derating of conduit and wire run above roofs

[mshields]

Table 310.15(B)(c) gives the derating factors where one is running conduit and wire above a roof. The last category with the least amount of derating is for a distance of 12 inches to 36. What if you are 4 feet off the roof. Is derating NOT required for 36 and 1 inch above a roof?

 

[iwire]

Table 310.15(B)(c) gives the derating factors where one is running conduit and wire above a roof. The last category with the least amount of derating is for a distance of 12 inches to 36. What if you are 4 feet off the roof. Is derating NOT required for 36 and 1 inch above a roof?

That is how I understand it.

 

[david luchini]

Table 310.15(B)(c) gives the derating factors where one is running conduit and wire above a roof. The last category with the least amount of derating is for a distance of 12 inches to 36. What if you are 4 feet off the roof. Is derating NOT required for 36 and 1 inch above a roof?

Table 310.15 (B)(3)(c) doesn't give you derating factors, it gives you temperature adders. You would add that temperature to your ambient before applying the ambient correction factor.

 

[luckylerado]

Is derating NOT required for 36 and 1 inch above a roof?

T310.15(B)(3)(c) does not apply at over 36" but 310.15(B)(2)(a) does.

Good design practice would include applying the correction factor based on ambient temp for your area. ASHRAE Handbook is sited as one source of this information but there are others including all the major weather info providers.

As is often the case I learned something while looking into this.
Per 310.15(B)(3)(c) 2014

"Exception: Type XHHW-2 insulated conductors shall not be
subject to this ampacity adjustment."

 

[don_resqcapt19]

I am somewhat surprised that there is not an adder for any conduit that is exposed to the sun. The conduit gets much hotter than the ambient temperature. I know they looked at the proximity of the roof because the reflection of the heat off of the roof adds to the problem.

Have any of you picked a stick of conduit off the outside storage pile without gloves on a hot day? That conduit is much hotter than the ambient.

Of course then we would also have to look at the color of the conduit...white coated conduit would not be near as hot as the standard galvanized conduit.

PS: I just bought some shares of a copper ETF

 

[Carultch]

I am somewhat surprised that there is not an adder for any conduit that is exposed to the sun. The conduit gets much hotter than the ambient temperature. I know they looked at the proximity of the roof because the reflection of the heat off of the roof adds to the problem.

Have any of you picked a stick of conduit off the outside storage pile without gloves on a hot day? That conduit is much hotter than the ambient.

Of course then we would also have to look at the color of the conduit...white coated conduit would not be near as hot as the standard galvanized conduit.

PS: I just bought some shares of a copper ETF

My engineering practice would simply to be to use the temperature adder for the highest distance above the roof indicated in the table. OK, so the conduit is 48 inches above the roof. Let's assume its temperature to be similar to if it were 36 inches above the roof.

 

[luckylerado]

My engineering practice would simply to be to use the temperature adder for the highest distance above the roof indicated in the table. OK, so the conduit is 48 inches above the roof. Let's assume its temperature to be similar to if it were 36 inches above the roof.

Why? That is above and beyond what is required. There is no adder for 48" above a roof.

 

[Carultch]

Why? That is above and beyond what is required. There is no adder for 48" above a roof.

I know there isn't. It's simply a closest match to the best model we have, for the physical conditions we anticipate. But the title of that section says "raceway in direct sunlight"...right?

48 inches above the roof surface is still a raceway in direct sunlight. And we know the conduit still gets hot.


I know that if I were an inspector, this kind of judgement call is irrelevant. The code is the code. But as the person designing an installation, it is a completely reasonable judgement call.

 

[luckylerado]

I guess I do not understand why one would apply an arbitrary temp adder rather than just using the ambient temp as it the code states. There is no value to the customer in over engineering the conductors and in parts of the country the ambient correction factor may serve to reduce the size of the conductors. Adding 25? to the ambient for no reason just makes the job cost more. That is wasteful and I would call it taking advantage.

 

[don_resqcapt19]

My engineering practice would simply to be to use the temperature adder for the highest distance above the roof indicated in the table. OK, so the conduit is 48 inches above the roof. Let's assume its temperature to be similar to if it were 36 inches above the roof.

What about conduit exposed to the sun on a west or south facing wall? Would you derate those conductors too? These conduits will also get much hotter than the actual ambient temperature.

 

[Carultch]

What about conduit exposed to the sun on a west or south facing wall? Would you derate those conductors too? These conduits will also get much hotter than the actual ambient temperature.

I've always wondered about that as well.

One small difference there, is that it is exposed to sunlight for a lot fewer hours. But it is enough to matter, since the NEC prescribes 3 hours or more as a continuous condition.

It might have something to do with the nature of natural convection on a vertical surface compared to a horizontal surface in the same conditions.

The west wall is probably the most important, since it has the most direct sunlight at the hot time of the day.
The air hasn't heated up yet, when the sun is on the east wall.

The south wall also is exposed to less irradiance, at least during the summer. In winter of course, no one would care. The sun being somewhere between 60 and 70 degrees from the horizon heats the roof a lot more than it heats the wall at 20 to 30 degrees from the wall surface. That's 40% less to 60% less sunlight.

 

[ActionDave]

I've always wondered about that as well.

One small difference there, is that it is exposed to sunlight for a lot fewer hours. But it is enough to matter, since the NEC prescribes 3 hours or more as a continuous condition.

It might have something to do with the nature of natural convection on a vertical surface compared to a horizontal surface in the same conditions......

It might have something to do with the lack of any real world evidence of conductors on roof tops failing in all the decades that preceded temperature correction factors in the NEC.

 

[david luchini]

One small difference there, is that it is exposed to sunlight for a lot fewer hours. But it is enough to matter, since the NEC prescribes 3 hours or more as a continuous condition.

:? The NEC doesn't proscribe 3 hours or more exposure to sunlight as a continuous condition.

 

[Carultch]

:? The NEC doesn't proscribe 3 hours or more exposure to sunlight as a continuous condition.

What I mean is that the NEC sees 3 hours or more of operating at full load as a definition of a continuous load. My understanding is that this would be the amount of time it takes to heat a conductor to its steady state temperature, due to its internal heating. This is what I mean by defining it as a continuous condition. 3 hours or more = continuous.

By a similar concept, I can reason that 3 hours or more of continuous sunlight would be enough to say that the conduit is in direct sunlight continuously. I am well aware that it doesn't explicitly say this. I was using this to make my point that a wall is still in the sun long enough to heat up.

 

[Andrew445]

What about when conduit is supported on the ground above an asphalt parking lot? Do the requirements not exist for some reason? Does the elevation change really make that much a difference to incur the added temp.?

 

[Carultch]

What about when conduit is supported on the ground above an asphalt parking lot? Do the requirements not exist for some reason? Does the elevation change really make that much a difference to incur the added temp.?

I would consider a conduit above a parking lot to be in the same situation as a conduit above a rooftop, regarding derate factors.

 

[jumper]

Rumor has it that the roof top adder was deleted in 2017. It may be only applicable to raceways less than 3/4", 5/8" above the roof top in the future.

 

[Mrs. RA]

As I read Article #310.15 ...

As I read Article #310.15 ...

[h=2][/h]

• 05-13-16, 11:56 AM
  jumper
  
  
  "Rumor has it that the roof top adder was deleted in 2017. It may be only applicable to raceways less than 3/4", 5/8" above the roof top in the future."
  ​

As I read/understand Article #310.15 (B) (3) (c) Raceways and Cables Exposed to Sunlight on Rooftops:
"... raceways or cables shall be installed a minimum distance above the roof to the bottom of the raceway or cable of 23 mm (7/8 in.). Where the distance above the roof to the bottom of the raceway is less than 23 mm (7/8 in.), a temperature adder of 33ºC (60º F) shall be added to the outdoor temperature .....

Of course the section includes the existing Exception regarding XHHW-2

So, the "rumor" Jumper heard is accurate, surprisingly.

 

[infinity]

Pretty much unless your laying the conduit directly on the roof the entire concept of temperature adders is gone from the 2017 NEC. If you use 1.5" strut, wood or some other method to keep the conduit 7/8" or more above the roof no calculation is required. If you're less than 7/8" there is a single adder of 60° F.

 

[ggunn]

Rumor has it that the roof top adder was deleted in 2017.

Which makes no sense to me. Why change the ampacity of a wire for a difference of 5 degrees of ambient temperature but not for a 30 degree rise because it is laying on the roof in direct sunlight?