How the voltage drop calculation works
Current flowing through wire resistance loses voltage in proportion:
VD = 2 × L × I × R ÷ 1000
The factor of 2 accounts for the round trip — current travels out on the hot conductor and back on the neutral, and both contribute drop. L is the one-way run in feet, I the load current, and R the conductor resistance in ohms per 1000 ft (the calculator uses standard copper values at operating temperature; aluminium runs roughly 1.6× higher resistance for the same gauge). Drop as a percentage is simply VD ÷ source voltage.
Worked example
A 15 A load at the end of 100 ft of 12 AWG on a 120 V circuit: VD = 2 × 100 × 15 × 1.59 ÷ 1000 = 4.77 V, which is 3.97% of 120 V — slightly over the classic 3% branch-circuit recommendation, and the load sees about 115.2 V. Stepping up to 10 AWG cuts the drop to 2.50%. This is exactly why long runs to sheds, well pumps and RV hookups deserve a wire-size upgrade even when ampacity tables say the small wire "is fine".
The 3% and 5% guidelines
Common practice (reflected in NEC informational notes and the wiring of every reputable electrician): keep branch-circuit drop ≤ 3% and total drop from the service to the load ≤ 5%. Sensitive electronics and motors care — a motor fed 10% low draws more current, runs hotter and loses torque roughly with the voltage square.