IEC 60228
IEC 60228 is the International Electrotechnical Commission's international standard on "conductors of insulated cables". the current version is Third Edition 2004-11 Among other things, it defines a set of standard wire cross-sectional areas: In engineering applications, it is often most convenient to describe a wire in terms of its cross-section area, rather than its diameter, because the cross section is directly proportional to its strength and weight, and inversely proportional to its resistance. The cross-sectional area is also related to the maximum current that a wire can carry safely. This document is one  considered "fundamental" in that it does not contain reference to any other standard. Description. The document describes several aspects of the conductors for electrical cables Class. This refers to the flexibility and thermal effects i.e temperature of a conductor. Size. The "nominal" (see below) cross-sectional area for standard conductors including the following: Resistance. The maximum permissible resistance (in ohms per kilometre – Ω/km) of each conductor size, class and type (both plain copper and metal coated) Purpose of the document. This document and its precursors were created due to a need for a standard definition of cable conductor size . The main problem being that not all copper has the same resistivity value, so, for example, a 4 mm conductor from two different suppliers may have different resistance values. Instead this document describes conductors by their "nominal" size, determined by resistance rather than physical dimensions. This is a key distinction as it makes a standardized definition of conductors based solely on their electrical characteristics. Almost all characteristics of conductors, resistance, current carrying capacity etc. are independent of the physical dimensions of the conductor. However this document allows an easy reference whereby the standard conductor sizes and reference to physical dimensions  are maintained but given an exact meaning in terms of the electrical characteristics of a conductor.