Ambient Temp corrections

[knot stable]

Can anyone lead me to information on using these tables. I need to run some pipe on a roof top here in Rhode Island and I would like to get this correct. It's something new to me and I want to learn this.
Thanks, Dave:?

 

[Dennis Alwon]

Look at Table 310.15(B)(3)(c)--- Let's say the ambient temperature in Rhode Island is 100? F --- Now suppose your conduit is run 31/2" to 12" above the roof deck. In the table that would correspond to 30? F . So you have 100? and now you must add 30? to this and get 130?

Now you go to Table 310.15(B)(2)(a) and Look for 130? under the wire temp. rating-- I assume 90C so then the corresponding correction factor would be .76. What this means is that a wire rated 100 amps must now be multiplied by .76 and the ampacity of the conductor is only 76 amps

 

[Dennis Alwon]

I want to add that I believe art. 310.15(A)(2) exception should come into play. What this means if ou have a 100' run of conduit and only 10' is outside-- (10%) then there is no need to add that 30? to the ambient temp. I wonder how others see this. I also want to say if there is more than 10' outside it does not matter how long the run inside is because the correction must be figured in.

 

[knot stable]

Thanks Dennis, That makes sense. When would you use the "adders"?

 

[Dennis Alwon]

Thanks Dennis, That makes sense. When would you use the "adders"?

Suppose you have a run of 100' to a rooftop and then have to pipe 20' across the roof to a roof top unit. Now you are beyond the 10' as well as the 10% so the entire run would have to be based on the table for raceways in sunlight