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#200211 03/25/11 02:50 PM
Joined: Jan 2004
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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?


George Little
2017 / 2014 NEC & Related Books and Study Guides
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George:

May this be the answer to your inquiry:

2008 NEC 310.15 (A) (2)

"(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."


John
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I bet that is why he picked 10 feet wink

I do think it is funny that they give you a break on ampacity for a 10 foot section in this article but bust you for 1.5" if it is going through insulation in 334.80


Greg Fretwell
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You think George is giving a test??

The logic behind this is the roof "heat' will disapate in the remaining 90'????

Me, I would derate the run! Not that I would have to, but....


John
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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.


Greg Fretwell
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I think that George is going to lurk around and see what (hopefully) the lot of us say.


John
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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.


George Little
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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.


Greg Fretwell
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Quote
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.

Larry

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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 wink
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.


Greg Fretwell
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