HI,
For single-phase system (Phase & neutral), the load is at ‘X’ distance form the source and to calculate the voltage drop in the cable, shall I use 2X distance (X-for phase wire + X-for neutral wire) or just X-distance?
It depends on the data you are using for calculating the voltage drop. Some data sources are for circuit feet, some are for conductor feet. In your case circuit feet are half of conductor feet.