Train,
You need to be more specific about what conductor that you are referring to. Are you talking about the equipment grounding conductor or the grounded conductor? If it is the EGC, that is never counted for derating purposes. Derating if required only because of the heat that is produced in the current carrying wires. If you are talking about the grounded conductor, then the answer is, sometimes. If the grounded conductor is part of a multiwire circuit and there is not a lot of non-linear loads on the circuit, then it is not counted. If it is not part of a multiwire circuit, it is always counted. It is also counted when part of a multiwire circuit where the majority of the loads are non-linear. If you have a three phase system and are using only 2 phases and a grounded conductor, it is counted.
Don
