Every cable has resistance, so some voltage is lost along its length. On short runs it doesn't matter. On long runs to a garage, workshop, pump or EV charger, it can mean dim lights, motors that struggle and a run that fails inspection.
The formula
For single-phase: VD = 2 × K × I × L ÷ CM, where K is about 12.9 for copper, I is amps, L is one-way length in feet, and CM is the conductor's circular-mil area. For three-phase, use √3 instead of 2.
Typical limits
NEC informational notes recommend no more than 3% on a branch circuit and 5% overall, from service to load.
Voltage drop is only one check
Cable sizing also depends on current-carrying capacity, installation method, grouping, ambient temperature, protective device and fault protection. Voltage drop often decides the size on long runs, but it's not the only check. This work must be designed and installed by a licensed electrician to the NEC and local code.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check local codes and your inspector for your project.