Wire gauge refers to the diameter of a wire. The higher the gauge number, the thinner the wire. There are two standards: American Wire Gauge (AWG) and Metric Wire Gauge (MWG). Depending on its purpose, a thin wire gauge might be fine, but for other jobs, a thicker wire will do a better job and protect against shorts from melting caused by overheating. Since using the wrong size of wire can lead to electrical fires, all wiring must be legally inspected and approved during new construction or remodeling.
In the United States many building codes allow for #14 gauge wire throughout the house for branch circuits (lighting), but these codes represent the minimal safety standard. Many electricians recommend #12 AWG instead. The thicker wire has lower resistance and can result in less light flicker and steadier power with minimal heat loss. The thicker wire is also rated for 20-amp fuses, while lighting fuses are typically only 15-amps.
Lighting and appliance fuses are located in a circuit breaker panel secured to the outside of the structure. Fuses are designed to trip off if connected wiring overheats. This occurs if the electrical load on the wire is great enough to build up excessive heat. A thicker wire can pass greater electrical loads without overheating, compared to thinner wires. Improperly wired fuses can lead to continual tripping and present a possible fire hazard.
Most circuit panels house a variety of fuse types to serve different purposes. Larger fuses intended to run appliances or air conditioners require a thicker wire gauge than smaller fuses used for lighting. For example, a #10 wire might be standard for a 30-amp fuse, but a thicker #8 copper wire will provide better protection.
One of the most common reasons for a non-electrician to become familiar with wire gauges is when choosing speaker wire. Thicker wire will be able to maintain bass tones better, and over a longer distance than thinner wire. Thicker wire also tends to deliver fuller, cleaner sound in general, compared to thin wire. This is because as a signal travels through wire, resistance leads to signal degradation. A #12 wire gauge is a typical choice for high quality entertainment systems, while some prefer a #10 gauge wire for the subwoofer, or bass speaker.
Wire can be purchased by the foot (or meter) from hardware or home improvement stores. If purchasing speaker wire, oxygen-free wire is typically recommended. Be sure to overestimate your needs to avoid having to splice wire together. Always check local building codes before starting construction and be sure to have new wiring inspected.