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Subject: NEC Table 430.250


Author:
Ralph Cole
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Date Posted: 11:41:46 01/31/06 Tue

I am enrolled in a [Tennessee] technical college course for the LLE. The class was running through some voltage drop exercises last night. One problem involved a 10 h.p. motor load at 208 v., 3-ph. Consulting NEC Table 430.250, we found the load current to be 30.8 amps.

However, when we calculated the current as P/E we arrived at 27.8 amps for this same motor. I = (10hp x 1000w/hp)/(208v. x 1.73) = 27.79amps

Given the above discrepancy, our class would like to know how the values in Table 430.250 were derived. We were unable to resolve the difference in values even after consulting various references and trying different calculation methods. Since wire size depends on circuit current, it is important that we know why Table 430.250 is trustworthy..

Regards,
Ralph Cole
Crossville, TN

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