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 Post subject: limits on electric systemPosted: Sat Sep 19, 2009 10:04 pm

Joined: Sun Nov 16, 2008 12:01 pm
Posts: 44
Are there limitations to the size of the main elctrical system that can use the tasks tables for the arc flash study analyis? And if there is what size at 480 volt would be the limit where it is necessry to do a complete arc flash study?

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 Post subject: Posted: Sat Sep 19, 2009 11:00 pm
 Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
To use the tables, you first have to know if your system fits the limitations of available fault current and clearing time. You need this basic info from the utility to make the determination, irregardless of service size.

Hope it helps,
Alan

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 Post subject: Posted: Tue Sep 22, 2009 12:52 am
 Sparks Level

Joined: Thu Jul 19, 2007 7:54 am
Posts: 201
Location: St. Louis, MO
Basically,
Yes, there are limits. They are listed in the notes after Table 130.7(C)(9).

What "size" 480V system would this be? No idea. There are two determining factors, one is available short circuit current, which could, sorta, be limited to an upstream transformer size, if you ignore several factors. The other is time, and is not affected by the "size" of the system, but by the protective devices used and how they are coordinated.

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 Post subject: Posted: Tue Sep 22, 2009 5:00 am

Joined: Sun Nov 16, 2008 12:01 pm
Posts: 44
4000amp 480v . the main short circuit current is 100ka and the system s.s.c.is 85 ka. just read those numbers today off our system. 6 - 500kva trans. primery is 13.2

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 Post subject: Posted: Tue Sep 22, 2009 8:28 pm
 Sparks Level

Joined: Thu Jul 19, 2007 7:54 am
Posts: 201
Location: St. Louis, MO
PUCKMAN wrote:
4000amp 480v . the main short circuit current is 100ka and the system s.s.c.is 85 ka. just read those numbers today off our system. 6 - 500kva trans. primery is 13.2

Then you cannot use the tables on the 480V sections. The highest fault current the tables are good for is 65kA.

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