Here's my question - If the ambient temperature of the conductors installed on the roof is 129 degrees and the run is 100 foot long with 10 feet on the roof and 90 feet installed inside the building at an ambient temperature of 86 degrees what if any adjustment has to be made in the way of correction factor to the conductor ampacity?
"(2) Selection of Ampacity. Where more than one calculated or tabulated ampacity could apply for a given circuit length, the lowest value shall be used. Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less."
I believe the example in the handbook was a pipe running through a warmer room than the rest. I am with you, on the roof I would derate. If for no other reason, the heat in the pipe will rise to the highest point and that is probably the roof.
John- How in the world could you even think that I would ever do such a devious thing as that?.
This roof top heat thing is all over the map. What if the roof is white what if the roof is black or maybe a metal roof. What if the conductors are on the roof but in a shaded part of the roof due to structural features?
Yes the code reference is 310.15(A)(2) and I don't think it solves the problem.
In the mean time I hope John will humble himself and apologize for thinking poor little ole George was trying to be crafty.
This roof thing is still in it's infancy. I assume there will be adjustments to the language as we go along but I am happy that they are referencing it. Down here in sun country, walking around on a roof with an IR thermometer will really open your eyes. Metal is usually too hot to touch and 140-150f is not unusual. A built up "tar and gravel" roof will be gooey.
If for no other reason, the heat in the pipe will rise to the highest point and that is probably the roof.
RANT ON Heat does not rise! Heat travels from high temperature to low temperature. Gravity is not involved! Hot air rises because the denser cold air is pulled down by gravity. If the roof was colder than the building, would the cold fall to breaker panel? RANT OFF
Sorry, thermodynamics and nuclear power are triggers for over explinations.
OK at night time the cold air from the roof would be pulled down by gravity displacing the warmer air in the building. Happy now Air is still going to be the working fluid and it will be affected by gravity. The warmer air will be at the top.
When the sun is shining the roof will be the hottest spot unless this is a smelting plant.