AIC ratings for feed thru lugs

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mtskawasaki620

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I recently changed companies and I now quote Square D switchgear. I have been confused when building panel boards that their program doesn't allow for feed thru lugs when the AIC is higher than 22,000 on a main lug panel at 120/208V 3PH4W. In the past when quoting both Siemens and Cutler Hammer I do not remember their program not allowing for feed thru lugs above 22,000 AIC. This doesn't make sense to me as lugs do not have an over current protection so why would AIC ratings make a difference to lugs? The same could be said about lugs from Burndy, Ilsco, Penn Union, T&B.....however I have never seen an AIC rating for mechanical lugs. Below id the definition of AIC rating.

The AIC rating indicates the maximum fault current (in amps) that an overcurrent protection device (circuit breaker, fuse, etc.) will safely clear when a fault is applied at the load side of the overcurrent protection device.

Could you please clarify the reasoning behind not allowing for feed thru lugs when the AIC is above 22,000?
 
Bob: If that is the case then why is it permissible to have a 200 Amp MLO panel at 42K AIC, but it doesn't allow for the feed thru lugs on this same panel. They use the same lug kit whether it is for feed thru or the main. Same lugs, but not okay per Square D for feed thru above 22,000.
 
mtskawasaki620 said:
Bob: If that is the case then why is it permissible to have a 200 Amp MLO panel at 42K AIC, but it doesn't allow for the feed thru lugs on this same panel. They use the same lug kit whether it is for feed thru or the main. Same lugs, but not okay per Square D for feed thru above 22,000.
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My guess is that, since the panel does not have any overall OCPD before the bus, they are counting on the maximum sum of branch/feeder amps that the panel can hold to provide an upper limit on the cumulative time/current curve the bus has to withstand.
Once you add feed through lugs there is no such assurance.
If, for example, you connected a solid bolted short circuit to the feed through lugs and the service could supply 42K amps indefinitely, the bus would be destroyed.
Not that the AIC is by definition what the assembly can safely interrupt.
For an MLO panel that interruption has to be done by the installed feeder/branch breakers.
Without any breakers installed the bus has a maximum short circuit current it can carry until something else interrupts it, but no interrupting capacity at all.

What I wonder about is how they can give an AIC at lower values for an MLO panel with feed through lugs in the first place?
Possibly on the assumption that whatever is attached to the feed through lugs has 22KAIC or better?

Addressing your other specific question: in an MB panel they know that the lugs connect immediately to OCPD with a specific characteristic. No such guaranty for feed through.
Remember that this is not a calculated value. It is a tested value. How can they test MLO feed through without knowing what is connected to the lugs?

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Lugs do not and cannot have an "AIC rating"; the I is "Interrupting" and lugs don't interrupt.

But, the AIC rating of the individual OCPDs in a panel indirectly indicates what the fault WITHSTAND rating of the panel itself is. More recently, the term "Withstand rating" has been replaced with the Short Circuit Current Rating (SCCR), which is the amount of fault current that the bracing of the bus bars and other current carrying components can stand up to without damage. So obviously the issue is that Sq. D has not listed their panels with feed through lugs in them at an SCCR higher than 22kA, which is a panel in which you would use breakers rated for 22kAIC.

Actually, I would be willing to bet that if you go back and look at whatever document you saw that said this, you will find that the "IC" was actually not in that statement, it said 22kA and your brain, used to seeing that in the context of "kAIC", filled in the missing pieces for you.
 
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