# Conductor sizing - 90-degree C ampacity? Eng. Pro Guides Full Exam Question 78

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#### akyip

##### Well-known member
Hey guys,

I have a question specifically about using the 90-degree C ampacity from the conductor ampacity tables in NEC Article 310. This comes from Eng. Pro Guides Full Exam Problem 78, which asks for the 90-degree C ampacity required for a combination motor and continuous non-motor load.

Here, the required conductor ampacity is 45 A (1.25 x 26 A + 1.25 x 10 A). Then, in the solution for Problem 78, it simply uses Table 310.15(B)(16) to find the required copper size at 90 degree C.

My question is: doesn't NEC Article 100.14(C)(1)(a) prevent you from using the 90-degree ampacity in this situation? This article reads:

"Termination provisions of equipment for circuits rated 100 amperes or less, or marked for #14 AWG through #1 AWG conductors, shall be used only for 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 degrees C (167 degrees F) or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75 degrees C (167 degrees F) ampacity"

This is something I learned from another practice exam, from Zach Stone's Electrical PE Review. His exam has a similar NEC problem where the circuit's rated current exceeds 100 A, and per NEC 110.14(C)(1)(b), for circuits over 100 A you can only use the 75-degree C ampacity at most to size conductors for circuits rated over 100 A. I'm trying to see if a similar logic with NEC 110.14(C)(1)(a) for circuits rated 100 A or less holds for this problem.

NEC 110.14(C)(1) also makes me think and question - when can you actually use the 90-degree C ampacity column for properly sizing conductors?

Let me know what you guys think... thank you!

If the equipment is not specifically rated for 90 C (lugs, insulation, terminations, breaker etc), you cannot feed it with a 90 C rated conductor based on the 90 C ampacity value. The equipment will exceed its temperature ratings well before the 90 C conductor will.

While it is fairly common for lugs to be rated 90 C, most equipment is not rated for 90 C, especially circuit breakers which is the first termination point in the circuit before even making it to the load.

This is easily one of the most commonly misunderstood aspects of insulation temperature applications I've come across in various practice exams throughout the years and in the general industry.

Last edited by a moderator:
Thanks @akyip  &amp; @Zach Stone, P.E.,  I have made the revision to the exam and added it to the errata.

Thank you for the responses!

Hey guys,

I have a question specifically about using the 90-degree C ampacity from the conductor ampacity tables in NEC Article 310. This comes from Eng. Pro Guides Full Exam Problem 78, which asks for the 90-degree C ampacity required for a combination motor and continuous non-motor load.

Here, the required conductor ampacity is 45 A (1.25 x 26 A + 1.25 x 10 A). Then, in the solution for Problem 78, it simply uses Table 310.15(B)(16) to find the required copper size at 90 degree C.

My question is: doesn't NEC Article 100.14(C)(1)(a) prevent you from using the 90-degree ampacity in this situation? This article reads:

"Termination provisions of equipment for circuits rated 100 amperes or less, or marked for #14 AWG through #1 AWG conductors, shall be used only for 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 degrees C (167 degrees F) or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75 degrees C (167 degrees F) ampacity"

This is something I learned from another practice exam, from Zach Stone's Electrical PE Review. His exam has a similar NEC problem where the circuit's rated current exceeds 100 A, and per NEC 110.14(C)(1)(b), for circuits over 100 A you can only use the 75-degree C ampacity at most to size conductors for circuits rated over 100 A. I'm trying to see if a similar logic with NEC 110.14(C)(1)(a) for circuits rated 100 A or less holds for this problem.

NEC 110.14(C)(1) also makes me think and question - when can you actually use the 90-degree C ampacity column for properly sizing conductors?

Let me know what you guys think... thank you!
View attachment 19265

View attachment 19266
Just a follow up question. So since the conductor circuit is less than 100amp and did not state in the problem that the termination or device is 90 degrees, we should use the 60 degrees column under T310.15(B)(16), correct? ,and the answer should be #6AWG?

@akyip

Last edited by a moderator:
Just a follow up question. So since the conductor circuit is less than 100amp and did not state in the problem that the termination or device is 90 degrees, we should use the 60 degrees column under T310.15(B)(16), correct? ,and the answer should be #6AWG?

@akyip
I just found out that there was an errata issued in which the problem statement is revised to state that all conductor terminations and devices are rated at 90 degrees C.

https://www.engproguides.com/powerexamerrata.pdf

So after doing the PPI exam as well, I am now confused about one other thing.

Apparently, if you have to derate the conductor ampacity due to more than 3 conductors in one raceway/cable or due to ambient temperature, you can select the higher column ampacity (higher than the lowest temperature rating in the circuit) for adjustment/derating purposes.

These photo attachments are from PPI Exam questions 33 and 99, and their respective solutions. (Question 99 uses a THHN conductor, which per Table 310.15(B)(16) is a 90-degree C rated conductor.)

PPI Exam Question 33:

The given solution in PPI exam question 33 acknowledges that the 90-degree column of Table 310.15(B)(16) cannot be used for determining the conductors' final ampacity since the given terminals are rated for 60 degrees C... But then afterwards, it states that the 90-degree C column can be used for ampacity adjustment and/or correction.

So based on that, it starts off with #10 AWG 90-degree copper, which has an initial ampacity of 40 A, and then derates that 40 A based on ambient temperature and 5 conductors in a raceway. The resulting ampacity is 26.2 A, which is greater than the required 24.5 A, and so the solution chooses #10 AWG based on derating the 90-degree ampacity...

PPI Exam Question 99:

Here the lowest temperature rating given in the circuit is 60 degrees, and this time the solution references NEC 110.14(C)(1)(a) and that the 60-degree ampacity column must be used here. The solution chooses 30 A because it is the #10 AWG copper wire's 60-degree ampacity...

The solution then shows that trying to use the #10 AWG 90-degree ampacity (40 A) and then derating it for the ambient temperature (derating factor is 0.82) leads to 32.8 A, which exceeds the 60-degree ampacity of the #10 AWG wire and thus is incorrect...

So, I'm now a little bit confused when derating for over 3 current-carrying conductors and ambient temperature comes into play.

My understanding is this:

• If you do not have to adjust the ampacity for over 3 current-carrying conductors and/or ambient temperature, then just use the ampacity column that corresponds to the lowest temperature rating in the circuit.
• But if you do have to derate the conductor ampacity due to over 3 current-carrying conductors and/or ambient temperature, then you have to start considering the conductor's temperature rating, even if it is higher than the lowest temperature rating in the circuit... and that is where I am confused...
Does anyone have any thoughts or input on this?