A wiring method exposed to sunlight and very close to the roof surface will be hotter than a wiring method with a bit of distance from the rooftop. The closer the wiring method is to the roof surface; the more indirect heat is reflected back to the wiring method from the roof surface. As the wiring method gets hotter, wiring in the raceway or cable cannot continuously carry the same ampacity and must be derated.

According to NEC^® 310.15(B)(2), where the distance above the roof to the bottom of the raceway is less than 3/4 in., a temperature adder of 60°F shall be added to the outdoor temperature to determine the applicable ambient temperature for application of the correction factors in Table 310.15(B)(1)(1) or Table 310.15(B)(1)(2).

Example: A rooftop raceway is installed ½ inch above the roof surface and exposed to direct sunlight and the ambient temperature is 90°F. The raceway contains three 8 AWG, type THWN copper conductors which have an ampacity of 50 amps in the 75°C column of NEC^® Table 310.16.

Since the raceway is less than 3/4 inch above the hot roof surface, a temperature of 60°F is added to the normal ambient temperature of 90°F to equal 150°F.

At 150°F, NEC^® Table 310.15(B)(1)(1) would require a correction factor of .33 from the 75°C column to be applied to the conductor ampacity of 50 amps as follows:

50 amps X .33 = 16.5 amps.

The 8 AWG conductor now only has an ampacity of 16.5 amps after adjusting it for the revised ambient temperature since it is in a raceway exposed to direct sunlight, closer than 3/4 inch to the roof surface.

See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to NFPA 70.