Source is a home 120/240VAC with pole transformer. I want to direct bury a 500 foot 120V line to run some battery chargers and dont need more than 10 amps. Why cant I only run a single large conductor [4 awg gives a 3% drop] and pick up neutral and earth ground at the load end [where I already have a 8 ft ground rod for a generator that feeds an RV] This would save me $$ on running a 2 or 3 conductor 4 awg, as they are harder to find. I am an Electrical Engineer, so dont need any basic help, just need help understanding the down sides to creating my own ground/return path reference 500 feet away from the source breaker panel/ transformer pole.
Follow the National Electrical Codes - use NMC or IMC rated for burial with a live, neutral, and ground and wire it accordingly.
Bury the NMC cable at least 18" and the IMC at least 6" and put protection above it before closing the trench - a treated lumber 1x4 works well.
Codes exist to insure the safe use of electricity and should not be ignored for the sake of saving money on cabling.
Even if your area does not require adherence to codes (which is completely idiotic) you should still follow them.
Example - "Gee, Mr. Inspector, I didn't know that the reason you are required to plug any unused cable openings in boxes is to prevent a fire from getting out of the box. So, that's why my house burnt down?"
This sort of thing happens, for want of a 25 cent plug, a house is lost.
Follow the codes.