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Monday, February 13, 2017
Question:
Re: CQD answer published Tuesday, January 31, 2017 -Conductor Ampacity1
Referring to the 1/30/17 answer on conductor correction factors. Over the years I have heard two opinions on this. The first being the answer given. The second being, that correction factor calculations were not subtractive. That the lowest value calculation was to be used as described in 310.15(A)(2). If the calculation values are subtractive, as you replied, what does 310.15(A)(2) refer to?
Thank you, Jeff Hetchler
A
Answer:
Hey Jeff thanks for your follow up question. The factors are "subtractive" from the starting value but are both applied (additive) if both conditions exist. Table 310.15(B)(16) is based on two "conditions of use" - not more than three current carrying conductors and -an ambient temperature of 86 degrees F. If a higher ambient exists the appropriate correction factor is applied from Table 310.15(B)(2)(a). If there are more than three current carrying conductors we apply the appropriate adjustment factor from Table 310.15(B)(3)(a). The concept is about heat, if we start at a higher temperature we are limited to less current to get to the insulation temperature rating. Adding more current carrying conductors provides more heat sources and also reduces heat dissipation.
If a conductor "run" is located where different conditions apply then the lowest value is used as stated in 310.15(A)(2). For example if part of a conductor length is in an air conditioned space not exceeding 86 degrees F and part of the conductor length is in a space with a higher ambient temperature the part in the higher ambient temperature location is used to determine the conductors ampacity (by applying the appropriate correction factor) as it is "worst case".