SCCR rating - engineering supervision 1

[eeprom]

Hello,
I'm trying to get a solid understanding of NEC article 409.22 stating that an industrial control panel must be labeled with the SCCR rating. I've been reading through UL508A SB4 which (sort of) explains how to determine the SCCR rating of a panel.

In using their table for calculations, you could never make a panel with an SCCR of greater than 10kA, and 5kA is more likely. There are very few industrial applications with fault currents this low, and so this calculation becomes impractical.

So what I'd like to know is where in the code does is say I can use engineering supervision to make this calculation? I know I can get around this SCCR deal with a good fuse, but I'd like to know that I can refer to the code when an inspector says the SCCR is too high.

EE

 

[eeprom]

fab1961,
I think that 10kA is a small amount of fault current for a 480V transformer to supply. If you figure a 1MVA transformer with a 5% Z, the single line to ground short circuit current (for an infinite bus) is nearly 25kA. Many facilities with large MCCs have larger than 1MVA transformers, and thus larger fault currents. But I did not take into account feeder cables which of course would reduce the fault current. But it would take a fair amount of cabling to reduce fault current by more than 50%.

DRWeig
You make a good point. We are not trying for a UL508A rating. We are trying to design panels in accordance with the SCCR markings as required by NEC. And NEC refers to the UL508A method of determining SCCR as one the the "approved" methods.

If you just consider the purpose of the rating - which is to make sure panel components are sized properly to withstand faults - it is almost easier to do your own calculations and pay no attention to the UL reference. This is very similar to NEC 110.9 (I think) which states that fault protection must be sized so that it can interrupt whatever fault current is available.