IEC is performance based, NEMA is manufacturing based.
What this means:
With IEC, you can build it anyway you want too as long as it passes the tests. NEMA on the other hand, basically says that if you build it a certain way, it will perform a certain way.
Short circuit calculations for IEC are based on IEC 60909, which is the European methods, versus IEEE short circuit calculations. The impact is dramatic, and the short circuit numbers from the analysis mean completely different things. Therefore, you CAN NOT, and I will repeat, you CAN NOT use equipment with an IEC rating in place of NEMA rated equipment unless it has a dual rating. There is not a "factor' that can be applied to go from one to the other. Many IEEE papers have been written dispelling this myth.
A few years back, there was a fuse manufacturer going around and showing people, by use of a video, that the IEC equipment was cheap, and could not withstand the same fault current as NEMA equipment and that's why you needed to use fuses in place of a breaker.
The fact was, that they were showing you e.g. a motor starter that had say a 65kA rating in IEC, and a 65kA NEMA starter and the IEC equipment was destructing. The reason, 65kA in IEC does not mean the same as 65kA in NEMA.
More and more manufacturers are coming out with dual rating equipment. Many manufacturers, for example Schneider Electric a.k.a Square D, bought companies so they could specifically offer both lines. GE did the same thing. But the bottom line, is that if doesn't have a NEMA rating, it should not be UL approved, and therefore you should stay clear.
"Just because you're paranoid, doesn't mean they're not out to get you"