Don't throw any tomatoes at me, but I put your question in a search engine which led me to the idea that the answer can be found here:
article 310.3 for wires in a raceway
So I put that in the search engine and found this under 310.3:
310.3 Stranded Conductors. Where installed in raceways, conductors of size 8 AWG and larger shall be stranded.
Exception: As permitted or required elsewhere in this Code.
Hope this is helpful!