Derating

[sparkyboys]

I would like to thoroughly understand how to use my code book when derating copper wire when ambient temperatures are above 30 deg C.
And how to better understand this formula for derating:
Voltage Drop per Foot:
V = I x L x (R/1000) x 1.004 x (T-20)

 

[Dennis Alwon]

I would like to thoroughly understand how to use my code book when derating copper wire when ambient temperatures are above 30 deg C.
And how to better understand this formula for derating:
Voltage Drop per Foot:
V = I x L x (R/1000) x 1.004 x (T-20)

Go to Table 310.16. Let's look at #2 wire at 90c (thhn wire)

We see it is rated 130 amps. Now go to the correction table under the ampacity table and look at --ohh let's say 46-50C -- Follow the line to the 90C column and you see .82

130 x .82= 106.6 amps. That's all there is to it. Of course, if the derating is still above the 75C column then you are limited to the 75C rating anyway. The 90C column is only good for derating purposes.

 

[sparkyboys]

That was really easy and thorough. We did not get into it that much in school at all. Thanks a lot for your help.