Among the many things associated with electrical wiring, it's a good idea to know what type of wire you are using, the size of the wire, and what each color of wire is used for. To some, it may seem that it shouldn't matter what color the wire is, they should all work for the installation. After all, a wire is a wire, right?
Well, I can tell you that I have been taught that green or bare wires are used for ground wires.
That rule has been in effect for as long as I have been an electrician, but wait! While working on a school project I opened a 400-amp panel, only to find green wires connected to the circuit breakers! What! Yes, it seems that someone had their own ideas about wire and didn't care what color they used. Image someone going to a junction box, opening the box and expecting the green wire to be ground wires. Connecting a grounded wire to this connection would make a big "pop" sound for sure.Then again, touching it would surely shock you.
As you can plainly see, the color of the wire coating is used to inform every one of the wire's use. If you don't have the right color wire for your purpose, you can still make the wire's use clear. A good example is using black wire in place of a red wire. By simply wrapping red electrical tape on each end of the pulled black wire, it is easily identified as a red wire.
Likewise, if you are using an NM cable with a black and white wire and using it to connect to a type of color coding method for wire markings, one wire would be the "hot" and the other for the switch leg. If you put red tape on the white wire, you can identify it as the switch leg, thus using the white wire, used any other time as a neutral wire, now being used for the switch leg.
Let's take a look at some colored wire selections, their uses, and electrical wire markings that can help identify each.
Electrical Wire Markings
Electrical wires have markings stamped or printed on the outside sheath of the cable. These markings tell what type and size of wire that you have. But looking deeper, the color of the wires inside of the sheath, like in type NM cable, will reveal that different color wires serve different purposes.
Black wires are always used for hot wires. These wires may feed a switch or outlet and are often used as switch legs. Never use a black wire for a neutral or ground connection.
Red wires are also used for hot wires, switch legs (like to a ceiling fan), and are the second hot wire in 220-volt installations. Another useful application is the interconnect wire between two hardwired smoke detectors.
Blue and Yellow Wires
Blue and yellow wires are used as hot wires. These wires are usually pulled in conduit. The blue wires are generally used for travelers in three-way and four-way switch applications. They also are used as switch legs to things like lights and fans. Yellow wires are generally used for switch legs. These control things like light, fans, and switched outlets.
Green and Bare Copper Wires
Green wires and bare copper wires are used only for grounding. These wires will ground devices and shall be bonded to junction boxes and appliance connections for safety.
Wire Color Exceptions
In certain instances, wire colors may be used for connections that don't follow these general rules. For instance, a white wire in a two-conductor cable may be used for the second hot wire on a 240-volt 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. Simply mark the end of the wire with black or red electrical tape. That way, no one will be confused and accidentally use it for something else.
Hopefully, this gives you a detailed look at wire colors and markings and how we electricians can identify the different colored wires and their uses in new and existing installations, as long as the color codes are followed appropriately.