How to calculate voltage drop across multiple wire gauges?
Each piece of wire is a resistor.  Add up the resistance of all the wire that the current to the motor runs thru.  Remember to consider both directions.

You find the resistance of each piece of wire by looking in a wire chart to get the resistance per unit length.  Multiply that by the actual length of the wire segment to get the resistance of that segment.

Add up all the individual resistances of the individual wire segments.  That's the total wire resistance in your installation.  Multiply that by the current, and you have the voltage drop caused by the wire.