The temperature inside conduit installed on rooftops is considerably higher than the ambient air surrounding the building; the closer the conduit is to the roof, the higher the temperature. This has caused problems for some rooftop raceway installations. To deal with the elevated temperatures on rooftops, the 2008 NEC added section 310.15 (B)(2)(c) and Table 310.15(B)(2)(c) to provide a means to calculate ampacity of conductors that are in conduits exposed to direct sunlight on rooftops. The effect of this new table is to reduce the ampacity of conductors in such installations. The temperatures given in table 310.15 (B)(2)(c) are to be added to the ambient temperatures to determine the correction factor for the "rooftop" temperature. Table 310.15(B)(c) gives adders (correction temperatures) for conduits installed at 4 different heights above a rooftop.

For example, if a conduit is installed 6 inches above a roof top in an area where the average ambient temperature is 75 degrees F, the adder from the Table is 30 degrees F. So the temperature inside the conduit is assumed to be 105 degrees F (75 + 30 = 105 degrees F). The correction factor at the bottom of Table 310.16 for 90 degrees C wire (THHN) at 105 degrees F is 87%. If the wire inside the rooftop conduit was rated for 100 amps before derating it could only carry 87 amps after derating.