Cable tray vs THHN single conductor

[rthreesuns]

I cannot find anywhere that states that you can not use single conductor THHN in cable tray. This is in a general purpose building, CME brand 12 awg THHN, we have multiple circuits of 120 volt in cable tray, We went off the customer specs and they requested the THHN. So can THHN wire be ran in cable tray?

 

[tkb]

If the THHN is TC rated and 1/0 or larger, and only in industrial establishments.
NEC 2017 392.10(B)(1)

#12 single conductors are not allowed to be installed in cable trays.