A common wire is either a connecting wire or a type of neutral wiring, depending on the electrical circuit. When it works as a connecting wire, the wire connects at least two wires of a circuit together. Standard wiring design in U.S. homes involves a neutral wire at the ground potential and two hot wires of 120 volts each.
The term "common" can have two different meanings when it comes to electrical wiring. It can refer to either a neutral wire, or a wire that is connected to more than one other wire. For example, on a relay, you would connect a "common" (hot, not neutral) wire to a terminal.
Circuit common is considered to be a separate concept, with the symbol . My understanding is that "ground" means that the node can and should be tied to the earth. "Common" implies an arbitrary voltage reference with no connotation about its voltage relative to earth. One might have multiple commons in a schematic, but earth is earth.
The symbol used on diagrams for common is different than for safety ground, and rarely is it connected to the service neutral wire. The safety ground is not an electrical part of the aparatus, normally just a connection made to the exterior metal parts to prevent them from accidental electrical energization.
In electrical engineering, ground or earth is the reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the earth. Electrical circuits may be connected to ground (earth) for several reasons.
The white wire is the common or neutral wire in a 100-volt service, but the wire can also be gray in color. A white or gray wire can only be connected to another white or gray wire. Knowing the color code is essential for setting up the correct configuration of an electrical system and for safety.
While electrical work is well within the ability of the average homeowner, it's important to do it right. Taking shortcuts could result in electrocution, short circuits, and even fires. In fact, according to Homeadvisor, 67,800 house fires are caused by electrical problems each year.
Wiring electrical outlets (properly called receptacles) and switches involve many of the same basic techniques. Making safe, long-lasting connections requires properly preparing the circuit wires that will connect to the device and secure each wire to the correct terminal.