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Thread: Cable Ampacity Rating Based on Termination Temperature Rating (NEC)

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    Cable Ampacity Rating Based on Termination Temperature Rating (NEC)

    In summary per NEC 110.14, you must base your cable ampacity on the temperature rating of the conductor equivalent to the temperature rating of the terminal that the cable would be terminating to (I.E. motor, molded case breaker, switchgear, etc.). Here is my scenario:

    I am trying to provide power to a 3-ph 460V 125HP motor. The FLA rating of this motor is 148A. Taking a 125% increase on the FLA for safe practices brings it to 185A. Per ampacity charts that we have standarized from IEEE&IPCEA industry standards, 3 - 1/C #2/0 cables rated at 90degree C are rated for 204A. The design firm we are contracting this work to is suggesting we increase our conductor size to a 3/0. Their reason is based on since the motor which is of NEMA design B, and the molded case breaker we are feeding this motor from, their terminals are rated at 75degree Celsius, we need to correct the sizing of our conductor and rate it at 75degree C. In turn, the #2/0 cable rated at 75degree C ampacity rating of 178A is less than the 125% FLA rating of 185A.

    Can anyone provide a logical reasoning as to why there is such an importance on going off the rating of the terminals? It has been standard practice at my company to base everything off of a 90degree C cable in a 40degree C ambient; we have never worried about terminal ratings in the past. I understand the fact of proper heat dissipation with sizing your cable according to the terminal rating. Typically motors arent sized to run at 100% full load anyway so the chances of getting to those high of currents would be rare.

    What implications would arise, if any, if we were to use a 2/0 rated at 90degree C connected to a 75degree C rated terminal, and we did achieve 100% FLA on the motor? I dont suspect the terminal would fail. In my judgment I feel we would be okay using the 90degree C as we have continuously done in the past. Any explanations/justifications about this matter would be greatly appreciated. Thanks.

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    Quote Originally Posted by eg6791 View Post
    In my judgment I feel we would be okay using the 90degree C as we have continuously done in the past. Any explanations/justifications about this matter would be greatly appreciated. Thanks.
    Simple. It is wrong and a code violation as you already know.
    "Electricity is really just organized lightning." George Carlin


    Derek

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    Yup, your conductor is only as good as it's termination! If your lug's etc. are only rated at 75, so's your conductor! Or 60 C for that mater. Only as strong as the weakest link, yeah?

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    Quote Originally Posted by Johnnybob View Post
    Yup, your conductor is only as good as it's termination! If your lug's etc. are only rated at 75, so's your conductor! Or 60 C for that mater. Only as strong as the weakest link, yeah?
    I guess what puzzles me is as to why are these lugs/terminals based off 60C and 75C...is it that they didnt have a means of testing them at 90C? I dont see a metal lug melting at 90C (196F).

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    Quote Originally Posted by eg6791 View Post
    I guess what puzzles me is as to why are these lugs/terminals based off 60C and 75C...is it that they didnt have a means of testing them at 90C? I dont see a metal lug melting at 90C (196F).
    There are plenty of 90c terminals available.

    What you will not find is 600 volt or less electrical equipment that is listed for use at 90c.

    For instance you may see a 90 c terminal on a breaker but the breaker itself was only tested and listed for operation at 75 c.

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    Quote Originally Posted by eg6791 View Post
    I guess what puzzles me is as to why are these lugs/terminals based off 60C and 75C...is it that they didnt have a means of testing them at 90C? I dont see a metal lug melting at 90C (196F).
    Blame the 'standards groups' like NEMA and UL.

    But, the answer is: the conductors are often used as heat sinks.
    According to a 1986 edition of NEMA AB1, for molded case circuit breakers, device terminals are limited to a maximum heat rise of 50C based on an ambient of 25C.
    Just because you can, doesn't mean you should.

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    Quote Originally Posted by eg6791 View Post
    ... I dont see a metal lug melting at 90C (196F).
    It's not about the lug or even conductor insulation melting. What is tested at the rated termination temperature is the integrity of the connection through multitudes of expansion and contraction from changes in current through the connection. A proper termination will maintain its integrity as long as the conductor does not exceed the rated temperature.

    And there are ways to use the 90°C rated ampacity of a conductor... but not as long as the equipment being connected to has a lower termination temperature rating.

    http://www2.schneider-electric.com/r...B9901R2-02.pdf and see page 4
    I will have achieved my life's goal if I die with a smile on my face.

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    Quote Originally Posted by jumper View Post
    Simple. It is wrong and a code violation as you already know.
    Yes I would agree, this is a clear code violation as stated.

    As I understand it the code is mostly a Heuristic, a "Rule of Thumb". It will not always "make sense" it just "is."

    However, at least in one case, as in 310.15 (C) Engineering Supervision, you can use an "engineered approach to a code issue, in this case conductor ampacity."

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    Quote Originally Posted by Mike Furlan View Post
    ...However, at least in one case, as in 310.15 (C) Engineering Supervision, you can use an "engineered approach to a code issue, in this case conductor ampacity."
    Yes, but even going that route, the method of calculating conductor ampacity is proscribed. I have no idea whether you gain or lose, on average, by going that route... but I don't see a lot of engineers taking that route, so that in itself tells me it isn't generally worth the added effort.
    I will have achieved my life's goal if I die with a smile on my face.

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    Quote Originally Posted by Mike Furlan View Post
    ...
    However, at least in one case, as in 310.15 (C) Engineering Supervision, you can use an "engineered approach to a code issue, in this case conductor ampacity."
    While that may change the ampacity of the conductors themselves, it has no effect on the rule for the terminations.
    110.14(C)(1) Equipment Provisions. The determination of termination provisions of equipment shall be based on 110.14(C)(1)(a) or
    (C)(1)(b). Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.15(B)(16) as appropriately modified by 310.15(B)(7).
    Don, Illinois
    (All code citations are 2017 unless otherwise noted)

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