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 Post subject: Impact of conductor length
PostPosted: Fri Jul 27, 2012 7:43 am 
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Joined: Fri Jul 06, 2012 11:57 am
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Location: Fort Worth, Texas
Excuse me if this question does not make much sense. What effect if any to the arc flash calculations would be the length of the conductor? For example: Three phase transformer delta primary, y secondary 480/277 1000KVA, 4.5% IZ. The available ssca is 21,384 with close coupled and 20,628 with 15' distance from transformer to main breaker.


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PostPosted: Fri Jul 27, 2012 8:14 am 
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Joined: Tue Feb 24, 2009 7:05 am
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Ron Baggett wrote:
Excuse me if this question does not make much sense. What effect if any to the arc flash calculations would be the length of the conductor? For example: Three phase transformer delta primary, y secondary 480/277 1000KVA, 4.5% IZ. The available ssca is 21,384 with close coupled and 20,628 with 15' distance from transformer to main breaker.

Once you leave the main swithgear, sections are smaller and distance adds up. If you don't count that, then you have the same SCC everywhere. A smaller cable with a length of 0 won't affect the SCC.


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PostPosted: Fri Jul 27, 2012 8:19 am 
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Joined: Fri Jul 06, 2012 11:57 am
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Location: Fort Worth, Texas
Thank you. I was wondering if there was any significant impact on the arc flash rating. The SCC reduction was only about 750 less.


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PostPosted: Sun Jul 29, 2012 5:07 pm 
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Location: Spartanburg, South Carolina
Lower short-circuit current for the same arc duration has lower arc incident energy. The arc duration may be longer, however, so the incident energy may be higher. You could calculate the length of cable required to lower the current to the instantaneous or short-time trip setting, and then assume the same incident energy as at the source for shorter cable lengths.


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