cm (6 inch) piece of bare copper conductor, make a three-way splice with the bare, copper grounding conductor in each box. Make this splice the same as you did for the neutral except you don't have to tape this one. Using the needle-nose pliers, form a clockwise loop in the free ends of each conductor in each of the switch boxes.
Install the first 3-way switches. Connect the black wire bringing power from the service panel to the dark coloured screw by placing it around the screw in a clockwise direction and tightening the screw. Connect the "Travelers," the red and black wires carrying the power to the 4-way switch(es), to the bright, brass-coloured screws on the 3-way switch. The polarity of the travellers doesn't matter so the black and red wires can be connected to either screw. Connect the bare, copper grounding wire to the green coloured screws.
Wrap the switch terminals with tape and push the switch carefully into the box. Secure the switch in place with the 6-32 screws through the switch's plaster ears. Install the last 3-way switch. The connections here are the same except that the black wire carrying power to the lighting outlet(s) is connected to the dark coloured screw.
Install the 4-way switch(es). Again, the polarity of the travellers does not matter, as long as you don't mix up the incoming travellers with the outgoing travellers. The incoming pair has to connect to one set of terminals on one end of the switch while the outgoing travellers have to connect to the pair of terminals on the other end of the 4-way switch.
Install the new switch circuit in the service panel.
When installing the circuit in the service panel, use caution because parts of the service panel will always be hot even if you turn off the main service, disconnecting circuit breaker.