CJS, I don't think you fully understand derating and you're not alone. I have had this same discussion with several inspectors. You derate from the 90deg column. Take a closer look at the ampacity tables and you will see what IWIRE and Don are talking about. I agree with Don, EMT over PVC. PVC needs to be supported at closer intervals to avoid a sloppy saggy install. I always pull an EGC in my EMT and bond it in every j-box. I install a 14 lug ground bar in the pull boxes to avoid a rat's nest of grounds/wirenuts. MW branch circuits installed in this manner present no more of a hazzard than any other wiring system and in 99% of the circuits they eliminate the neutral as a current carrying conductor for derating purposes.
