Installing in an outdoor environment @ 40 degrees c - 1200a AC Disconnect/Safety Switch that has conductors coming in from a xtrmr from buried conduit. In the past I have used the ambient temp correction factor 90 degree column of .91 to derate conductor ampacity . The only areas subjected to the temperature would be the underground conduit entries and the enclosure interior. My thought would be, the only time NOT to apply this derate is when the entire run is not subjected to over 30 degrees anywhere in the run. Any thoughts?