Wire Gauge Calculator
Find the right wire size for any run. Enter the current, distance and voltage to get the conductor cross-section and the copper or aluminum AWG that keeps voltage drop within your target. Or pick a gauge and check the actual drop, end voltage and resistance, with an optional wire cost estimate.
Formula
Worked example
15 A over a 30 m one-way run on 120 V single-phase copper, 3% target: allowed drop = 0.03 × 120 = 3.6 V. A = 2 × 1.724e-8 × 30 × 15 / 3.6 = 4.31 mm². The next copper size up is 10 AWG (5.26 mm²), which gives an actual drop near 2.46%.
How wire gauge relates to voltage drop
Every conductor has resistance, and that resistance is inversely proportional to its cross-sectional area: double the area and you halve the resistance. As current flows along the cable, Ohm’s law turns that resistance into a voltage drop that is wasted as heat before the electricity reaches the load. By fixing a target drop, commonly 2%, 3% or 5% of the supply voltage, and rearranging Vd = k·ρ·L·I / A, you can solve directly for the smallest cross-section A that keeps the loss inside that budget. The factor k is 2 for single-phase and DC circuits, where current travels out to the load and back along the return conductor, and √3 for a balanced three-phase circuit, where the line-to-line drop is reduced.
Size mode and check mode
In size mode you give the current, distance, voltage and a target drop, and the calculator returns the required cross-section and the nearest standard gauge that is at least as thick, because rounding down to a thinner wire would push the drop back above target. In check mode you pick a specific gauge, from 18 AWG up to 4/0, and it reports the actual voltage drop, the percentage, the voltage that arrives at the load, and the conductor resistance. Both modes report the same supporting outputs so you can compare a candidate wire against the recommendation. Switching the conductor material between copper and aluminum updates the resistivity, and aluminum, being about 64% more resistive, usually needs one to two sizes larger for the same result.
From cross-section to an AWG number
American Wire Gauge is a geometric scale: every six gauge numbers the cross-sectional area roughly halves, and the diameter of an n-gauge wire is 0.127 × 92^((36−n)/39) millimetres. A lower gauge number means a fatter conductor, so 10 AWG is far thicker than 14 AWG, which is the opposite of what intuition first suggests. Above 1 AWG the scale continues as 1/0, 2/0, 3/0 and 4/0 (written 0, 00, 000, 0000), each a step thicker again. The suggestions here assume annealed metal at around 20 °C; real cable resistance rises with temperature, so a hot conductor drops slightly more than these figures show.
Why the target drop matters, and its limits
Electrical guidance commonly recommends keeping voltage drop on a branch circuit near 3%, with about 5% total across feeders and branches combined, so equipment receives a voltage close to its rating. Sensitive electronics and long charging runs often aim for 2% or less, while motors and heating loads tolerate up to 5%. Sizing for voltage drop, however, is only half the job: a wire must also be rated to carry its current without overheating, a separate ampacity limit set by insulation type, grouping and ambient temperature. On short runs the ampacity rule usually governs; on long runs the drop calculation here often demands the larger wire. Always pick whichever rule calls for the thicker conductor, and verify against the wiring code that applies where you are.
AWG cross-section reference
| AWG | Diameter (mm) | Area (mm²) |
|---|---|---|
| 4/0 | 11.68 | 107.2 |
| 2/0 | 9.27 | 67.4 |
| 1/0 | 8.25 | 53.5 |
| 2 | 6.54 | 33.6 |
| 4 | 5.19 | 21.2 |
| 6 | 4.12 | 13.3 |
| 8 | 3.26 | 8.37 |
| 10 | 2.59 | 5.26 |
| 12 | 2.05 | 3.31 |
| 14 | 1.63 | 2.08 |
| 16 | 1.29 | 1.31 |
Nominal cross-sectional area of solid conductors. Use the next size up from the required area.
Frequently asked questions
Does a lower AWG number mean a thicker wire?
Yes. AWG runs backwards: the smaller the number, the larger the conductor. A 6 AWG wire has about four times the cross-sectional area of a 12 AWG wire, so it carries more current and drops less voltage over the same distance. Above 1 AWG the scale continues as 1/0, 2/0, 3/0 and 4/0, each thicker again. Picking a lower number than the suggestion is always safe for voltage drop.
How is three-phase different from single-phase here?
Single-phase and DC circuits count the run length twice, because current flows out to the load and back, so the formula uses a factor of 2. A balanced three-phase circuit uses a factor of √3 (about 1.73) on the line-to-line drop, which is why three-phase needs a smaller conductor than single-phase for the same load. Select the circuit type and the calculator applies the right factor automatically.
Should I use copper or aluminum?
Copper has lower resistance per unit area, so it gives the smallest wire and the least drop. Aluminum is lighter and cheaper but about 64% more resistive, so it typically needs one to two AWG sizes larger for the same voltage drop. Switch the conductor material in the calculator to see the size change. Aluminum also needs rated terminations and anti-oxidant compound where codes require it.
Is voltage drop the only thing that sets wire size?
No. A wire must also be rated to carry its current without overheating, its ampacity, which depends on the insulation, how the cable is installed, and ambient temperature. This tool sizes for a target voltage drop only. Always check the conductor also meets the ampacity required by your local electrical code and use the thicker of the two results.