IEC versus NEMA

[kingpb]

True, but you make it sound like it is impossible for an IEC device to have to have two different sets of short circuit ratings.

Square D circuit breakers carry both UL Listed and IEC ratings on their nameplates as do Telemecanique contactors. This way a single device can be applied worldwide.

Straight out of the SqD Catalog for a breaker:

View attachment 396

You'll take note that there are different columns for UL/CSA and IEC, with different values.

The point I am trying to illustrate here is that when it says 65kA under the UL column, that means it is good for 65kA calculated by IEEE methods. The rating under the IEC column, which in fact also says 65kA for some models applied at 415Vac/240Vac, is based on current calculated by IEC 60909. The 65kA IEC is not equal to 65kA IEEE because of how the calculation methods differ. So, you cannot plug an IEC breaker into a panel, where the fault current is calculated to be 65kA (IEEE method) and expect it to always perform properly. Whats interesting is that SqD explains interrupting and withstand ratings, but the fact of the matter is that IEC uses short circuit breaking current, and peak current/making current (which is usually only a 1- sec rating, bus has a 3-sec rating). So, if the IEC peak/make rating is 65kA, the breaking current rating is only going to be around 26kA (2.5x). Whereas the UL/CSA equipment can withstand 65kA making or breaking.


I also looked at the SqD free software for IEC and NEMA contactor sizing (2 different sizing spreadsheets). Intersestingly enough they do not mention short circuit ratings at all. Which smells of mackerel to me.

Rule of Thumb: If it's a US or Canada standard project, then use NEMA, ANSI, IEEE, UL, CSA, etc. If it's international, use IEC. Do not mix apples and oranges.

 

[RBI14]

IEC contacts can withstand 10-12 times rated current without welding. NEMA can withstand 40-50 times rated current without welding. When a motor draws 6-10x current at start up, the IEC works just fine. But the higher efficiency motors can draw 16-20x current. Have seen in industrial applications where Allen Bradley was active in selecting their IEC device to run process lines. AB ended up swapping out all contactors due to welding. Quite a safety issue when you hit estop and line continues to run. This is not a slam on AB, just what I was involved in

Do you want it to last and cover all application problems? NEMA. Want to spend some time selecting and it still might not work and have a shorter life -- and also about 1/2 price? IEC

When I have been in a price crunch will use IEC, but always go 1 size up. Best contact life I have seen is with Cutler Hammer's latest IT starter. It outlasts others NEMA devices. They can explain why

All of the control vendors have white papers comparing IEC to NEMA that are probably on their respective websites

 

[jim dungar]

Any product must be applied based on the available system. If the electrical system is in the US then you need to use the IEEE (UL) SCCR values even if the product is of an IEC design. If the electrical system is in Germany then you need to use the VDE ratings even if the product is of NEMA design.

The typical mistake, leading to most examples of reduced operating life, is the idea that motor starters need to be sized based only on HP. This simplistic attitude lead NEMA to drastically oversize contactors.

For example:
The NEC says that at 480V a 10HP motor requires a circuit of 14A.
NEMA says to use a size 1 starter FVNR with a continuous rating of 27A.
UL allows an IEC 10HP starter to be rated 18A. (same tested electrical life as the NEMA device).
A 10HP definite purpose controller may be 25A.

And for a 10HP plugging/jogging application:
NEMA says to use a size 2 with a continuous rating of 45A.
IEC allows the 10HP 18A device to be used, but they advise you to actually chose the starter based on operating life curves instead.
A 10HP hoist contactor may be rated 30A.

And for those that say bigger/heavy/beefier is better.
Do you always use GRC and never any of that whimpy EMT?
Do you install 20"w x 5-3/4"d panelboards in residential applications?
Do you always drive a 24' straight-back truck instead of a 4x4?
Or, do you evaluate your options?

 

[eng_waelhamdy]

Thanks For Your Efforts