Sizing for a branch circuit

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RTL1977

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If I am installing a 20 Amp branch circuit in support of a continuous load and intend on installing THHN, can I use the 90 degree ampacity from table 310-16 in my de-rating calculations even if I don't know my termination ratings (110-14C indicates I can)? I am currently taking an "Understanding the NEC" course at my local college and the instructor (the city Electrical Inspector) says I can as long as my calculated result does not exceed the 75 degree amount in the table 310-16. If I read on 110-14, I get the impression 60 degree ampacity is the figure to use.

Thank you
 
Re: Sizing for a branch circuit

Generally speaking, if you are terminating on wiring devices you are limited to a 60 degree ampacity.

But other factors are involved here. Are there other conductors in the raceway? What is the ambient temperature?
 
Re: Sizing for a branch circuit

No other conductors in raceway (only this single phase, 20 amp circuit), ambient temperature 72 degrees.

Thanks
BL
 
Re: Sizing for a branch circuit

The answer to the question that I think you are asking is, "yes, you start with the 90C column, and then derate from there." Once you get that result, you campare with the 75C column, and use the lower of the two values. As to whether you also have to limit the final answer to the 60C column, I'll have to leave that to someone else to address. I have always been confused by the word selection in the various paragraphs under 110.14. The phrases "determination of termination provisions of equipment" and "determining equipment termination provisions," and in specific the word "provisions," have always eluded my grasp.
 
Re: Sizing for a branch circuit

Section 210.19(A) addresses continuous loads. The actual load that is supplied is multiplied by 1.25 to arrive at the continuous load as defined in Article 100. Since you are useing THHN, Cu with 90 degree C insulation, any derating or correction factors are allowed to be applied to the 90 degree ampacity column. However, the resultin ampere value, after applying correction or derating factors, cannot be less than the load and the value listed in table 310.16 under the temperature rating of the overcurrent device AND the load termination.
 
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