Opening up an outlet or light switch box, you might be confronted with a bewildering array of wires of different colors. Black, white, bare copper, and other colors closely intermingle, yet each one has its own specific purpose. Knowing the purpose of each wire will keep you safe and your house's electrical system in top working order.
Electrical Cable and Wire Color Markings
Non-metallic (or NM) 120 V and 240 V electrical cable comes in two main parts: the outer plastic sheathing (or jacket) and the inner, color-coded wires. The sheathing binds the inner wires together, and its outer markings indicate the number of wires and size of wire (gauge) within the sheathing. The color of the sheathing indicates recommended usages. For example, white sheathing means that the inner wires are 14-gauge and yellow sheathing indicates that they are 12-gauge.
But looking deeper, the color of the wires inside of the sheathing reveals that different colored wires serve different purposes. The U.S. National Electrical Code (NEC) says that white or gray must be used for neutral conductors and that bare copper or green wires must be used as ground wires. Beyond that are general, industry-accepted rules about wire color that indicate their purpose. For your safety, consider wire color to be supplemental information to checking a wire's live status with a voltage tester.
Black Wires: Hot
Black coating is always used for hot wires.
The term "hot" is used for wires that carry a live electrical current. These wires may feed a switch or an outlet and are often used as switch legs. Even though you are permitted to use a white wire as a hot wire by marking it with electrical tape, the opposite is not recommended or allowed. In other words, do not use a black wire as a neutral or ground wire, or for any purpose other than carrying live electrical loads.
Red Wires: Hot
Red wires are also used for hot wires.
Red wires are used as switch legs (like to a ceiling fan), and as the second hot wire in 220 V installations. Another useful application is as an interconnecting wire between two hardwired smoke detectors.
White Wires With Black or Red Tape: Hot
When a white wire is augmented with a red or black color marking, this may indicate that it is being used for a purpose other than neutral. Typically, this color is indicated with a short wrap of electrical tape over the wire's plastic coating.
For instance, a white wire in a two-conductor cable may be used for the second hot wire on a 240 V appliance or outlet connection. Another application is using the white wire for a switch leg for lighting or running a three-way switch application. This white wire should be properly marked to show that it is being used for something other than a neutral.
Bare Copper Wires: Ground
Bare copper wires are the most common type of wire used for grounding.
All electrical devices must be grounded. In the event of a fault, grounding provides a pathway for electricity to travel besides through your body. The current literally passes back to the ground or earth. Bare copper wires will ground should be bonded to metal electrical boxes and to appliance connections for safety. For plastic boxes, the copper ground wire should be bonded to the device itself, such as the outlet or light switch.
Green Wires: Ground
Green coated wires are sometimes used for grounding.
Ground screws on electrical devices are often painted green, too. Never use a green wire for any purpose other than for grounding.
White Wires: Neutral
White indicates a neutral wire.
For your safety, when examining a white wire, make certain that it has not been wrapped in electrical tape. This would indicate a hot wire. Older wires sometimes may lose their electrical tape wrapping. So, if the box has a loose loop of tape inside of it, there is the possibility that it may have come off of the neutral wire.
Gray Wires: Neutral
Gray may indicate a neutral wire, though gray is used less often than white, especially in NM cables.
Blue and Yellow Wires
Blue and yellow wires are sometimes used as hot wires in conduit.
Rarely will blue and yellow wires be found in NM bundled cables. The blue wires are generally used for travelers in three-way and four-way switch applications. They also are used as switch legs for devices like lights and fans.