Voltage drop is the reduction in electrical potential along a conductor when current flows through it. All conductors have resistance. As current passes through a wire, some energy is lost as heat according to Ohm's Law.

In low-voltage DC systems — 12V and 24V marine, off-grid solar, automotive, motorsport, RV — even small voltage losses can cause significant performance issues. Motors may run hot or weak, lights may dim, and sensitive electronics may brown out. Because supply voltages are relatively low, voltage drop is proportionally more impactful in 12V systems than in higher-voltage installations.

For DC systems: V_drop = 2 × I × R × L. Where I = current (amps), R = resistance per meter (Ω/m), L = one-way cable length (meters). The factor 2 accounts for the round-trip path (outbound and return conductor).

This calculator uses published copper resistance values at 20°C. Resistance increases with temperature; in engine rooms or hot environments, actual voltage drop may be higher than calculated. The result is provided in volts, percentage of system voltage, and estimated power loss.

Use the AWG sizing calculator to find the minimum gauge for your current, length, and max drop percentage.

In marine and vehicle electrical systems, voltage drop is commonly limited to 3% maximum for critical circuits (navigation lights, bilge pumps, safety electronics) and up to 10% for non-critical loads. These limits are consistent with guidance from ABYC E-11 and other industry best practices.

3% of 12V = 0.36V. 3% of 24V = 0.72V. Exceeding these limits can cause motor overheating, reduced pump performance, electronic instability, and charging inefficiency.

Load: 10A. Length: 10m one-way. Wire: AWG 14 copper. Resistance ≈ 0.0085 Ω/m.

V_drop = 2 × 10 × 0.0085 × 10 ≈ 1.7V. Percentage: 1.7V / 12V = 14.2%. This exceeds the 3% recommendation for critical circuits. A larger conductor such as AWG 10 would be required to meet a 3% target.

See the fuse sizing tool for overcurrent protection.

This calculator assumes copper conductors, 20°C reference temperature, steady DC load, and proper terminations. Real-world factors that increase effective resistance: elevated temperature, poor crimps or corrosion, long harness runs with connectors, undersized return conductors. Always size conductors for worst-case load and environmental conditions.

Voltage drop guidance varies by jurisdiction and application. Relevant references include ABYC E-11 (marine DC), NEC (U.S.), SAE J1128 (automotive), IEC conductor standards. This calculator is a design aid and does not replace professional engineering review or code compliance verification.