We are working with an electrician to install permanent wiring to a large trailer unit - it will be a 120/240 V split-phase system (the trailer itself uses a NEMA 14-50 dryer plug) delivering 50 amps at a distance of 300 ft.
I (think I) understand that this wiring set up effectively gives us three circuits - two 120 V/50 A circuits and a single 240 V/50 A circuit.
What I am struggling to understand is how I determine the proper wiring size for the system, given the parameters of the circuit and our voltage drop needs ( <3% ). Following Ohm's Law, I have been using the formula to calculate the expected voltage drop for particular wire gauges (4 AWG or 1 AWG):
Vd = 2 * L * R * A
Vd: voltage drop
L: one way distance (in thousands of ft)
R: resistance of wire (in Ohms per 1000 ft)
A: load current (in amperes)
Ex. 1 1 AWG wire, 300ft, 50A
Resistance of 1 AWG wire / 1000ft = .124 Ohms
Vd = 2 * 50 * (.3) * (.124)
Vd = 3.72 V
Ex. 2 4 AWG wire, 300ft, 50A
Resistance of 4 AWG wire / 1000 ft = .249 Ohms
Vd = 2 * 50 * (.3) * (.249)
Vd = 7.47 V
Based on this information, my numbers suggest that we would need to use 1 AWG wire, but the electrician has said that 4 AWG is more than sufficient. I have complete confidence in this electrician, so I am sure there is something that I am missing. How does the presence of the split-phase 240 V system affect voltage drop considerations (if at all?).
Any comments or help is greatly appreciated!