wire ampacity

[fjm]

going from 6/3 nM in attic to # 6THHN. To a pull box to pvc outdoor underground raceway with other circuits in pipe beside the 50A circuit. Does my ampacity for derating begin with the 60 deg because the NM?

 

[infinity]

Although you size NM cable according to the 60° C ampacity it does have 90° C conductors so you can use the 90° C ampacity for derating.

 

[fjm]

Thanks! started to second guess pipe size if i had to go to #4

 

[suemarkp]

To expand on your question, each segment of cable is treated to its conditions. Sounds like you have a run of NM in the attic, a run of THHN in the attic, and a run of THHN bundled with other circuits. The attic sections have an ambient temperature derating. The bundled section has a bundling derating. If you bundled in the attic, that would be the worst portion and you would apply both factors. But as was pointed out by Infinity, both NM and THHN use the 90C rating for derating purposes so type doesn't matter.