310.15(B)(3)(c) Circular raceways exposed to sunlight on rooftops?

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tx2step

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To avoid the ampacity derating required by 310.15(B)(3)(c) "Circular raceways exposed to sunlight on rooftops", & Table 310.15(B)(3)(c) -- have any of you installed a shade over the conduits? I've been thinking of puting 1"x4" boards or maybe sheetmetal strips lengthwise over the conduits to shade them. Have any of you done something like that? Any thoughts about it?
 
tx2step said:
To avoid the ampacity derating required by 310.15(B)(3)(c) "Circular raceways exposed to sunlight on rooftops", & Table 310.15(B)(3)(c) -- have any of you installed a shade over the conduits? I've been thinking of puting 1"x4" boards or maybe sheetmetal strips lengthwise over the conduits to shade them. Have any of you done something like that? Any thoughts about it?
Click to expand...

Maybe you would like to consider a wireway instead as this only applies to conduit.
 
Yes, these're on a rooftop running to rooftop a/c units. The minimum circuit amps is 75 amps, 3 phase. I can use #4 thhn in 1" emt without the additional derating. If I have to derate for the direct sunlight I'll have to upsize to #3 thhn in 1-1/4"? Shading would seem to eliminate the upsizing? Right???
 
tx2step said:
To avoid the ampacity derating required by 310.15(B)(3)(c) "Circular raceways exposed to sunlight on rooftops", & Table 310.15(B)(3)(c) -- have any of you installed a shade over the conduits? I've been thinking of puting 1"x4" boards or maybe sheetmetal strips lengthwise over the conduits to shade them. Have any of you done something like that? Any thoughts about it?
Click to expand...

Why are there different crietira for circular raceways vs a square or rectangular

wireway? Sheet metal strips, now thats creative.
 
John120/240 said:
Why are there different crietira for circular raceways vs a square or rectangular

wireway? Sheet metal strips, now thats creative.
Click to expand...

My guess would be the free air around the conductors, but you would think that a square raceway would have more surface area to heat up.

I see many plans come through here that state all conduits shall be run below the roof.
 
tx2step said:
Yes, these're on a rooftop running to rooftop a/c units. The minimum circuit amps is 75 amps, 3 phase. I can use #4 thhn in 1" emt without the additional derating. If I have to derate for the direct sunlight I'll have to upsize to #3 thhn in 1-1/4"? Shading would seem to eliminate the upsizing? Right???
Click to expand...

What would cost less, the upsizing of the feeders, or creating a shade that will keep the sun off the total run of conduits all day and in all seasons?
You will have to make sure that your shade doesn't become a sail and get blown off by the wind.
 
I once saw some plastic covering on some conduits running across a roof. I wondered why. I thought maybe it was for corrosion protection at the time, but that made little sense to me. Now I wonder if it was to prevent direct exposure of the conduit to the sun. has this requirement been around long?

It was wrapped around it like you would put on pipe insulation. It looked like a thick version of caution tape to me. It was yellow. My other thought at the time was that it was to show a tripping hazard but it seemed like painting the conduit would have made more sense than this stuff.
 
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tkb said:
What would cost less, the upsizing of the feeders, or creating a shade that will keep the sun off the total run of conduits all day and in all seasons?
You will have to make sure that your shade doesn't become a sail and get blown off by the wind.
Click to expand...

honestly, i'm too lazy to look it up, but isn't the derating a function of the
height of the conduits off the roof? one of the local manufacturers here is
making prefab racking to keep the conduits up in the air and avoid the
derating.

edit:

i was shamed into looking it up.... the cutoff point is 36" from the
bottom of the conduit to the roof surface.... no derating required
above that, based on what i read.
 
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Fulthrotl said:
honestly, i'm too lazy to look it up, but isn't the derating a function of the
height of the conduits off the roof? one of the local manufacturers here is
making prefab racking to keep the conduits up in the air and avoid the
derating.

edit:

i was shamed into looking it up.... the cutoff point is 36" from the
bottom of the conduit to the roof surface.... no derating required
above that, based on what i read.
Click to expand...

Even though you don't have to do the additional derating for conduits above 36", you still have to derate for the ambient temp on the roof.
Table 310.15(B)(16) is based on an ambient of 86 F. The ambient outside is about 95 F depending on the locationas per 310.15(B)(2)(A) and ASHREA.
 
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