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Thread: Selecting ambient temperature correction factors from Ta

  1. #1
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    Selecting ambient temperature correction factors from Ta

    I want to calculate wire ampacity in a 46C enviroment. I am using 90C wire but all terminals/lugs are rated for 75C. Do I use the 90C column or the 75C column?

  2. #2
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    Re: Selecting ambient temperature correction factors from Ta

    This is an excellent question.

    A 75C terminal's thermal rating, is based on the assumption that it will be connected to a conductor with a 75C temperature rating and an ampacity “…based on Table 310.16 as appropriately modified by 310.15(B)(1) through (6).” See 110.14 (C)(1) and 110.14 (C)(1)(b).
    "Bob"
    Robert B. Alexander, P.E.
    "I know that you believe you understand what you think the NEC says, but I am not sure you realize that what you read is not what it means." (Corollary to Charlie's Rule)

  3. #3
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    Re: Selecting ambient temperature correction factors from Ta

    Bob, thanks for the quick response. I think that your answer is, to use the 75C column correction factor??

    However 110.14(C)says " Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction or both."

    So, does this mean, use the 75C correction factor and the ampacity for the 90C wire as long as the resulting calculated ampacity does not exceed the 75C wire ampacity??

  4. #4
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    Re: Selecting ambient temperature correction factors from Ta

    Yes, I did mean to use the 75C based ampacity at the terminal.

    I think 110.14 is due for a rewrite as a whole. I'm just not sure I'm up to it.

    Section 110.14(C) is basically saying that you are not required to calculate the ampacity of the conductor (in this case a 90C) throughout its entire run based on the terminal limitations; i.e., it is not required to treat it as a 75C conductor in all "conditions of use." [See the definition of Ampacity]

    For example the terminations may be in a 30C ambient, but a major run of the circuit is in the 46C ambient. At the terminations a #1/0 would have a 150A rating whether the conductor was 75C or 90C rated. But, "in the field," assuming no other factors in 310.15(B)(1) through (6) needed to be addressed, a 90C conductor would have an ampacity of 170 x 0.82, or ~139A and a 75C conductor would have an ampacity of 150 x .75 or ~ 113A.

    Therefore the circuit's ampacity would be 139A with a 90C conductor or 113A with a 75C conductor.

    If the ambient of the circuit were uniformly 30C the circuit's ampacity would be 150A for either a 75C or 90C conductor. [See Section 310.15(A)(2)]

    Edit Add: I know the circuit's ampacity is based on the OCPD, but lets not go there for the time being.

    [ March 31, 2005, 02:29 PM: Message edited by: rbalex ]
    "Bob"
    Robert B. Alexander, P.E.
    "I know that you believe you understand what you think the NEC says, but I am not sure you realize that what you read is not what it means." (Corollary to Charlie's Rule)

  5. #5
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    Re: Selecting ambient temperature correction factors from Ta

    Here is the process that I would use:
    </font>
    • <font size="2" face="Verdana, Helvetica, sans-serif">Let’s presume you mean that the whole environment is at 46 degrees C.</font>
    <font size="2" face="Verdana, Helvetica, sans-serif"></font>
    • <font size="2" face="Verdana, Helvetica, sans-serif">Let’s presume you have a conductor rated at 90 C, and terminations rated at 75 C.</font>
    <font size="2" face="Verdana, Helvetica, sans-serif"></font>
    • <font size="2" face="Verdana, Helvetica, sans-serif">Start with the 90C column in Table 310.16, and look up the ampacity for the size of your conductor.</font>
    <font size="2" face="Verdana, Helvetica, sans-serif"></font>
    • <font size="2" face="Verdana, Helvetica, sans-serif">Look up the temperature correction factor from the same 90C column. That would be, as Bob used in his example, 0.82.</font>
    <font size="2" face="Verdana, Helvetica, sans-serif"></font>
    • <font size="2" face="Verdana, Helvetica, sans-serif">Multiply the tabulated ampacity by the correction factor. Let’s call that result “Answer 1.”</font>
    <font size="2" face="Verdana, Helvetica, sans-serif"></font>
    • <font size="2" face="Verdana, Helvetica, sans-serif">Now look up the ampacity for the same conductor in the 75 C, and multiply it by the temperature rating from the 75 C column (that would be 0.75). Let’s call that result “Answer 2.”</font>
    <font size="2" face="Verdana, Helvetica, sans-serif"></font>
    • <font size="2" face="Verdana, Helvetica, sans-serif">Your final ampacity is the lower of the two answers.</font>
    <font size="2" face="Verdana, Helvetica, sans-serif">
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  6. #6
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    Re: Selecting ambient temperature correction factors from Ta

    Charlie's point is well taken - escpecially the bold emphasis, I should have included a uniform 46C set of ampacities too.

    BTW the Section 310.15(A)(2), Exception does NOT apply to the terminations. If it did we wouldn't need most of 110.14

    [ March 31, 2005, 04:14 PM: Message edited by: rbalex ]
    "Bob"
    Robert B. Alexander, P.E.
    "I know that you believe you understand what you think the NEC says, but I am not sure you realize that what you read is not what it means." (Corollary to Charlie's Rule)

  7. #7
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    Re: Selecting ambient temperature correction factors from Ta

    OK, I'm confused now. This is the way I have always done this:

    Step 1: Multiply the ampacity of the 90 degree rated wire by the temperature rating from the 90 degree column and write it down. (Answer 1)

    Step 2: Write down the ampacity of the 75 degree rated wire (no derating). (Answer 2)

    Step 3: If Answer 1 is greater than Answer 2, you can use that size wire. If Answer 1 is less than Answer 2, you must increase your wire size and calculate again.

    Have I been doing this incorrectly all of these years???

    Laura

  8. #8
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    Re: Selecting ambient temperature correction factors from Ta

    Charlie and Bob
    I agree with lauraj. I see no reason to adjust
    the conductor for 90C and 75C. The reason for using the 90C conductor is to take advantage of the higher rating. As an example take #3 thhn rated at 110 amps at 90C and 100 amps at 75C.
    Adjusting for temp 36-40 the factor is .91.
    0.91 x 110 = 100 amps. This conductor rating does not exceed #3 thhn rated at 100 amps 75C and could be used with a 100 amp breaker.
    If you adjust for 90C and 75C the 90C figure is always greater than the 75C in this example.

  9. #9
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    Re: Selecting ambient temperature correction factors from Ta

    The ambient affects the thermal rating of each element of the circuit, including the terminals.

    Simply stated, the ampacity of a 75C terminal or connector in a specified ambient is that of the 75C conductor[s] approved to be connected to it and corrected for the ambient.

    If multiple ambients apply to a circuit, the conductor's ampacity may be calculated beginning with its own 30C ambient ampacity "...based on Table 310.16 [and] as appropriately modified by 310.15(B)(1) through (6).” Note: "Based on Table 310.16" includes the Ambient Correction Factors at the bottom of the Table.

    After all computations are done the lowest computed ampacity applies to the entire circuit per 310.15(A)(2).
    "Bob"
    Robert B. Alexander, P.E.
    "I know that you believe you understand what you think the NEC says, but I am not sure you realize that what you read is not what it means." (Corollary to Charlie's Rule)

  10. #10
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    Re: Selecting ambient temperature correction factors from Ta

    I have to disagree. The derating coefficients from Table 310.16 are for conductors, not terminations.

    110.14(C) then states that "Conductors with temperature ratings higher than specified for terminations (90 degree conductor, 75 degress termination) shall be permitted to be used for ampacity adjustment, correction, or both."

    This means that we only derate the wire, not the termination.

    The March/April 2005 IAEI has a great article concerning this and explains it very clearly.

    Laura

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