Wire gauge and voltage drop: choosing a wire that delivers the volts
A wire is a resistor, just a small one, and over a long run at a high current it eats a surprising share of the supply: lights dim, motors labour, chargers cut out, and the wire itself gets warm. The loss is simple Ohm's law once you know the wire's resistance, which comes from its cross-section and its material. This calculator gives the resistance of a run in AWG or metric sizes, copper or aluminium, the voltage lost and what reaches the load, the heat, whether the wire can carry the current at all, and the smallest size that keeps the drop within your limit.
How to use the voltage drop calculator
- Pick the wire from the AWG or metric list and the material (copper unless you know it is aluminium).
- Choose the circuit: DC or single-phase AC count the return conductor (2 × length); balanced three-phase uses √3 × one conductor.
- Type the one-way length (metres, or 16 ft, 250 cm), the current and the supply voltage.
- Read the drop in volts and percent, the voltage at the load, the heat and the run's resistance; the drawing shows the conductors as thick as the wire, and the bar where the drop lands against 3% and 5%.
- Set a limit (3% by default) and the smallest wire in the same family that meets it, and carries the current, is suggested with a one-click swap. Every gauge tabulates the whole family on this run; click the wire for its reach and neighbours.
The formula
Vdrop = 2 × I × ρ × L / A (√3 instead of 2 for three-phase)
ρ is the resistivity of the metal: 1.724 × 10⁻⁸ Ω·m for copper, 2.82 × 10⁻⁸ for aluminium, at 20 °C (add about 0.4% per degree above that). L is the one-way length and A the cross-section, so ρL/A is one conductor's resistance; the current flows out and back, hence the 2. Multiply by the current for the volts lost, and by the current again for the watts of heat. Everything that makes the drop worse is in the numerator — more current, longer run, aluminium — and only a fatter wire fights it.
Your run 2 × 10 A × 41.42 mΩ = 828.5 mV, 6.9% of 12 V.
AWG and mm²
American Wire Gauge numbers run backwards: a bigger number is a thinner wire. The sizes are a geometric series, each gauge 1.123 times the diameter of the next, so three gauges halve the area (and double the resistance), six gauges halve the diameter, and ten gauges divide the area by ten. Beyond 0 the sizes are 1/0, 2/0, 3/0 and 4/0. Metric cables simply state the copper area in mm². Rough equivalents: 18 AWG ≈ 0.75 mm², 16 ≈ 1.5, 14 ≈ 2.5 (a little under), 12 ≈ 4, 10 ≈ 6, 8 ≈ 10, 6 ≈ 16, 4 ≈ 25.
| AWG | Diameter | Area | Resistance | Free-air limit |
|---|---|---|---|---|
| 4 | 5.19 mm | 21.15 mm² | 815 µΩ/m | 135 A |
| 6 | 4.12 mm | 13.3 mm² | 1.3 mΩ/m | 101 A |
| 8 | 3.26 mm | 8.37 mm² | 2.06 mΩ/m | 73 A |
| 10 | 2.59 mm | 5.26 mm² | 3.28 mΩ/m | 55 A |
| 12 | 2.05 mm | 3.31 mm² | 5.21 mΩ/m | 41 A |
| 14 | 1.63 mm | 2.08 mm² | 8.28 mΩ/m | 32 A |
| 16 | 1.29 mm | 1.31 mm² | 13.2 mΩ/m | 22 A |
| 18 | 1.02 mm | 0.82 mm² | 20.9 mΩ/m | 16 A |
| 20 | 0.81 mm | 0.52 mm² | 33.3 mΩ/m | 11 A |
| 22 | 0.64 mm | 0.33 mm² | 53 mΩ/m | 7 A |
| 24 | 0.51 mm | 0.2 mm² | 84.2 mΩ/m | 3.5 A |
The 3% rule
Most wiring codes and most equipment makers settle on the same numbers: keep the drop to a branch circuit under 3%, and the whole way from the supply to the load under 5%. At 3% a motor still starts cleanly, an LED driver stays in regulation and a battery charger reaches its target. The rule bites hardest at low voltage: 0.36 V is 3% of 12 V, and 10 A through 5 m of 14 AWG loses more than twice that, which is why car, boat, solar and LED-strip wiring looks so thick for its current. At 230 V the same wire and current lose a negligible 0.4%.
Voltage drop is not the only limit
A wire also has a current limit (ampacity) set by how hot it may get, which depends on the insulation and on whether it is in free air, bundled or buried. The figures here are the common free-air chassis-wiring table and are generous; a cable in conduit or a wall carries roughly half, and building wiring must follow the local code's tables. The two limits are independent: a short run can be within 3% and still overheat, a long run can be cool and still lose 10%. The calculator checks both and recommends the smallest wire that passes both.
Your run, step by step
- Conductor: 14 AWG (2.08 mm²), copper: 8.285 mΩ/m, so 41.42 mΩ over 5 m.
- Run: × 2 = 82.85 mΩ; at 10 A that is 828.5 mV (6.9%), leaving 11.17 V at the load and 8.28 W of heat in the wire.
- Current limit: about 32 A in free air — fine.
Worked example: 12 V, 10 A, 5 m of 14 AWG
14 AWG copper is 2.08 mm², so 8.28 mΩ/m; 5 m out and 5 m back is 10 m, 82.8 mΩ. At 10 A the wire drops 828 mV, 6.9% of 12 V, and the load sees 11.2 V while 8.28 W warms the cable. To stay within 3% (0.36 V) the run may have at most 0.036 Ω, which needs 4.79 mm²: 10 AWG (5.26 mm²). The same 10 A over 5 m from a 230 V supply through the same 14 AWG would lose only 0.4%.
Questions
Does the length mean one way or the whole loop?
One way, from the source to the load; the calculator doubles it for the return conductor (or applies √3 for three-phase). If you have the total cable length of a two-core cable, halve it first.
Why is aluminium wire thicker for the same job?
Its resistivity is about 1.6 times copper's, so it needs 1.6 times the area for the same drop — roughly two AWG sizes up. It is lighter and cheaper per ampere, which is why overhead lines and large feeders use it, but its terminations need care (it creeps and oxidises).
What about temperature?
Copper's resistance rises about 0.39% per °C. A wire running at 60 °C has 16% more resistance than the 20 °C figure used here; add that margin for hot engine bays, roof spaces or wires near their current limit.
Can I run two wires in parallel instead of one thick one?
Yes: two identical conductors in parallel halve the resistance, like the parallel resistor calculator shows, and codes allow it for large sizes if the lengths and terminations match. For small wires it is usually simpler and safer to buy the next size up.