Wire Gauge Calculator

Wire size is determined by the required cross-sectional area, which depends on the current (amps), wire length, and allowable voltage drop. The formula is: A = (2 × ρ × L × I) / ΔV, where ρ is resistivity, L is one-way length, I is current, and ΔV is the allowable voltage drop in volts. For three-phase circuits, the factor 2 is replaced by √3. The resulting area is then matched to the nearest standard AWG size.

AWG (American Wire Gauge) sizes are inversely numbered — the smaller the AWG number, the thicker the wire. After calculating the minimum required cross-sectional area in mm², match it to the AWG table and always round up to the next larger (lower AWG number) wire to ensure the wire handles the current safely. This calculator does that matching automatically.

Longer wire runs have more resistance, which increases voltage drop. Use the full round-trip distance (twice the one-way length) in the calculation. For long runs, you often need to go one or two AWG sizes larger than ampacity alone would require, to keep voltage drop within the 3% NEC recommendation.

The NEC recommends a maximum of 3% voltage drop for individual branch circuits and 5% total for feeders plus branch circuits combined. For sensitive electronics or motors, staying at or below 2% is often advisable. Higher voltage drop can cause equipment to run inefficiently, overheat, or fail to start.

Copper has lower resistivity (~1.72×10⁻⁸ Ω·m) than aluminum (~2.82×10⁻⁸ Ω·m), so copper wire carries more current for a given size. Aluminum wire requires a larger gauge to carry the same current as copper. Aluminum is lighter and less expensive, making it common for large feeder circuits, but it requires anti-oxidant compound at connections.

For three-phase circuits, the voltage drop formula uses a factor of √3 (~1.732) instead of 2 for single-phase, because three-phase systems distribute current across three conductors more efficiently. The required cross-section is: A = (√3 × ρ × L × I) / ΔV. This calculator handles both single-phase and three-phase automatically based on your selection.

Motors draw higher starting currents — typically 6–8× their running current. The NEC requires motor branch circuit conductors to be sized at 125% of the motor's full-load current (FLA). Enter 125% of the motor's running amps into this calculator to find the appropriate wire gauge for motor circuits.

Yes. A higher insulation temperature rating (90°C vs 60°C) allows the wire to carry more current (higher ampacity) at the same gauge. However, even with 90°C-rated insulation, the NEC often requires using the 60°C or 75°C ampacity column for termination points (like breakers and outlets) that are not rated for higher temperatures.