A pair of three-way switches controls a fixture from two locations, such as the top and bottom of a stairway. A three-way switch has three terminals: two traveler terminals and one common terminal, which is darker in color. The toggle is not marked ON and OFF.
About 4 hours to run cable and install a light and two switches (not including cutting a pathway and patching walls)
Voltage tester, drill, saw, hammer, screwdriver, ladder, fish tape, strippers, long-nose pliers, lineman's pliers
Stripping, splicing, and connecting wires to terminals; installing boxes; running cable through walls and ceilings
Check that a new fixture doesn't overload the circuit and tap into a power source. Lay a drop cloth on the floor below.
Two-wire and three-wire cable, ceiling fixture, two three-way switches, switch and fixture boxes, cable clamps, wire nuts, electrician's tape
Wiring Three-Ways: Switch, Fixture, Switch
Traveler wires run from switch to switch. Common terminals connect to the power at one switch and the black fixture lead at the other.
At the first switch, connect the black power wire to the common terminal, splice the white wires, and connect red and black wires to traveler terminals. At the second switch, connect the traveler wires to the traveler terminals and the black wire from the fixture box to the common terminal. At the fixture box, splice the white fixture lead with the white wire and splice the black fixture lead with the black wire from the switch's common terminal.
At the fixture box, splice the black power wire to the black wire that runs to one switch box and splice the white wire to the white fixture lead. Splice the black fixture lead to the black wire coming from the other switch box. In each switch box, connect the black wire to the switch common terminal, connect the red wire to a traveler terminal, and mark the white wire as hot, then connect it to the other traveler terminal on the switch.
In the fixture box, splice the black wires, splice the white power cable wire to the white fixture lead, and mark the other white lead as hot and splice it to the black fixture lead. At the first switch, connect the black wire from the fixture to the common terminal, connect the red wire to a traveler terminal, and mark both white wires black and connect as shown. Wire the second switch as shown. Connect all grounds.
To control a fixture from three or more locations, install three-way switches at each end and one or more four-ways in between. A four-way switch has only traveler terminals and no common terminal. Connect wires from the first switch to the "input" terminals; wires connected to the "output" terminals lead to the third switch.