Derating wire in PVC pipe

[Bacchus]

Came across someone today who said you don't need to derate multiple wires in a PVC pipe. His explanation was that the derating was because of harmonic problems. It didn't make much sense to me and thinking he might have known more than me I decided to give him the benefit of the doubt. The AHJ might give them a pass on this too FAIK.

I haven't looked up this code in a while, not running much pipe, so I tried to find the reference in the code as to why. I was always taught it was for heat dissipation. Even if you ran a 3" pipe for anything more that 3 CCC's regardless of size you had to use Table 310.15(B)(3)(a) for derating.

Can anyone give me the reference where I'd find the 'why' behind ampacity derating?

And did you notice this little gem at the end of 310.15? The exception? New to the 2014 code you can now use XHHW-2 wire without the adjustment factor. That's great news! Finally releif from the curse of derating wires in pipes. Yahoo, I feel so relieved!! It doesn't look like it's too much more expensive than THHN either. Maybe 50% higher. YMMV.

Bacchus

 

[infinity]

Whoever is providing you this information is incorrect. In general if you exceed 3 CCC's in a conduit or tubing you need to apply derating factors. The size of the circular raceway is irrelevant for derating purposes.

 

[Smart $]

... The size of the circular raceway is irrelevant for derating purposes.

And the reasoning is that on horizontal runs, all the conductors nest in the bottom of the raceway. This also the base reason why we don't have to derate for several conductors in a wireway or gutter, as the don't nest side-by-side (unless you bundle 'em or otherwise install 'em that way intentionally).