Voltage drop is a function of current, wire size and material, temperature, and length of run.
All things being equal; less current means less voltage drop. However, all things are never equal. The closer to the max rated current you get the more heat is generated and the higher the resistance of the run which increases the rate of heat generation.
Using a wire rated for 30 amps and drawing just 20 will reduce the effective voltage loss by 33%. So, if the voltage drop was going to be 6 volts at 30 amps it will be 4 volts at 20, for example.
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