terminal temperature rating

[ohmboyz]

Hey all

I am coaching a student who is preparing to sit for the California electrician certification. My student is using Tom and Tim Henry's 2017 Exam Questions and Answers. The relevant question (and this issue crops up time and again in their other questions) is question number three on the chapter two, end of chapter test on page 59. The query is: " The ampacity of copper # 10 THWN-2 is ____ when there are three current carrying conductors in the conduit and the ambient temperature is 70 degrees F.

Their answer here and in many other instances, assumes a terminal temperature rating based on the insulation. In this case that would be the 90 degree C column. I referred my student to 110.14 C (1) (a) (3). As I have understood this for years, it means that unless a higher terminal temperature rating is indicated on both ends of the termination, one must begin with the 60 degree column and can only use the 90 degree column when adjusting for conduit fill and ambient temperature adjustments. The Henrys, in this case and in other instances in their book, default to the 90 degree column for a starting point.

Anybody care to comment? Have I been doing this wrong for the past 400 years?

 

[JFletcher]

3 CCC and 70*F means no derating. Terminals are often rated for just 75*C even tho the wire is rated for 90*C.

I would be looking at 240.4(D) and go with 30A as my answer.

 

[jumper]

3 CCC and 70*F means no derating. Terminals are often rated for just 75*C even tho the wire is rated for 90*C.

I would be looking at 240.4(D) and go with 30A as my answer.

30A is the breaker size according to that rule and is the limit of the load one may put on the conductor, but as the question is written, that is not the ampacity of the conductor.

If no derating, adjustment, or correction is needed then the ampacity is the 90C, 40A.

One is limited by the terminal rating but that does not mean the actual ampacity is changed.

 

[infinity]

What about Table 310.15(B)(2)(a) and a correction factor of 1.04 for 70° F?

 

[electrofelon]

30A is the breaker size according to that rule and is the limit of the load one may put on the conductor, but as the question is written, that is not the ampacity of the conductor.

If no derating, adjustment, or correction is needed then the ampacity is the 90C, 40A.

One is limited by the terminal rating but that does not mean the actual ampacity is changed.

I agree. One common thing people say about taking tests is don't read more into the question than is there, just answer it as is.

 

[JFletcher]

30A is the breaker size according to that rule and is the limit of the load one may put on the conductor, but as the question is written, that is not the ampacity of the conductor.

If no derating, adjustment, or correction is needed then the ampacity is the 90C, 40A.

One is limited by the terminal rating but that does not mean the actual ampacity is changed.

So they basically want the value from 310.16?

 

[jumper]

So they basically want the value from 310.16?

In a literal interpretation of the question, yes. If no adjustments are factored in.

Rob asked above about temp, I did not factor any in. I stated that in post.

 

[infinity]

In a literal interpretation of the question, yes. If no adjustments are factored in.

Rob asked above about temp, I did not factor any in. I stated that in post.

Given the fact that it is part of the question IMO you cannot ignore it because it changes the more obvious answer of 40 amps.

 

[jumper]

Given the fact that it is part of the question IMO you cannot ignore it because it changes the more obvious answer of 40 amps.

I was not trying to ignore it by any means, my point to Fletch was that the answer of what a conductors ampacity is, and what a conductors usable ampacity is, are two different things.

I was not answering OPs question directly.

 

[jumper]

l just looked. The temperature correction factor for 70F is 1.04.

40 x 1.04 = 41.6.

This could be used for calculating when derating or other purposes but even using 90C terminals the ampacity is 40 IMO. I do not think you could be a 41A load on the conductor.

 

[JFletcher]

I think I'm just going to have to agree to disagree here. Due to other rules in the NEC, mainly 110.14(a) and 240.4(D), #10 THWN-2 can only be loaded to 30A. Regardless of its 40A (or 41.6A here) capacity as a starting point for derating, there's not a single NEC compliant piece of #10 THWN-2 carrying more than 30A, save for the first few moments of a motor starting, welders, etc.

That #10 in a conduit at 70*F is connected to a load and a source, and even with 90* terminations on both ends, it's still limited to 30A.

Let me put it another way: short of exceptions in 240.4(e-g), when can a #10 wire actually carry 40A?

 

[jumper]

Legal?

 

[JFletcher]

Legal?

Yes, with conductors that dont fall under the 'small conductor rule' of 240.4(D). Yes, using a size down for a feeder using say 250MCM between the 90*C terminations and 300 or 350MCM from the 75* to 90* terminations is legal provided all conductors are rated for the ampacity at which they are used.

You could not use say #8 from the 75* to 90* terminations, and #10 from 90 to 90*, if the breaker size was 40A.

 

[jumper]

Read the question.

The ampacity of copper # 10 THWN-2 is ____ when there are three current carrying conductors in the conduit and the ambient temperature is 70 degrees F.

Nothing about loads, breakers, terminals or such.

Yes, we would consider those things when actually installing but who knows what the person who wrote it factors in. Crappy question by the author IMO.

 

[JFletcher]

Read the question.

The ampacity of copper # 10 THWN-2 is ____ when there are three current carrying conductors in the conduit and the ambient temperature is 70 degrees F.

Nothing about loads, breakers, terminals or such.

Yes, we would consider those things when actually installing but who knows what the person who wrote it factors in. Crappy question by the author IMO.

That I can agree with! and yes, we consider the entire code when doing an install. In lieu of pertinent information, how much is one to assume or disregard with a test question like the OP gave? Is the author looking for a table value from 310.16, or 110.14(A), 240.4(D), some combination of, or something else? again, I'll agree with you that it's too vague and crappy.

 

[jumper]

Let me state it like this:

The actual ampacity is 40A.

For correction/adjustments, the calculated ampacity is 41.6A.

The most commonly used ampacity is 30A.

Under certain conditions, the permitted ampacity is 35A.

 

[infinity]

Let me state it like this:

The actual ampacity is 40A.

For correction/adjustments, the calculated ampacity is 41.6A.

The most commonly used ampacity is 30A.

Under certain conditions, the permitted ampacity is 35A.

I agree, if you ask what's the ampacity of a specific conductor you look up the size and insulation type and find the corresponding ampacity in one of the tables in Article 310.

 

[jumper]

I agree, if you ask what's the ampacity of a specific conductor you look up the size and insulation type and find the corresponding ampacity in one of the tables in Article 310.

Yep and if your calculations result in a higher ampacity then the actual, you could never use that calculated ampacity to install. 110.14(C)

At best, one could do that trick above and use the 90C.

I am hoping whatever answer is in the text reflects that. Using 70F with no other adjustments/corrections was kinda screwy IMO.

 

[ohmboyz]

let's focus this a bit more

let's focus this a bit more

Thanks to everyone for weighing in. I did not include the Henry's answer. Perhaps it's a good thing I did not, for the posts reveal something that bothers me about this type of question--the answers were all over the place I suspect because the question is so vague. First of all, neither the Henry's nor I asked or said squat about OCPD's. The question is exactly as stated and one of you, i forget which, noted that since the question is simply asking for ampacity, therein lies the reason for defaulting to the 90 degree column. If the query was and only was, what is the ampacity?, then perhaps I could be convinced to go to the 90 degree column. The problem is that since the Henry's supplied additional information---70 degrees F----and then proceeded to answer the question including that information {per table 310.15 B (2) (a)} they are walking down the road of solving the problem as if it were a real world one requiring a thorough and complete application of the code. As I have understood 110.14 C for many years, in accordance with the interpretation of the code in North Carolina, Illinois, Michigan and Pennsylvania, unless you are informed that the terminal temperature rating on both ends is 75 degrees, you have to begin your rating of conductors, regardless of the insulation, in the 60 degree column.

If the purpose of these tests is to help students prepare for the real world, and not merely word play, then 110.14 C (1) (a) (3) should apply just as does 310.15 B {2) (a).

 

[jumper]

What was the answer in the text?