Derating for outside temps

[jmfelectric]

I see in the 08 code, Table 310.15(B)(2)(c) gives adjustments for conduits Exposed to sunlight on or above rooftops. The adders are based on averaging of the ambient temps. That seems pretty clear.

What about conduits installed outside, on a wall, that see a wide range of temps from morning shade to afternoon sun?

At what point are the correction factors found in table 310.16 implemented?

Is there a specific length of time above 86F allowable before derating begins?

Is the ambient temp found by averaging the location's temps over a specific time period?

Any specific code articles that apply here?

Thanks

 

[macmikeman]

Keywords are exposed to sunlight. Soooooooo... Not many will like my thinking but a plywood teepee over the conduit, and its no longer exposed to sunlight anymore and code is code......

 

[Smart $]

Keywords are exposed to sunlight. Soooooooo... Not many will like my thinking but a plywood teepee over the conduit, and its no longer exposed to sunlight anymore and code is code......

Less costly to just paint it...

...and going with the intent of the code, use only white paint

 

[Chamuit]

Less costly to just paint it...

...and going with the intent of the code, use only white paint

We talked specifically about those remedies in my 08 class. The PE teaching the class said you could try it, but he thought the risk wasn't worth it if you had to deal with the AHJ.

He was of the opinion that they may extend this at some point to anything on the outside of a building, which would cause a lot of havoc here in Phoenix Metro area. Design temp here is 114?F - 121?F. You add the adder to a ?" clearance and you can't use any wire.

 

[iwire]

What about conduits installed outside, on a wall, that see a wide range of temps from morning shade to afternoon sun?

Are the conduits 'on or above a rooftop'?

If not the table does not apply even though common sense tells us it should apply.

 

[Smart $]

We talked specifically about those remedies in my 08 class. The PE teaching the class said you could try it, but he thought the risk wasn't worth it if you had to deal with the AHJ.

The wording does say exposed to direct sunlight, and when painted the conduit is no longer "exposed" nor will sunlight be "direct", and white paint will reduce the direct radiant heating effect.

He was of the opinion that they may extend this at some point to anything on the outside of a building, which would cause a lot of havoc here in Phoenix Metro area. Design temp here is 114?F - 121?F. You add the adder to a ?" clearance and you can't use any wire.

FPN to Table 310.15(B)(2)(c): The temperature adders in Table 310.15(B)(2)(c) are based on the results of averaging the ambient temperatures.

According to World Climate, the yearly 24-hour average temperature for Phoenix is 21.5?C (70.7?F) :roll:

http://www.worldclimate.com/
http://cdiac.esd.ornl.gov/r3d/ushcn/ushcn_r3.html

 

[jmfelectric]

Thank you for all the comments. So as a practical matter, using the AVG 24 hour/yearly area temp and derating according to table 310.16, would be the most diligent method. That makes sense to me. Thank you again.

 

[Chamuit]

The wording does say exposed to direct sunlight, and when painted the conduit is no longer "exposed" nor will sunlight be "direct", and white paint will reduce the direct radiant heating effect.

I realize what the wording says. The speculation as to whether an inspector would consider paint as sufficient to exclude the conduit from the adder was in question. This was before any of the jurisdictions adopted the 08. So, no one had had the opportunity to test this.


Thanks for the pointer on the ambient temp. I have just taken peoples word about it. I know the PE said 121? though . . .

 

[Chamuit]

De-rating: Outdoor Temperatures for Various U.S. Cities

De-rating: Outdoor Temperatures for Various U.S. Cities

From Copper.org

To help determine the temperature insulation will be subjected to?under unloaded conditions?CDA has compiled a table of Outdoor Temperatures for Various U.S. Cities, as well as temperatures inside conduit on and at various heights above rooftops.

 

[Smart $]

I realize what the wording says. The speculation as to whether an inspector would consider paint as sufficient to exclude the conduit from the adder was in question. This was before any of the jurisdictions adopted the 08. So, no one had had the opportunity to test this.

My comment on painting the conduit, while technically correct, was made mostly tongue-in-cheek.

Thanks for the pointer on the ambient temp. I have just taken peoples word about it. I know the PE said 121? though . . .

Can't really enforce an FPN referring to Ashrae temps... and I imagine that is what the PE and the "table" is base their claims on (but I and others see Copper.org as a biased source for information .

Is it worth knowing what the "2% design temperature" means?

Note the temperatures were based on averages for the hottest months only... and the NEC FPN does not say that.