How do you know the ambient temperature of a roof?

[mike1061]

310.15(B)(2)(a) is the table for temperature corrections. How do you determine the temperature for a conduit on a roof? I’m proposing the put a single emt conduit on 4x4’s Accross the roof to a junction box. The conduit would have 6 current carrying conductors and one grounding conductor. The loads are 3 air conditioner compressors. With minimum circuit ampacity of 42 amps, max breaker 45 amps. 240 volts 1 phase. I was using the 90* column of of 310.15(B)(2)(b). I have to derate for 9 conductors, so multiply by .70 per 310.15(B)(3)(a) them multiply by 310.15(B)(2)(a). How do you determine the temperature to use?
I found this website. Would that be correct, or is there another method? I’m in Chicago and using that table, I come up with #6 conductors for a 45 amp circuit. 75x.70x.82=43.05.

https://www.copper.org/applications/electrical/building/pdf/rooftop.pdf

 

[tom baker]

I had a slide cart from the copper development association that listed air temp for various cities.

 

[Ponchik]

I had a slide cart from the copper development association that listed air temp for various cities.

That is the link that he has posted.

 

[Ponchik]

How high above the roof surface is the conduit?

 

[tom baker]

That is the link that he has posted.

I missed that I am old school, not used to all the information on line

 

[tom baker]

310.15(B)(2)(a) is the table for temperature corrections. How do you determine the temperature for a conduit on a roof? I’m proposing the put a single emt conduit on 4x4’s Accross the roof to a junction box. The conduit would have 6 current carrying conductors and one grounding conductor. The loads are 3 air conditioner compressors. With minimum circuit ampacity of 42 amps, max breaker 45 amps. 240 volts 1 phase. I was using the 90* column of of 310.15(B)(2)(b). I have to derate for 9 conductors, so multiply by .70 per 310.15(B)(3)(a) them multiply by 310.15(B)(2)(a). How do you determine the temperature to use?
I found this website. Would that be correct, or is there another method? I’m in Chicago and using that table, I come up with #6 conductors for a 45 amp circuit. 75x.70x.82=43.05.

https://www.copper.org/applications/electrical/building/pdf/rooftop.pdf

Yes that is how you determine the temp. There is a was an IN in 310, but a few code cycles all the derating for temp went away and now its only required if you are closer than 7/8" seee 310.15 B 2 in the 2020 NEC. All the derating requirements that started in 2008 were based on a study by the IAEI and Copper Development Association that were unrealistic.
And a note, the 2008 language was for conduit above the roof and EMT is not conduit so that rule was flawed. That got changed in 2011.

 

[mike1061]

Thanks. So if the emt is 3 1/2” above the roof (on 4x4’s) I don’t have to derate for the heat, just the 9 conductors?

 

[Ponchik]

Thanks. So if the emt is 3 1/2” above the roof (on 4x4’s) I don’t have to derate for the heat, just the 9 conductors?

Yes you do need to derate for the heat. These screen shots are from 2011 NEC. As I don't have my 2017 with me (it is in the truck) I don't think much has changed between 2011 and now.

 

[Fred B]

It would seem that some changes occurred since the 2011.
2017 NEC 310.15(B)(3)(c) Raceways and Cables Exposed to Sunlight on Rooftops. Where raceways or cables are exposed to direct sunlight on or above 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 33°C (60°F) shall be added to the outdoor temperature to determine the applicable ambient temperature for application of the correction factors in Table 310.15(B)(2)(a) or Table 310.15(B)(2)(b).
Exception: Type XHHW-2 insulated conductors shall not be subject to this ampacity adjustment.
Informational Note: One source for the ambient temperatures in various locations is the ASHRAE Handbook — Fundamentals.

 

[mike1061]

I think I got it now. If you’re less then 7/8” above the roof you add 33* Celsius to the the temperature. I read that part of the code a few times and didn’t understand it.
Thanks everyone

 

[Ponchik]

The attachments in my post #8 are not valid unless your state or local jurisdiction is still under 2011 NEC.

 

[suemarkp]

What is special about XHHW-2 that it is excluded (as compared to THWN-2 or any of the other 90C wet rated conductors)?

The ASHRAE Manual J book has a table in the back of summer design temperatures for many cities. If you have an HVAC friend that does Manual J calculations, they could give you the temp in the book for the city nearest you.

 

[tom baker]

XHHW

What is special about XHHW-2 that it is excluded (as compared to THWN-2 or any of the other 90C wet rated conductors)?

The ASHRAE Manual J book has a table in the back of summer design temperatures for many cities. If you have an HVAC friend that does Manual J calculations, they could give you the temp in the book for the city nearest you.

 

[tom baker]

XHHW-2 is a cross linked polymer insulation and far better than PVC for high temps,the insulation is thicker. It was my standard for underground runs, seen many cases of failed PVC