Temperature Adder for Rooftop Installations

I see that in Article 310.15(3)(c) it states that there is a 33 degree C (60 degree F) temperature adder for raceways mounted on a roof top, in direct sunlight, less than 7/8" from the rooftop. The problem I have is that 33 degrees C is actually 91.4 degrees F, not 60 degrees F. So, which temperature do we add to the ambient to get the correct ampacity of the conductors within the raceway?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I see that in Article 310.15(3)(c) it states that there is a 33 degree C (60 degree F) temperature adder for raceways mounted on a roof top, in direct sunlight, less than 7/8" from the rooftop. The problem I have is that 33 degrees C is actually 91.4 degrees F, not 60 degrees F. So, which temperature do we add to the ambient to get the correct ampacity of the conductors within the raceway?
Either one, but you need to add Celsius degrees, not degrees Celsius. A Celsius degree is about 9/5 as big as a Fahrenheit degree, so 33 Celsius degrees is about 60 Fahrenheit degrees. What's throwing you off is the 32 degree offset, i.e., zero degrees Celsius is 32 degrees Fahrenheit.
 
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