In general if you treat your starters nice and use them within their design ratings the IEC devices are great, however if you tend to abuse them as is commonly done here in the US, stick with the NEMA devices.
Treat the IEC ratings as a limit, not a recommendation. I usually go up a frame size from the theoretical requirement because the small difference in capital cost is far outweighed by the saving in breakdowns.
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If we learn from our mistakes I'm getting a great education!
NEMA starters were designed primarily for the Automotive Industry who wanted to be able to use the control gear from last year's production equipment on the next model change, but not have to hire an engineer to make sure every application was precisely selected. In other words, they grossly over sized the components to allow relatively low cost electricians to re-apply the devices rapidly.
IEC starters are designed to be applied by a staff Engineer who will know a lot of information about the intended use on each and every piece of equipment, using a system of charts that look at expected life span, ambient temperature, number and frequency of operations, voltage level stability etc. etc. etc.. Technicians are allowed to exchange like sized components, but generally anything new is intended to be engineered every time.
I'm sure you can find photos showing comparison between NEMA and IEC contactors for the same hp motor. There is no comparison, really. The NEMA contactors have much larger contacts.
In many cases the NEMA contactor is complete overkill, since the baseline requirement is for a a very tough duty cycle.
With a NEMA contactor, if you have a 10 hp 460V motor, you buy a Size 1 starter and forget about it. It will probably outlive you. With IEC, you need to be aware of duty cycle, environment, and other factors and make an appropriate selection based on how long you expect it to last.
It isn't that IEC contactor are low quality, it's just that the basis of design is different and more complicated with IEC contactors.
If you follow the brute force approach of over-sizing the contactor by a frame size which our site adopted because of the tough life our control gear has, the contactor sizing ends up very similar to the NEMA equivalent.
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If we learn from our mistakes I'm getting a great education!
From what I can tell, the older design NEMA contactors were overbuilt even for NEMA standards. Much like a lot of the older equipment was overbuilt at one time.
I'm looking at a contactor here.
NEMA size 5 480-600V = 200hp
AC3 690V = 200A
AC3 500-550V = 250A
These ratings look about the same to me.
Now, in the same contactor line the first 2 frame sizes do not compare as closely - up to NEMA size 1. The IEC current ratings are higher than the expected NEMA current would be.
But then, I'm not sure what a NEMA size 1 is rated for current wise. For example, a size 1 is rated 10hp 480/600V which is typically 14A/11A. But the NEMA rating may have higher current numbers than this to account for the smaller motors having their FLA all over the place.
If you want something reasonably close to a "rule of thumb", if you use the AC-4 IEC rating on a contactor is will be roughly equivalent to NEMA ratings. That will typically translate to price as well.
