Enter the current, voltage, wire size and one-way length to get the volts the wire loses, that loss as a percent of the supply, and the voltage left at the load. It also gives the longest run that stays within 3% and within 5%, and the smallest wire that would at your length, never smaller than the current needs on an AC circuit. For example, 15 A at the end of 50 feet of 14 AWG copper on 120 V loses 4.61 V (3.84%), leaving 115.4 V at the load.
Voltage drop is the current multiplied by the wire's resistance, counted over the whole path the current travels. On a single-phase or DC circuit that path goes out and comes back, so the drop is 2 × I × R × L, where I is the current, R the resistance per foot and L the one-way length. On a balanced three-phase circuit the line currents balance with no return current, and the line-to-line drop is √3 × I × R × L, with √3 about 1.73. A 4 AWG aluminum feeder carrying 40 A for 200 feet on a 208 V three-phase system loses 6.89 V, or 3.31%. R is taken at 75 °C, a conductor running warm under load, where 10 AWG copper is 1.21 ohms per 1000 feet.
The 3% and 5% figures come from informational notes to NEC 210.19 for branch circuits and 215.4(A)(2) for feeders (215.2(A)(2) in the 2023 edition). They suggest at most 3% on a branch circuit or feeder, and 5% in total from the service to the farthest outlet. Informational notes are recommendations, not requirements, so a run at 4% isn't a code violation. It means the equipment runs below its rated voltage. For a resistance heater the loss compounds: its power goes with the square of the voltage, so a 3% drop costs it about 6% of its heat.
Low-voltage DC is where drop matters most, because the volts lost are measured against a much smaller supply. 15 A through 20 feet of 10 AWG copper loses 0.73 V at any voltage. That is 0.61% of 120 V and 3.04% of 24 V. On a 12 V system it is 6.07%, and only 11.3 V reaches the load. Keeping that 12 V run within 3% takes 6 AWG copper, at 2.40%; 8 AWG still comes in at 3.82%. Small DC loads such as LED strips often use 14 to 20 AWG, so the size list goes down to 20 AWG.
Heavier wire and higher voltage work the other way. At 40 A on 240 V, 8 AWG copper stays within 3% for 117 feet one way, and 6 AWG copper for 187 feet. A load built for 240 V draws half the current of the same wattage at 120 V, and each volt it loses is half the share, so its percentage drop falls to a quarter.
The figures are for solid wire. Stranded wire of the same gauge has a little more resistance: NEC Chapter 9, Table 8 gives 1.98 ohms per 1000 feet for stranded 12 AWG copper at 75 °C, about 2.5% above solid. Its drop is higher by the same share, so a stranded run close to the limit can need the next size up.
Voltage drop doesn't size a wire for heat. A short run can show a tiny drop on a wire that is too small for the current: 30 A through 10 feet of 14 AWG copper drops only 1.54%, yet NEC 240.4(D) limits 14 AWG copper to 15 A. On AC circuits, when the current is more than the wire's allowed amps in the 75 °C column, for sizes Table 310.16 rates, the result says so. NM-B cable is held to the 60 °C column, which can be lower still. To size a circuit for both ampacity and drop at once, use the wire size calculator.