Electrical wires and screw terminals are color-coded to help you match each wire to the correct terminal. But this doesn't mean that color-coding is always a reliable guide. For example, sometimes white wires are used in place of black wires, and some devices, like outlets and lamps, can be wire backward and still work (even though this creates a potential shock hazard). Understanding the basics of electrical circuits and color-coding will help you assess existing wiring and avoid some common mistakes with new installations.
Basic Wire Color-Coding
A simple standard electrical circuit has a black or red "hot" wire that carries power from the power source to the device (e.g., switch, fixture, outlet, appliance), a white neutral wire that carries the power back to the power source, and a green or bare copper ground wire that connects the device to the home's grounding system.
A black or red-hot wire usually connects to a brass-colored screw terminal or black wire lead on electrical devices. A white neutral wire usually connects to a silver-colored terminal or white wire lead. A green or bare ground wire almost always makes a ground connection—to a ground screw on a device, electrical box, or appliance case or to a green wire lead.
Of course, there are always exceptions to the rule, and there are many legitimate and not-so-legitimate ways to wire devices that don't follow the basic color-coding, so never make assumptions based on color-coding alone.
Single-Pole Switch Terminals
Single-pole switches have only two terminals, plus a ground screw. The terminals connect only to the hot wires in a circuit and are interchangeable, so the terminals are the same color. These switches don't typically connect to the neutral, so there is no terminal for the neutral wire.
Three-Way Switch Terminals
Color-coding on three-way switches is very important. These switches have two light-colored terminals and one dark-colored terminal, plus a ground screw. The light-colored terminals are the traveler terminals and are interchangeable. The dark-colored terminal is the common terminal and brings power from the source to the light fixture. As with single-pole switches, neutral wires do not connect to three-way switches. When replacing a three-way switch, the wire connected to the common terminal on the old switch must be connected to the common terminal on the new switch.
Outlets, or receptacles, typically have two brass-colored screw terminals and two silver-colored terminals. The brass terminals are for the hot wires, and the silver terminals are for the neutral wires. If there is only one hot wire and one neutral wire in the electrical box, the hot wire can connect to either brass terminal; the neutral can connect to either silver terminal. Each terminal pair is connected electrically by a metal connecting tab. You can remove this tab for a special wiring configuration called split-wiring.
White Wire Labeled as Hot
Sometimes a white wire is used as a hot wire—not a neutral—in a switch leg, or switch loop, between a switch and a light fixture. In one common scenario, a switch is added to a fixture that is wired without a wall switch (as might be the case with a pull-chain fixture). The power is fed up to the light fixture, so there is a hot, neutral, and ground wire already there. A new cable with a black, a white, and a ground wire is run from the fixture box to a newly installed switch.
The black wire from the new cable connects to the black hot wire in the fixture box and to one of the terminals on the single-pole switch. The white wire from the new cable connects to the fixture's hot wire terminal or hot wire lead and to the other screw terminal on the switch; it serves as the second hot wire in the switch loop. To clearly indicate that the new white wire is used as a hot wire, it should be wrapped with a band of black or red electrical tape near both ends of the wire. This means the white wire is "coded for hot."
The ground wire in the new cable connects to the switch and the fixture. If either the switch or fixture box is metal, the ground also connects to a pigtail attached to each box (metal boxes must be grounded).
Lamp Cord Wiring
Most lamp cords have only two wires—a hot wire and a neutral wire. If you look closely at the cord, one half has slight ridges on the cord insulation, while the other half is smooth. The ridged half is the neutral wire. There is a right and a wrong way to connect these two wires, even though the lamp will light up either way.
The hot wire (smooth insulation) should connect to the brass-colored terminal on the light socket; this is connected to a little metal tab inside the socket, which delivers power to the light bulb. The neutral wire (rigid insulation) should connect to the silver-colored terminal, which is joined to the threaded metal sleeve of the bulb socket where the light bulb screws in.
If you get the wiring backward and connect the hot wire to the neutral terminal, you would energize the metal sleeve. If someone unscrews the bulb and touches the bulb base and the sleeve at the same time, their body would become part of the circuit and they could get a shock.