The International Electrotechnical Commission (IEC) is a Europe-based organization made up of national committees from over 60 countries. There are basically two major mechanical and electrical standards for motors: NEMA in North America and IEC in most of the rest of the world.
Dimensionally, IEC standards are expressed in metric units. Though NEMA and IEC standards use different units of measurements and terms, they are essentially analogous in ratings and, for most common applications, are largely interchangeable. NEMA standards tend to be more conservative—allowing more room for “design interpretation,” as has been U.S. practice. Conversely, IEC standards tend to be more specific, more categorized— some say more precise—and designed with less overload capacity. As an example, a NEMA-rated motor starter will typically be larger than its IEC counterpart.
Sources: Electric Motors and Control Systems-Frank D. Petruzella