Derating conductors for the environment where they are installed is an important consideration for selecting the type and size of conductor for a particular installation. When installing a system on a rooftop, special consideration must be given to the ambient temperature inside the wire way or conduit that contain the conductors to prevent heating beyond the rating of the insulation of the conductors.
Conductors installed in raceways on rooftops can be subject to higher temperatures than they would be just from the heating effect of solar energy absorbed by the wall of the raceway. This effect becomes more evident when the proximity is very close to the roof or the raceway is in contact with the roof surface.
Anyone who has been on top of a roof on a hot summer day knows what it’s like to stand in the middle of an oven. Extreme temperatures from the sun can bake everything in sight, from shingles to appliances, and even electrical conductors and conduit.
Conductor insulation is meant to provide long lasting protection from expected heat due to the flow of electricity and normal environmental factors. However, excessive heat from solar radiation can damage conductors and cause premature failures.