Question about NEC 110.14(C)(1): Temperature Limitations and Equipment Provisions

[akyip]

Hey guys,

I have a question that I just thought about while re-visiting the code questions from Zach Stone's Electrical PE Review practice exam.

NEC 110.14(C)(1)(a) - Temperature Limitations - Equipment Provisions states that:

"Termination provisions of equipment for circuits rated 100 amperes or less, or marked with #14 AWG through #1 AWG conductors, shall be used for only one of the following:

(1) Conductors rated 60 degrees C (140 degrees F)

(2) Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on the 60 degrees C (140 degrees F) ampacity of the conductor size used.

(3) Conductors with higher temperature ratings if the equipment is listed and identified for use with such conductors.

(4) For motors marked with design letters B, C, or D, conductors having an insulation rating of 75 degree C (167 degrees F) or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75 degrees Celsius (167 degrees F) ampacity.

Now, in many other practice exams and problems, I have seen common questions regarding sizing motors with FLC x 1.25% being less than 100 A, in which the solution uses the 75 degrees C ampacity to size conductors for these motors under 100 A even with the 125% multiplier. I guess the reason the 75 degrees ampacity is allowed in these common sizing scenarios is specifically because of 110.14(C)(1)(a)(4), correct?

I'm just asking the question, because one of the Electrical PE Review NEC question(question 4 on the practice exam, which asks about sizing a 90-degree rated conductor for a 3-phase motor) clearly highlights the importance of conductor ampacity temperature ratings not to exceed the lowest temperature rating of any connected termination, conductor, or device. All this is based on NEC 110.14(C), from what I understand of his solution.

Thanks, everyone!

 

[Zach Stone P.E.]

Now, in many other practice exams and problems, I have seen common questions regarding sizing motors with FLC x 1.25% being less than 100 A, in which the solution uses the 75 degrees C ampacity to size conductors for these motors under 100 A even with the 125% multiplier. I guess the reason the 75 degrees ampacity is allowed in these common sizing scenarios is specifically because of 110.14(C)(1)(a)(4), correct?

Exactly.

B, C, and D NEMA design motors are permitted to be fed with 75 C rated conductors even if the circuit is rated for 100A or less.

 

[akyip]

Exactly.

B, C, and D NEMA design motors are permitted to be fed with 75 C rated conductors even if the circuit is rated for 100A or less.

Zach, thanks for your response! I'm just trying to cement my understanding of conductor sizing per the NEC.

 

[Zach Stone P.E.]

Zach, thanks for your response! I'm just trying to cement my understanding of conductor sizing per the NEC.

My pleasure. I should clarify the previous statement that B, C, and D NEMA design motors are permitted to be fed with 75 C rated conductors at the 75 C rated ampacity even if the circuit is rated for 100A or less.

In other words, you do not have to size the 75 C conductor based on the 60 C ampacity rating.

 

[cagupta]

The NEC Article 110.14(C) acknowledges that conductors with higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction or both. It means that when you have a derating factor due to external environmental conditions (ambient temp, group derating factor etc), that factor needs to be employed on the 90degC ampacity rating of the conductor, since after derating, the ampacity will reduce to a value lower than the 75degC ampacity.