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 Post subject: Motor Contribution Too High
PostPosted: Wed Mar 30, 2011 1:20 pm 
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Joined: Tue Mar 29, 2011 4:54 am
Posts: 3
I'm using SKM Power Tools in a study. The facility has 15 Motors over 50hp. The Isc 3P feed back to the main breaker is calculated at 69,390 A. This seems way to high when the main feed is only 38,157 A.
Is this normal?


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PostPosted: Wed Mar 30, 2011 1:47 pm 
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Joined: Wed Jun 04, 2008 9:17 am
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Location: Spartanburg, South Carolina
Total HP? Voltage?


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PostPosted: Thu Mar 31, 2011 4:19 am 
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1200 HP Total, 480V


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PostPosted: Thu Mar 31, 2011 7:50 am 
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A rough approximation of motor contribution would be to assume 6 times FLA and HP = kVA, so 1200 HP would have 6*1200/0.48/sqrt(3) = 8,660 A fault contribution. There is a problem with the 69,390 A value.


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PostPosted: Thu Mar 31, 2011 8:43 am 
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Joined: Tue Feb 16, 2010 7:59 am
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Location: Minneapolis, MN
If you are using SKM, in your motor data pages check what you have in the LRA/FLA field (locked rotor to full load, the default is 5.88). Did you accidentally enter a HP value in one of these fields for a motor?

Bob Ragsdale P.E.


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PostPosted: Fri Apr 01, 2011 4:51 am 
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Thank you both for your help. I entered the FLA in the LRA/FLA field for the motors. Once changed, it looks much better. Thanks


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PostPosted: Fri Apr 01, 2011 6:29 am 
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Not that I've ever done that.


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PostPosted: Fri Apr 01, 2011 11:13 am 
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Location: Midwest
;)
Me either!


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