NEC 2005, Article 409

[StanleySmith]

I'm doing short circuit current rating for my panel as required ny NEC 2005, article 409 (first time doing this so please bear with me). I have some questions:
1) According to article 409, power transformer's not included in the SCCR calculation. Why ? and I assume this means that I don't have to find out the SCCR for my transformer disconnect and control transformer (?)
2) The circuit breakers I current used are rated at 5KA SCCR, and the panel has to be rated at least 60KA of SCCR. I think I have to use fuses instead since fuses have better SCCR rating.
3) Does anyone have any other advices for me ?
Thank you,

 

[VxA]

Hi,

Ok, first let me say that these are my first impressions, and I am no expert on this subject (so hopefully some of the guys/gals that are will add their $0.02 to this one). I'm not totally familiar with Article 409, so I can't comment on your power transformer question, though it would seem to me that in order to calculate the SC current availabel, you would need to know the kVA and impedence (in percent) of the transformer just upsteam of where you are calculating your SC current at.

If you really need an iterrupting capacity rating of 60kA Seems to me the best way to appraoch this might by to add one set of current limiting fuses on the main panel feed. If you can get the let-through current of the fuses below 5kA, then I'd guess you wouldn't need to change out all your CBs. On the other hand, there are CBs with higher interupting capacities out there. I think a 3-phase 480VAC MCCBs (molded-case circuit breakers) with IC ratings up to 100kA are readily available (a bit pricey I think though). This route might be more attractive, as you could end up with coordination issues where the fuses trip out your whole panel before the CB one the one faulted circuit trips. Anyway, just some thoughts.