liquid tight flex conduit strapped to roof top exhaust duct

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j2020

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Is it permissible to strap a liquid-tight flex conduit to the side of a rooftop exhaust shaft that is about 12 feet tall?
I have to install an obstruction light on top of the rooftop exhaust shaft being that there is a helipad on the roof.
If not permissible, how would you recommend strapping a 3/4" conduit to run it up 12 feet to the obstruction light? Thanks.
 
That's the reason why I am asking. I wasn't sure if there would be a code violation screwing in straps to the exhaust fan shaft housing going that far up.
 
No idea what type of exhaust, but regardless, I would use some sort of stand-off hanger to make sure there is an air space between the exhaust pipe and the conduit.
 
Let's be sure we understand what we have here. It this a shaftway, say 2'x2' that contains an exhaust duct, say 1'x1'? Or is the shaftway the exhaust duct, or are the shaftway walls tight to the duct?
 
Srv52761 said:
No idea what type of exhaust, but regardless, I would use some sort of stand-off hanger to make sure there is an air space between the exhaust pipe and the conduit.
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Are you suggesting this to prevent heat transfer to the conduit itself? Just curious. Like, I understand this principle for transformers and whatnot and ensuring air flow to ensure cooling/heat dissipation... but is it really an issue for the conduit itself, too?
 
oldsparky52 said:
I thought of something like these when I read his post. Just fasten a stand off strap to the magnet and slap it on the duct.

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I would never even think of something like this... would that be acceptable? Would that still be considered "fastened to the building structure?"
(N.B. "Fastened to the building structure" is probably not verbatim to code, but I always think of code strapping requirements that way).
 
Yes exactly the magnets or similar.
Magnets are commonly used to fasten large coax runs and cellular antennas to water towers, they are sized for seismic and wind loads.
 
tom baker said:
Yes exactly the magnets or similar.
Magnets are commonly used to fasten large coax runs and cellular antennas to water towers, they are sized for seismic and wind loads.
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Huh, I would have never thought using magnets would be acceptable... in any circumstances...
 
Jerramundi said:
Are you suggesting this to prevent heat transfer to the conduit itself? Just curious. Like, I understand this principle for transformers and whatnot and ensuring air flow to ensure cooling/heat dissipation... but is it really an issue for the conduit itself, too?
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Yes, It’s on a roof, so whether it’s a kitchen exhaust, a boiler exhaust or a restroom exhaust, we can expect temperature increase from solar gain.

Going up 12 feet and not laying directly on the roof, I would not hazard an opinion if there is any temperature adjustment needed for ampacity. But if it is a heat carrying exhaust, and most restroom exhausts are not that high so I would think it may very well be heat related, I would think temp adjustment may be in order.
But then, how many amps will an LED consume? But 12 feet exceeds the 10’ limit for the exception on multi-ampacity wire, so the entire circuit may have to be adjusted if there is anything else on it.
 
Srv52761 said:
Yes, It’s on a roof, so whether it’s a kitchen exhaust, a boiler exhaust or a restroom exhaust, we can expect temperature increase from solar gain.

Going up 12 feet and not laying directly on the roof, I would not hazard an opinion if there is any temperature adjustment needed for ampacity. But if it is a heat carrying exhaust, and most restroom exhausts are not that high so I would think it may very well be heat related, I would think temp adjustment may be in order.
But then, how many amps will an LED consume? But 12 feet exceeds the 10’ limit for the exception on multi-ampacity wire, so the entire circuit may have to be adjusted if there is anything else on it.
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I thought those temperature adjustment factors for "roof runs" only applied when the raceway was run parallel to the roofs surface and of a particular distance from the surface of the roof? I forget what it is exactly, but it's something like within inches of the surface of the roof (I think).

If you're suggesting a similar, but vertical run on a structure on the roof such as an exhaust stack (esp. a heat carrying exhaust), could be subject to similar effects, I'm not necessarily in complete disagreement, just trying to clarify my understanding of the relevant code requirement(s).

It seems to me, and correct me if I'm wrong, but what you're suggesting takes the code a step further than what it actually reads... which isn't necessarily a bad thing, because the code is technically a required minimum.
 
Jerramundi said:
I thought those temperature adjustment factors for "roof runs" only applied when the raceway was run parallel to the roofs surface and of a particular distance from the surface of the roof? I forget what it is exactly, but it's something like within inches of the surface of the roof (I think).

If you're suggesting a similar, but vertical run on a structure on the roof such as an exhaust stack (esp. a heat carrying exhaust), could be subject to similar effects, I'm not necessarily in complete disagreement, just trying to clarify my understanding of the relevant code requirement(s).

It seems to me, and correct me if I'm wrong, but what you're suggesting takes the code a step further than what it actually reads... which isn't necessarily a bad thing, because the code is technically a required minimum.
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All y'all need to re-read the OP. He's talking about the exhaust shaft. It's not clear that the shaft is carrying the exhaust or is built around the actual exhaust duct and there's no clue as to the temperature involved. Or to misquote Mark Twain “There is something fascinating about electrical contractors. One gets such wholesale returns of conjecture out of such a trifling investment of fact.”
 
Not to try to throw a wrench into things, but I have to ask, wouldn't the vent stack be considered "equipment" as opposed to "structure or building?"
Don't ask me if I have any other suggestions on how to run it, because I don't, lol.
 
gadfly56 said:
All y'all need to re-read the OP. He's talking about the exhaust shaft. It's not clear that the shaft is carrying the exhaust or is built around the actual exhaust duct and there's no clue as to the temperature involved. Or to misquote Mark Twain “There is something fascinating about electrical contractors. One gets such wholesale returns of conjecture out of such a trifling investment of fact.”
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Not sure how this applies to MY comment asking @Srv52761 for clarification about his comment regarding roof temperature adjustments...

...but yes, that would be an important distinction... especially in regards to my subsequent question... as to whether "the shaft" is "equipment" or a "structural member."

The OP title says "duct," but the OP post says "shaft," which could be structural. I suppose it would have to be structural to run 12' vertically.
 
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