Wire Resistance Calculator
Wire resistance depends on the conductor material, length, and cross-sectional area (determined by the AWG gauge). This calculator computes the resistance of a wire run, the voltage drop at a given current, and the maximum safe current capacity according to standard ampacity ratings.
Wire resistance is calculated using R = ρL/A, where ρ is the resistivity of the conductor material, L is the wire length, and A is the cross-sectional area. Copper has a resistivity of 1.724 × 10-8 Ω·m, making it the most common conductor. Aluminum has higher resistivity (2.65 × 10-8 Ω·m) but is lighter and cheaper, often used for power distribution.
The American Wire Gauge (AWG) system defines wire diameters using the formula d = 0.127 × 92(36-AWG)/39 mm. Lower AWG numbers mean thicker wire with lower resistance. Common household wiring uses 14 AWG (15A circuits) and 12 AWG (20A circuits).
Voltage drop is a critical concern in long wire runs. The NEC recommends no more than 3% voltage drop for branch circuits and 5% total from service entrance to the farthest outlet. Excessive voltage drop wastes energy as heat and can cause equipment to malfunction. To reduce voltage drop, use thicker wire (lower AWG) or shorter runs.