Fedup,
When the wire size is increased by the derating (more than 3), one is reducing the resistance of the wire, per foot, to reduce the heat per foot, to protect the insulation from being overheated.
The derating is kind of a red herring, that is, difficult not to think about once it comes to mind.
The derating protects the insulation by reducing the resistance per unit length of current carrying conductor.
When the resistance is lower, the conductor voltage drop is lower. Colateral effect. A good one, especially when the run is long. Derating is not concerned with length of run, or the type of load. In your case, the motor start current and heavy load current won't drag the voltage down as much. In this case, the derating of the conductor introduces a little more headroom for the motor's nonlinear use of voltage and current.
Al
