Using 30deg vs 40deg celcius for temperature correction factors

[mull982]

I notice that NEC 2011 has two temperature correction factor tables 310.15(B)(2)(A) and 310.15(B)(2)(B) based on 30deg C and 40dec C. Most of the designs I have seen use the 30deg C table for de-rating both 75deg and 90deg rated conductors. Is the 30deg table typically used for correcting cable ampacity for ambient temperatures? When is the 40deg table used? Can someone explain the difference between the two?

 

[lauraj]

I believe the 40C correction table would be used as adjustment factors for the ampacity tables with a 40C ambient: 310.15(B)(18) - 310.15(B)(21).

 

[jumper]

The ampacity tables tell which one to use.

 

[Twoskinsoneman]

I notice that NEC 2011 has two temperature correction factor tables 310.15(B)(2)(A) and 310.15(B)(2)(B) based on 30deg C and 40dec C.

This is no real change, accept adding more extreme temps. The correction factors had once been at the bottom of the tables 310.16-310.20 in the 2008

 

[weressl]

I notice that NEC 2011 has two temperature correction factor tables 310.15(B)(2)(A) and 310.15(B)(2)(B) based on 30deg C and 40dec C. Most of the designs I have seen use the 30deg C table for de-rating both 75deg and 90deg rated conductors. Is the 30deg table typically used for correcting cable ampacity for ambient temperatures? When is the 40deg table used? Can someone explain the difference between the two?

At the top of each 310.15 series Allowable Ampacities table the reference is given for the temperature that is used as the BASE for the listed ampacities. It is either 30*C or 40*C, so you will use table 310.15(B)(2)(a) for the former and 310.15(B)(2)(b) for the later.

 

[mbeatty]

UL508A control panels

UL508A control panels

We base our wire sizing on the fact that UL508A Industrial control panels are rated for a maximum of 40 degrees C ambient temperature.