How are electrical wire sizes measured?

The amperage capacity of a wire is determined by the cross sectional area of the wire.

Another Answer

Electrical conductors are expressed in terms of their cross-sectional area. In Europe, for solid conductors, this is expressed in square millimetres. Stranded conductors are expressed in terms of the number of strands, and the diameter (in millimetres) of each strand, expressed in the format A/B (where A = number of strands, and B = diameter of strand) -from which the total area, in square millimetres can be determined.

In North America, the cross-sectional area is expressed in circular mils (round conductors) or square mils(rectangular conductors). A 'mil' is one-thousandth of an inch. A 'circular mil' is the diameter of a conductor, expressed in mils, squared -so it is not really a measure of area (as it doesn't take pi into account), but a figure that represents an area.