Wire Voltage Drop Calculator
Estimate voltage drop, percentage drop, and power loss in a copper or aluminium wire run from AWG gauge, length, and current.
Voltage drop in a wire
Every conductor has a small resistance, so current through a long run loses a little voltage as heat before it reaches the load. Drop grows with length and current, and shrinks with a thicker (lower-AWG) wire.
V_drop = I × R, with R = ρ × (2 × length) / area
The length is doubled because current flows out and back. Aluminium has about 1.6× the resistivity of copper, so it drops proportionally more for the same gauge. A common rule of thumb keeps drop under 3% of the supply.
Worked example: a 12 V pump 5 m from the battery
A bilge pump draws 10 A at 12 V, sitting 5 m from the battery. Enter 16 AWG copper, 10 A, 5 m, 12 V: the round-trip resistance is 131.7 mΩ, so the drop is 1.317 V — 11% of 12 V, well past the 3% rule of thumb and enough to leave the pump gutless.
Step up to 12 AWG and the drop falls to 521 mV (4.34%) — still borderline. 10 AWG gives 328 mV (2.73%), comfortably under. That's the wire to run.
Related
Look up a wire's diameter and area with theAWG wire gauge calculator, or size a PCB track with thePCB trace current calculator.