There are two major systems of wire gauge measurement. The "Brown and Sharpe" measurement system, commonly referred to as "American Wire Gauge" (AWG), uses wire numbers to describe inches. Most other countries use the "Metric Wire Gauge" (MWG), which measures wire diameter in terms of millimeters.
Wire gauges have whole number names. For example, an AWG number 20 wire has a diameter of .03196 inches. An MWG is typically expressed in millimeters, so a number 10 wire would measure 10 millimeters in diameter.
Depending on their purpose, electricians will use a variety of sizes. A large wire connecting to the main power to an electrical box may use a number 3 wire, measuring .26 inches in diameter, while electronic wiring may use number 16 wire, which measures only .058 inches in diameter.
On the Metric Wire Gauge, the common wire range goes from .5 to 35 gauge. With the American Wire Gauge system, numbers range from 1 to 20.
The AWG and MWG systems measure wire differently. With the American gauge system, higher number wires are thinner than low numbered ones. For example, a number 20 wire is a fraction of the size of a number 1 wire.
With the MWG system, the opposite occurs. Higher number wire gauges are thicker. A .5 gauge is skinnier than a 30 gauge wire.
Standardized wire gauges make it easier for electricians to do their job. Residential houses typically use the same gauge of wiring, which allows repairs to be easily made. Using different gauges of wire in the same house or business could lead to overheating, which may start a fire.
Standard wire gauge measurements lower wire prices. By setting wire gauges, companies compete with one another on price, because they all produce the same product.
Wire gauges also lower the price of electrical repairs, since electricians do not have to redo the entire electrical system throughout the house, due to varying wire sizes.
Usually, hardware and supply stores clearly mark wire gauge measurements on their packaging. Electricians usually can identify wire gauge from experience.
A wire gauge size tool exists, which resembles a modified comb. This tool has notches for all common wire gauge sizes. By placing the wire in the correct slot, you can read the diameter of the wire.