What is the common wire in electrical wiring?

The common wire is a trick that saves wiring and simplifies electrical panel design. If you recall your science classes in school, you probably built simple circuits using a battery and a light bulb. Your house wiring works on the same principle, just on a larger scale and at higher voltages.

This is an updated version of the first arrangement. Because the electrical code as of the 2011 NEC update requires a neutral wire in most new switch boxes, a 3-wire cable runs between the light and switch. The red and black are used for hot and the white neutral wire at the switch box allows for powering a timer, remote control, or other programmable switch.

The most common requirement of any hardwired automated light switch is a neutral wire. Yes, there are a few…(read very, very, very few) switches that don’t require a neutral, but those will limit you to incandescent only. For those of us using LED, Fluorescent or some other energy efficient bulbs under 20w a neutral is REQUIRED!

Typical 3-Way Switch Wiring, NM Cable. In the 1st diagram below, a 2-wire NM cable supplies power from the panel to the first switch box. The black (line) wire connects to the common terminal of the first 3-way switch. A 3-wire NM connects the traveler terminals of the first and second 3-way switch together.

The only one of the three wires to keep track of and identify if replacing a three way switch is the wire connected to the common terminal. Here is a wiring diagram for a light switch (three way light switch). The third type of switch you may encounter in the home is the four way switch. The 4 way switch has four terminals.

Switch Wiring Diagrams A single switch provides switching from one location only. “Single-Pole” may sound simple, but there are different ways to wire a Single-Pole Switch. The power can come from either the switch box or the fixture box and a set of electrical switch wiring diagrams will explain each of these scenarios to you clearly.

3-Way Switch Wiring Diagram with LINE and LOAD in the Same Switch Box. In the above diagram the white wire must be re-identified as a hot wire at each switch location. The white wire between switches is not being used as a neutral. An example of three-way switch wiring with the LINE and LOAD in the same 4″ square electrical box.

At the first switch location, the feeder wire from the power source is a 2-wire cable with ground. This means that there is a black hot wire, a white neutral wire, and a bare copper grounding wire. At this first switch, the black feed wire is connected to the common screw on the switch.

The switch still breaks the circuit, but in this case the smart switch can tap into the neutral wire to keep power flowing through the switch - the electronics in the switch still break the circuit to the light, but the switch maintains a circuit so it is always receiving power. Here is a diagram of a standard switch with a neutral wire:

Smart switch with neutral. Below is a diagram of a smart switch installed in the first configuration (with neutral). Notice that regardless of whether the switch is on or off, there is a clear path from line to neutral that includes R1.