310.15(B)(2)(a) or 310.15(B)(2)(b)?

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ggappa

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what is the difference between the 310.15(B)(2)(a) and 310.15(B)(2)(b) charts? and what is the best situation to use one versus the other? thanks. out of the 2011 code book of course.
 

david luchini

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what is the difference between the 310.15(B)(2)(a) and 310.15(B)(2)(b) charts? and what is the best situation to use one versus the other? thanks. out of the 2011 code book of course.

One table is for Ambient Temperature Correction Factors for ampacities based on 30deg ambient, the other is for ampacities based on 40deg ambient.

Which table to use for Ambient Temperature Correction depends on the listed ambient temperature in the ampacity table of the conductor you need to adjust. For example, T310.15(B)(16) is based on a 30deg ambient, so T310.15(B)(2)(a) would apply to it. T310.15(B)(18) is based on 40deg ambient, so T310.15(B)(2)(b) would apply to it.
 
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