Effect of Rooftop Exposure in Direct Sunlight on Conduit Ambient Temperatures

The interiors of electrical conduits located on rooftops in full sunlight become considerably hotter than those located in shaded areas. Differentials between these conduit interior temperatures and outside air temperatures (the latter always measured in the shade) can easily reach 39 °C (70 °F) for conduits lying directly on a dark roof. As the height of the conduit above the roof increases, this temperature differential decreases, to perhaps 15 °C (27 °F) at a height of 0.9 m (36 in.) above the roof. Lighter-colored roofs, though they keep the interior of the buildings cooler, actually reflect more heat onto conduits located more than a few centimeters above the roof. This makes these conduit interiors hotter as compared to conduits located above dark-colored roofs. In practice, the actual temperatures inside conduits are seldom taken into account when electrical ampacity calculations are made. This can lead to serious overheating and even failure of electrical cables inside the conduits.