DC Wire Size Calculator

Direct current circuits experience voltage drops along a conductor due to internal resistance. The formula for the required wire cross-sectional area (A) is derived from Ohm's law and resistivity equations: A = [ρ × (1 + α × (T - 20)) × I × 2 × L] / (V × V_drop), where ρ is the conductor resistivity at 20°C, α is the temperature coefficient, T is the operating temperature, I is the current in amperes, L is the one-way distance, V is the source voltage, and V_drop is the allowable voltage drop percentage. The calculator then converts this cross-sectional area into standard wire gauge sizes and determines the conductor diameter using d = √((4 × A) / π).

Consider a 12V DC solar power setup supplying a 15-amp refrigerator located 20 feet away from the battery bank. We want to keep the voltage drop under 3% (0.36 volts) using standard copper wire at an ambient temperature of 25°C. Copper has a baseline resistivity of approximately 0.0175 Ω·mm²/m and a temperature coefficient of 0.00393 /°C. Plugging these values into our equation: A = [0.0175 × (1 + 0.00393 × (25 - 20)) × 15 × 2 × 20 (converted to meters) / (12 × 0.03)]. The resulting minimum cross-sectional area is approximately 3.5 mm², which corresponds roughly to a 12 AWG wire. Selecting this size ensures safe and efficient power delivery.

Always account for the total round-trip distance of your circuit—meaning both the positive and negative conductor lengths—unless your calculation specifically incorporates a multiplier. Additionally, if your wire runs through engine bays or hot attics, factor in higher ambient temperatures because elevated heat increases electrical resistance and reduces safe current capacity. When in doubt between two wire gauges, always step up to the thicker wire size to minimize energy losses and future-proof your installation.