When should I worry about voltage drop?

Worry about voltage drop once a 120V run passes roughly 100 feet, and much sooner on low-voltage systems. Distance is the whole game: drop grows with every foot of wire.

The loads that suffer first are motors, EV chargers, and heating elements at the end of long runs. Detached garages, well pumps, and gate openers are the classic cases.

At 240V the same wire and load see half the percentage drop, which is one reason long runs are usually fed at 240.

The fix is one wire size up, decided before you buy cable. Doubling gauge after the trench is closed is the expensive version of this lesson.

The distance wire size calculator tells you at what length your circuit needs the next gauge up.