Wire Size Calculator

Wire sizing involves two separate questions: whether a conductor can safely carry the expected current and whether it can deliver acceptable voltage at the load. This calculator focuses on the second question. It estimates the minimum copper cross-sectional area needed to remain within a selected voltage-drop percentage. The calculation uses conductor resistance, Ohm's law, and the full outgoing-and-returning path of a typical two-wire circuit. The permitted voltage loss is found by multiplying circuit voltage by the chosen percentage. The required area follows A = 2ρLI/Vdrop, where ρ is copper resistivity near 20°C, L is one-way distance, I is load current, and Vdrop is the allowed loss in volts. The factor of two accounts for current traveling to the load and back. After finding area in square millimetres, the calculator determines an equivalent solid-wire diameter and converts it to the American Wire Gauge scale. AWG sizes are discrete. The displayed recommendation rounds toward the larger conductor, represented by the lower gauge number, so its nominal area is not less than the continuous mathematical requirement. Voltage-drop sizing can recommend a much larger cable on long runs or low-voltage systems because the same absolute voltage loss represents a greater percentage of a low supply voltage. Higher current also increases the required area in direct proportion. This voltage-drop estimate does not replace an ampacity calculation. A conductor must also satisfy electrical-code limits for insulation rating, ambient temperature, raceway fill, bundling, terminal temperature, load duration, and installation method. Motor circuits, continuous loads, renewable-energy systems, and services may have additional rules. The simplified model assumes copper and direct-current resistance near room temperature; hot conductors have greater resistance. Use the recommendation as a preliminary engineering value and compare it with the next available commercial cable size and all governing code tables. If ampacity rules require a larger conductor, use that larger size. Safety-critical designs and permanent wiring should be reviewed by a qualified electrician or electrical engineer.