Wire Sizes

Cables Wires and Conductors
A wire is a single rod of metal with a small ratio of diameter to length.
A conductor is a wire suitable for carrying an electric current.
A stranded conductor is a conductor made up of a group of wires. These wires are usually twisted together.
A cable is either a single stranded conductor or a combination of conductors insulated from one another (a multi-core cable). Cables in the oil and gas and petrochem industries are generally always insulated and often protected with an armoured sheath. In general, stranded conductors are more flexible and less susceptible to fatigue-failure than solid wires.

Cable Sizing
Wires can carry only a limited amount of current safely. If the current flowing through a wire exceeds the current-carrying capacity of the wire, excess heat is generated. This heat may be great enough to burn off the insulation around the wire and start a fire. An increase in the diameter, or cross section, of a wire conductor decreases its resistance and increases its capacity to carry current.
Other reasons for choosing an increased cross sectional area of wire is to limit volt drop along its length - this is of particular concern in long cable runs, and in Intrinsically Safe (IS) circuits.

Limitations
Wires and cables are made in standard diameters. When selecting cables it is common to select the next standard size up from that calculated.
The terminals (e.g. Weidmuller, Phoenix etc) into which the cable or wire will terminate are made to accommodate a range of sizes. Be aware of any limitations this may place on your selection.

Cable Dimensions
Wire diameters are often specified in American Wire Gauge (AWG) rather than in mm or inches. The charts below give dimensions of common diameters and the corresponding AWG.
 
solid wire AWG sizes stranded wire AWG sizes

 

 

Further Reading - external links

For those who want to delve further into the world of wiring and electrical installations in hazardous areas, then the following may be of interest: