12V automotive wire sizing — ohms per foot is your whole world

A 120 V house circuit considers 3% voltage drop good practice. A 12 V vehicle can’t afford it the same way: 3% of 12 V is 0.36 V, and the difference between 11.6 V and 12 V at your headlights is visible to your eyes.

The math that matters

Everything reduces to one number: resistance of the copper run. Voltage drop is:

drop (V) = 2 × amps × Ω-per-ft × one-way length

Copper resistance in Ω per foot: 10 AWG is 0.000999, 12 AWG is 0.001588, 14 AWG is 0.002525. Double the length, double the drop. Double the amps, double the drop. The full gauge chart lists every size.

Worked examples

LED headlight conversion — 5 A over 12 ft (round trip 24 ft)

14 AWG: 2 × 5 × 0.002525 × 12 = 0.30 V — fine. 16 AWG: 2 × 5 × 0.004016 × 12 = 0.48 V — tolerable, margin disappears with corroded grounds.

80 A inverter — 6 ft apart

12 AWG: 2 × 80 × 0.001588 × 6 = 1.52 V (12.7%!) — the inverter will alarm or brown out. 2 AWG: 2 × 80 × 0.000156 × 6 = 0.15 V — clean. This is why serious inverter installs bolt directly to the battery with massive lugs.

Trailer tongue harness — 15 A at 20 ft

12 AWG gives 2 × 15 × 0.001588 × 20 = 0.95 V — right at the edge of “acceptable” for lights, too much for a brake controller. 10 AWG (0.60 V) is the safe answer.

Automotive reality: it’s the connectors, not the copper

Every crimp and every fuse block adds resistance, and the return path through the chassis adds more. Three practices make more difference than one gauge step:

1. Solder + adhesive-lined heat shrink on all added harness joints; crimps are allowed but adhesive-lined crimps only.
2. Run the ground with the same gauge — don’t rely on chassis return for power circuits over ~10 A.
3. Protect with a fuse — size the fuse by wire ampacity, not load current; the fuse protects the cable from short-circuit heating, not your device from overcurrent.

Temperature and bundling derate

An engine bay wiring run at 90 °C ambient has roughly half the ampacity margin it does at 20 °C. A harness taped inside a conduit bundle must also be derated. The calculator’s upsizing is a conservative mechanical default; the installed condition can still make you go one size up.

The 12V chart tabulates common loads by length so you can skip the formula, and the calculator does both limits for you.