Table 310.15(B)(2)(a) doesn't have "qualifiers", that permit field modification of the rule, so derating should apply.
It seems to me that derating these conductors to 50% (which would result in an ampacity of 15 amps), would be similar to using #14 wire as the switch legs to the lights on a 20-amp ckt. That is something we all have agreed that would be prohibited.
