Voltage Drop Calculator (AWG & mm²)

Works out how much voltage a cable loses on the way to the load. Choose DC, single-phase or balanced three-phase, copper or aluminium, and give the conductor size as an AWG gauge or a cross-section in mm², the one-way run length, the current, the supply voltage and the conductor temperature. The tool returns the drop in volts and as a percentage of the supply, the voltage left at the load, the power turned into heat in the cable, the resistance per kilometre and per 1,000 ft, and the smallest AWG gauge and the smallest standard metric size that would keep the drop within the limit you set. Resistance comes from first principles: copper at the IACS value of 1/58 Ω·mm²/m at 20 °C, aluminium at 61 % of that, corrected linearly for temperature, and AWG diameters from ASTM B258 — which reproduces the NEC Chapter 9 Table 8 resistances for solid conductors at 75 °C. The drop is 2·I·R for DC and single-phase (out and back) and √3·I·R per phase for a balanced three-phase load. What it leaves out: the reactance of AC cables, which matters from roughly 25 mm² (4 AWG) upward and adds a few percent at low power factor; the small extra resistance of stranded conductors; and skin effect at high frequency. Codes and standards set the limits — 3 % for a branch circuit and 5 % overall is a common recommendation in the NEC, and IEC 60364 uses 3 % for lighting and 5 % for other loads — and they also require a size that carries the current without overheating, which this tool does not check; use an ampacity table for that.