IMO, derating would apply, T310.15(B)(2)(a) would dictate 50% off of 310.16 ( 10-20 conductors), however note the math starts at the 90degC collum there.
some consideration for the cieling 'ambient' temp may also apply
The interesting thing is the JB vs. a trough, and the magic '29' conductor limit here;
376.22 Number of Conductors.
The sum of the cross-sectional areas of all contained conductors at any cross section of a wireway shall not exceed 20 percent of the interior cross-sectional area of the wireway. The derating factors in 310.15(B)(2)(a) shall be applied only where the number of current-carrying conductors, including neutral conductors classified as current-carrying under the provisions of 310.15(B)(4), exceeds 30. Conductors for signaling circuits or controller conductors between a motor and its starter and used only for starting duty shall not be considered as current-carrying conductors.