The terminology used for residential light switch wiring can be confusing. In North America, a two-way wall switch is a single-pole, single-throw switch -- in other words, a simple on-off switch. In Europe, though, a two-way switch is a single pole, double-throw switch capable of being wired with an identical one to power a fixture from two different locations. In North America, that type of switch is known as a three-way switch.
Two-Way Vs. Three-Way
There's no doubt about it -- when it comes to light switches, European terminology is more intuitive. North American electricians don't necessarily agree on the reason for calling each switch in a pair that powers a single light fixture a "three-way" switch, but one suggestion is that, in a three-way circuit, electricity can travel along three different paths. It may be arcane, but it's the terminology used by the National Electrical Code. So, as far as North American wiring is concerned, a two-way switch is a simple on-off switch, and it's usually the only switch in the circuit.
Two-Way Switch Basics
Two-way light switches are among the easiest of electrical devices to wire; you need a circuit cable with a hot, neutral and ground wire and an outgoing fixture cable with the same three wires. The switch has two brass terminals to which you connect the hot wires and a larger, green ground terminal for connecting a pigtail for the ground wires. The general wiring principle is to insert the switch in the hot leg of the circuit and bypass the switch with the neutral leg by splicing the white wires together. Twist the white wires together with pliers and screw on a wire cap.
Ground the switch by connecting the bare ground wires, which aren't wrapped in white or black plastic, to the green ground terminal via a pigtail. A pigtail, a 6-inch length of bare copper wire, connects the ground screw to the ground wires, all three wires held by a wire nut. You can also crimp the wires together with a copper crimp connector. Once the installation is complete, turn on the breaker and test the switch.
Switch Placement in the Circuit
A light switch can be at the beginning of the circuit -- that is, between the fixture and the panel -- or at the end of the circuit. It can also be placed between two or more fixtures that it controls. When the switch is between fixtures, it helps to draw a diagram of the circuit before doing the wiring to help you understand the path that the electricity takes. In mid-circuit wiring, you sometimes have to use one of the white wires in the switch box as a hot wire. When this is necessary, it's essential to identify the white wire as hot by wrapping black tape around it. This prevents confusion and accidents.
Replacing a Switch
Because the hot terminals on a two-way switch are interchangeable, you usually don't have to worry about identifying the black wires when replacing an old switch with a new one. Simply turn off the breaker, test the box with a non-contact circuit tester, unhook the wires from the old switch and connect them to the new one -- either wire can go on either terminal. Switch replacement doesn't require disconnecting the white wires from each other, but you will have to ground the new switch, again by connecting the ground wires to the green ground terminal via a pigtail.
Once the swap is complete, turn on the breaker and test the switch.