How this calculator works
Wire resistance is resistivity times length divided by conductor area. A two-wire circuit includes an outgoing and return path, giving a factor of two. The balanced three-phase line-to-line approximation uses sqrt(3). AC voltage-drop estimation projects the resistive drop using displacement power factor and neglects reactance, while conductor heating still uses I²R. AWG modes derive the bare conductor area from gauge; parallel-conductor mode assumes identical wires sharing current equally. Use the actual hot-conductor resistivity. This calculation does not determine ampacity, protective devices or compliance with an electrical code.
Formula
DC: deltaV = 2 rho L I/(A p). AC resistance-only approximation: deltaV = k rho L I PF/(A p), k = 2 single phase or sqrt(3) three phase, p = identical parallel conductors per leg. Pwire = nlegs I² rho L/(A p). AWG diameter d = 0.000127 × 92^((36-gauge)/39) m; A = pi d²/4.
Worked example
A 20 m one-way DC run with rho = 1e-8 ohm m, area 2e-6 m² and current 10 A drops 2 V. A 120 V source therefore gives an estimated 118 V at the load.
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Frequently asked questions
Do I enter the round-trip length?
No. Enter the one-way run. The two-wire return is already included.
Can this size a cable for installation?
It estimates area for voltage drop only. Thermal limits, installation conditions and electrical-code requirements need separate checks.
When is the AC estimate inadequate?
Long runs, low power factor or cables with appreciable reactance need a full phasor impedance calculation.