Do you derate for a three phase line and load conductors in the same conduit?

Yes, there will be a de-rate applied to this type of installation. In the electrical code book the amperage rating of conductors is for allowable ampacities for not more than three copperconductors in raceway or cable. This is based on an ambient temperature of 30 degrees C.

If the ambient temperature of the conductors where the raceway is installed is over 30 degrees C the conductor amperages have to be reduced.

Where the number of conductors are from 4 to 6 conductors in a raceway, the conductors amperage has to be reduced by .8