AIC rating, short circuit available and let through current

[RKyleM]

is the let-through current via a current limiting overcurrent protective device (such as a fuse) an acceptable gauge to what aic rating the equipment needs to have? i don't think it is and the available short circuit current must be used but i can't find this in the NEC. perhaps i just overlooked it or perhaps it is somewhere else.

 

[rbulsara]

Your thinking is correct and current limiting nature of a fuse or any over current protective device (OCPD) is not an acceptable method. NEC does not prescribe the SCC calculations methods. IEEE Red and Buff book and IEC standards (not familiar with exact standards) do and I am not aware of any standard permitting taking credit for the behaviour of any overcurrent device, let alone current limit fuse.

In fact, for the purposes of calculating available SC current, the OCPDs are ignored (considered closed or shorted).

Recognized testing labs such as UL also do not recognize taking credit for the current limiting nature of any OCPD, while determining SCC rating of an assembly.

Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

See Article 240.86 in NEC. The only way to take credit for CLF upstream is to have a tested combination.



David Castor

http://www.cvoes.com

 

[racobb]

As dpc said, unless it is a tested combination, you must consider the max without the CLF.

Alan

 

[RKyleM]

thanks for the reply! i agree with you but if you read the 2008 NEC handbook (not the code book itself but the explanation of the code), on page 43, it states in the colored box "when the available short-circuit exceeds the short-circuit current rating of an electrical component, the overcurrent protective device must limit the let-through energy to within the rating of that electrical component."

the handbook, written by the same author of the code itself, seems to permit let-through current.

 

[dpc]

First of all, the NFPA NEC handbook is definitely NOT the NEC and statements in there cannot be used to justify an opinion that is contrary to what is clearly written in the NEC. Since the Handbook is not the NEC, it doesn't permit anything.

Article 240.86(A) discusses allowing a licensed engineer to determine appropriate series rating combination in EXISTING INSTALLATIONS in lieu of using a tested combination. However, no licensed engineer in their right mind should be willing to accept this liability.

I'm not sure what you mean by "permit let-through current". If you're saying that the NEC allows use of the "Up-over-down" method of determining let-through current, I would have to disagree with you.

At the end of the day, the local AHJ will have the final say, but I feel the NEC is pretty clear on this.

Cheers,

Dave



David Castor

http://www.cvoes.com