Current-Limiting devices and how it relates to available fault current

[msteffin]

My understanding of a current-limiting device is that it will lower the amount of current that a load side piece of equipment will see. I also remember reading in the NEC that current-limiting devices are not to be taken into consideration when determining the available fault current at the equipment (Control Panel). I have 2 questions.

1. Is there a way, per NEC, to utilize the current-limiting characteristics of a fuse to reduce the available fault current at the equipment? (other then having it series rated and tested under UL 508A)

2. Where in the NEC does it say that current-limiting devices can't be used in the calculations when calculating the available fault current?

 

[Dennis Alwon]

Section 240.86 talks about series rating and current limiting however article 220 does not specifically allow or disallow the use the current limiting device in its calculation. I don't understand why a current limiting device would change a calculation except for perhaps an unusual situation of which I cannot readily think about.

 

[msteffin]

Section 240.86 talks about series rating and current limiting however article 220 does not specifically allow or disallow the use the current limiting device in its calculation. I don't understand why a current limiting device would change a calculation except for perhaps an unusual situation of which I cannot readily think about.

I guess the situation I'm thinking of would be for a company designing 5k AIC rated control panels for an unknown available fault current. The control panel gets installed into a system where the available fault current is 10,000 amps. Would the engineer be allowed to use NEC 240.86(A) to utilize the let thru of a current-limiting device to bring the fault current down to an acceptable 3000 amps at the equipment? Two uncertainties would be the Series rating of the control panel and if any device in the control panel would begin to open before the current-limiting fuse cleared the fault.