# Chelapati Review Manual error in NEC problem?



## GabeM (Jul 26, 2010)

The Chelapati review manual, problem 2.9.1 in the NEC section seems to have an error in it. I wanted to make sure that it is indeed an error and I am not missing something. The problem is the following:

"480 V, four wire service feeds a 3 phase motor of 185 kW at 0.8 PF and single phase resistive loads of 60, 72, and 50 kW connected on phases A, B, and C, respectively. If service conductors installed in conduit, what conductor sizes and insulation should be used?"

The solution uses an ampacity derating factor of 0.8 for the service conductors per NEC 2007 Table 310.15(B)(2)(a), since there are four current carrying conductors in the conduit. However, NEC 2007 310.15(B)(4)(a) states that the neutral conductor should not be counted for the purposes of determining the derating factor, since it is only carrying the unbalanced current.

Therefore, I believe that the ampacity derating factor should not be used in the solution in the book. Please let me know if I am missing something here.


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## aliki (Jul 26, 2010)

GabeM said:


> The Chelapati review manual, problem 2.9.1 in the NEC section seems to have an error in it. I wanted to make sure that it is indeed an error and I am not missing something. The problem is the following:
> "480 V, four wire service feeds a 3 phase motor of 185 kW at 0.8 PF and single phase resistive loads of 60, 72, and 50 kW connected on phases A, B, and C, respectively. If service conductors installed in conduit, what conductor sizes and insulation should be used?"
> 
> The solution uses an ampacity derating factor of 0.8 for the service conductors per NEC 2007 Table 310.15(B)(2)(a), since there are four current carrying conductors in the conduit. However, NEC 2007 310.15(B)(4)(a) states that the neutral conductor should not be counted for the purposes of determining the derating factor, since it is only carrying the unbalanced current.
> ...


The neutral conductor is carrying some current for the single phase loads. So we have a 4-current carrying conductor situation.


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## GabeM (Jul 26, 2010)

To clarify, 310.15(B)(4)(a) states the following: "A neutral conductor that carries only the unbalanced current from other conductors of the same circuit is not considered a current-carrying conductor."

So I am pretty sure that the neutral in this situation should not be counted for the derating factor per 310.15(B)(4)(a). However, I am wondering if there is another article in the NEC that takes exception to 310.15(B)(4)(a), which would basically be an exception to an exception. I don't know if there is such a thing but I can imagine it seeing as how convoluted these codes can be.


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## GabeM (Aug 6, 2010)

Update:

I received an email from Afshin Majd from Irvine Institute of Technology stating that I am correct about the error in the problem mentioned above.


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