National Electrical Installation Standards

Standards as High as Your Own

Monday, February 26, 2018


Re: CQD answer published Tuesday, February 13, 2017 -Terminal Ratings and Ampacity

I believe you have confused the issue on 90 Deg C. wire. In my 2017 code handbook, page 257, 310.15(A)(3) in the blue commentary under this section it states “Most terminations are designed for 60DegC and/or 75DegC maximum temperatures. The higher-rated ampacities for conductors of 90DegC, 105DegC, and so forth cannot be used unless the terminals at which the conductors terminate have comparable ratings. Your explanations of the adjustment factors is good, but unless the breaker terminals are rated at 90 DegC, you cannot use that column of the table, according to the above comment.

Alton Tupa



Hey Alton thanks for your comment and I often get confused. You are correct but the answer given is also correct. If there are no adjustment or correction factors to be applied then the values in the 60 degree C [for 100 amperes and less] and 75 degree columns [for over 100 amperes] of Table 310.15(B)(16) are what need to be used as maximums. But if conditions of use exist then the 90 degree C value can be used for applying adjustment and correction factors (formerly called derating) as stated in 110.14(C) if the insulation has that rating. If after applying the factors the value determined is lower than the 60 or 75 degree column values then it (the determined value) is the ampacity. If the ampacity determined by applying the factors results in a value that is higher the 60 or 75 degree column values then the 60 or 75 degree value is the limit. We don't get to "use" conductors at the 90 degree column values unless the terminations are listed and marked with that temperature rating but we do get to start from those values for applying the factors. The small word "IF" in the second paragraph of 310.15(B) helps explain this.

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