Wire Gauge & Ampacity Calculator
Determine the minimum wire cross-sectional area (in mm²) needed to safely carry a given current over a set distance without exceeding your allowed voltage drop. Essential for wiring solar systems, EV chargers, and long cable runs.
About this calculator
Voltage drop in a conductor is governed by Ohm's Law applied to the wire's own resistance: V_drop = I × R_wire, where R_wire = ρ × L / A (resistivity ρ, length L, cross-sectional area A). For a two-conductor run the total length is doubled: V_drop = 2 × I × ρ × distance / A. Rearranging for the minimum area gives A = 2 × I × ρ × distance / V_drop. The allowable voltage drop in volts equals voltage × (voltageDrop% / 100). Copper has a resistivity of 1.724 × 10⁻⁸ Ω·m; aluminium is 2.82 × 10⁻⁸ Ω·m. The calculator takes the ceiling of the computed area, ensuring the chosen wire is never undersized, which is critical for fire safety and compliance with NEC/IEC standards.
How to use
Scenario: run a 20 A, 12 V DC copper circuit 15 feet (one way) with a maximum 3% voltage drop. Enter current = 20 A, distance = 15 ft, voltage = 12 V, voltage drop = 3, wire type = copper (ρ = 1.724 × 10⁻⁸). Allowable drop = 12 × 0.03 / 2 = 0.18 V per conductor. Area = 2 × 20 × 15 × 0.3048 × 1.724 × 10⁻⁸ / 0.18 ≈ 1.74 mm². The calculator rounds up to 2.5 mm², which corresponds to 13 AWG — a standard size that safely handles the load.
Frequently asked questions
What is the difference between wire ampacity and voltage drop when sizing a cable?
Ampacity is the maximum continuous current a wire can carry without overheating, determined primarily by its cross-sectional area and insulation temperature rating. Voltage drop is a separate concern — even a wire with adequate ampacity can cause excessive voltage drop if the run is very long, reducing efficiency and potentially damaging equipment. Best practice is to calculate both: choose the wire size that satisfies whichever constraint is more demanding. For runs longer than about 50 feet at low voltages, voltage drop is almost always the limiting factor.
Why does a longer wire run require a larger gauge wire for the same current?
Resistance increases proportionally with conductor length (R = ρ × L / A), so a longer wire drops more voltage for the same current. To keep the voltage drop within an acceptable percentage, the cross-sectional area must increase to reduce resistance. This is why a 20 A circuit in a 200-foot run may need 4 AWG wire, while the same 20 A load just 10 feet away is safely served by 12 AWG. The trade-off is cost and weight of the larger conductor.
How much voltage drop is acceptable for residential or automotive wiring?
The NEC recommends a maximum of 3% voltage drop on any branch circuit and no more than 5% total from the service panel to the end device for residential wiring. Automotive and marine systems typically target 3% or less because 12 V systems are particularly sensitive — a 3% drop is only 0.36 V, but it can noticeably affect motor speed and LED brightness. For sensitive electronics or critical loads, aim for 2% or below. Always check the equipment manufacturer's voltage tolerance before finalising wire size.