Voltage drop occurs in electrical systems as current flows through system conductors. Like the analogy of water flowing through a pipe, the electrical current is synonymous with water flow, the impedance of the conductor is like the area of the pipe, and the voltage is similar to the pressure in the pipe. These currents and associated drops in voltage occur at varying degrees from sources to loads based on the impedances they flow through.
Code recommends voltage drop of 2% max for feeders and 3% max for branch circuits (5% total) from source to load. Poor voltage regulation will allow system voltages to fluctuate significantly with the amount of load on the system.
Strategies: Use higher voltages whenever possible for large loads and long runs. Not only do you reduce current for the same amount of power, the percentage drop will be proportionally less. Even without reductions in the amount of energy consumed, there can be significant reductions in conduit and wire cost of feeders and branch wiring. For lower voltages keep distribution system equipment and panels closer together. For 120V branch wiring, keep branch panelboards within 100-125 feet of loads served when possible to avoid increasing branch wiring. Understand loads that have high inrush currents (motors started across the line) and account for them. For engineers, I recommend using load flow analysis software such as ETAP for larger projects. Taking a few simple steps early on in a project can provide a more stout and more efficient distribution system.