luckyboy886
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- Oct 7, 2011
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Hi guys,
I came across the followng problem associated with the voltage drop and the circular mils calculation.
Can anyone give me some help? Thank you.
Question: What is the Circular Mils (CM) rating for the branch-circuit in illustration below?
(picture link: http://www.flickr.co...N06/6720463707/)
I used the following formula to calculate the CM:
CM = 1.732 x K x I x D / VD
(From EC&M Article: http://ecmweb.com/ne...nt_let_voltage/)
In this case, I determined K = 12.9, I = 82.5A, D = 125', VD = 3% x 480V = 14.4V. Then,
CM = 1.732 x 12.9 x 82.5 x 125 / 14.4 = 16,000 circular mils
Per NEC Chapter 9 Table 8, #8 copper should be used.
But the problem is, the allowable ampacity of #8 copper is only 50A per Table 310.16, which is smaller than the continuous load requirement in this example (82.5A).
Something is wrong in my above calculation.
But I couldn't figure out whether I applied the wrong formula or used the wrong parameters.
Can anyone give me some help?
Thank you.
I came across the followng problem associated with the voltage drop and the circular mils calculation.
Can anyone give me some help? Thank you.
Question: What is the Circular Mils (CM) rating for the branch-circuit in illustration below?
(picture link: http://www.flickr.co...N06/6720463707/)
I used the following formula to calculate the CM:
CM = 1.732 x K x I x D / VD
(From EC&M Article: http://ecmweb.com/ne...nt_let_voltage/)
In this case, I determined K = 12.9, I = 82.5A, D = 125', VD = 3% x 480V = 14.4V. Then,
CM = 1.732 x 12.9 x 82.5 x 125 / 14.4 = 16,000 circular mils
Per NEC Chapter 9 Table 8, #8 copper should be used.
But the problem is, the allowable ampacity of #8 copper is only 50A per Table 310.16, which is smaller than the continuous load requirement in this example (82.5A).
Something is wrong in my above calculation.
But I couldn't figure out whether I applied the wrong formula or used the wrong parameters.
Can anyone give me some help?
Thank you.