310.15(B)2 - Derating for more than 3 wires

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mike abbott

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NEC 310.15(B)(2) addresses the adjustment factors for more than three current carrying conductors in a conduit. However, I've run into a debate with another engineer regarding the initial ampacity of the conductor before applying the derating factor.

Take the following example: 4 current carrying #3/0 THHN in a conduit.
310.15(B)(2) requires us to derate 80%.


I believe the calculation allows for the maximum temperature rating of the wire (which is 90C for THHN). So, the calculation would be 225A X 80% = 180A.

He believes 110.14(C)(1)(b) restricts us to use the 75C column, thus the calculation would be 200A x 80% = 160A.

In either method, the overall system ampacity is still less than the 75C ampacity, so 110.14 would be satisfied.

What are your thoughts, and can you direct me to any specific code section or code examples to prove this?

Thanks!
 
NEC Article 110.14(C) below....

(C) Temperature Limitations. The temperature rating associated
with the ampacity of a conductor shall be selected
and coordinated so as not to exceed the lowest temperature
rating of any connected termination, conductor, or device.
Conductors with temperature ratings higher than specified
for terminations shall be permitted to be used for ampacity
adjustment, correction, or both.


The last sentence is what allows the conductor higher temperature ampacity(typically 90 deg C) to be where to start with the ampacity value, then derate for number of conductors in raceway, ambiant temperture....Get resultant ampacity required, then look in the column of the conductor temperature that will be used (typically 75 deg C)
 
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