AWG | Nema - Code

American Wire Gauge, short AWG, 

also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in North America for the diameters of round, solid, nonferrous, electrically conducting wire. The cross-sectional area of each gauge is an important factor for determining its current-carrying capacity. 
(Source Wikipedia)
 

For further Information about AWG (American Wire Gauge) please follow this Link.

National Electrical Manufacturers Association, short NEMA, 

The NEMA used as an American institution traditionally the "English" system of measurement "inch", while in the western world almost exclusively the metric system is used. When converting from inches to millimeters (1 inch equals 25.4 mm) caused rounding inaccuracies and errors. Therefore, the bolt patterns may vary slightly in some dimension drawings. The differences, however, are usually in such a small area that they are usually covered by the general tolerances. (Source Wikipedia)

For further Information about NEMA please follow this link.