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Thread: terminal temperature rating

  1. #1
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    terminal temperature rating

    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?

  2. #2
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    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.
    Electricians do it until it Hertz!

  3. #3
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    Quote Originally Posted by JFletcher View Post
    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.
    "Electricity is really just organized lightning." George Carlin


    Derek

  4. #4
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    What about Table 310.15(B)(2)(a) and a correction factor of 1.04 for 70° F?
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  5. #5
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    Quote Originally Posted by jumper View Post
    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.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  6. #6
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    Quote Originally Posted by jumper View Post
    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?
    Electricians do it until it Hertz!

  7. #7
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    Quote Originally Posted by JFletcher View Post
    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.
    "Electricity is really just organized lightning." George Carlin


    Derek

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    Quote Originally Posted by jumper View Post
    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.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  9. #9
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    Quote Originally Posted by infinity View 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.
    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.
    "Electricity is really just organized lightning." George Carlin


    Derek

  10. #10
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    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.
    "Electricity is really just organized lightning." George Carlin


    Derek

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