Calculating the voltage drop in a split-phase system
The 50A breaker pair feeding the NEMA 14-50 plug limits the current on any phase to ... 50A, period, no matter how you load your phases. The breakers are linked, so if you have a single-phase fault on L1, it will trip itself and L2.

Your electrician will use that breaker panel limit not only to calculate the IR drop, but also the allowable cable temperature rise given the ambient temperature, conduit type, wire material and insulation type, as these all affect the 'ampacity' of the wire.

As for the voltage drop, the NEC specifies a maximum drop of 5%, not 3%. See https://www.mikeholt.com/technnical-voltage-drop-calculations-part-one.php

At 300ft and 50A, #4 ga. copper will be less than that, about 3.5%. So, plenty.

Your electrician knows what they're doing.

BONUS: An IR drop calc, with a wire resistance chart: https://www.calculator.net/voltage-drop-calculator.html

tl; dr:


resistance of #4 ga. is 0.2485 ohm/1000ft
One-way IR drop for 300' at 50A = 50 * 300/1000 = 3.73V
Two-way IR drop is 7.46V, or 3.1%