12 V Wire Voltage Drop Calculator
Wire gauge math for car, camper and van 12 V circuits — why 0.36 V is already your whole 3 % budget and which AWG keeps the fridge running.
Preset: 15 A fridge/accessory circuit, 4 m route, 12 AWG copper. Compute with engine-OFF resting voltage (12.4–12.6 V), not the 14.4 V charging figure — loads meet the drop at night.
12 V Wire Voltage Drop Calculator computes voltage lost in car, camper and van 12 V wiring runs — free, instant and private in your browser. Van builders, overlanders and anyone adding 12 V accessories to a vehicle use it to skip the datasheet algebra: type your numbers, read the answer with the substituted formula shown step by step, and share an exact permalink of the calculation.
About 12 V Wire Voltage Drop Calculator
12 V Wire Voltage Drop Calculator computes voltage lost in car, camper and van 12 V wiring runs using the standard engineering relation: Vd = 2·I·(ρ/A)·L with copper at 1.724×10⁻⁸ Ω·m — the ×2 covers the return conductor. Worked live: 15 A over a 4 m route in 12 AWG drops ~0.31 V, already 2.6 % of a 12 V system. The result recalculates on every keystroke, the worked-example panel shows your numbers substituted into the formula, and the Copy permalink button encodes the inputs in the URL so a colleague opens exactly your calculation. Everything runs client-side — nothing you type leaves your device.
How to use 12 V Wire Voltage Drop Calculator
- 1Enter your values — the tool starts with realistic defaults for this exact use case, so the worked example is meaningful immediately.
- 2Read the live result and the worked-example panel, which substitutes your numbers into the formula step by step.
- 3Adjust any input to compare scenarios, then use Copy result or Copy permalink to share the calculation.
Why use 12 V Wire Voltage Drop Calculator?
- ✓Implements the real formula — Vd = 2·I·(ρ/A)·L with copper at 1.724×10⁻⁸ Ω·m — the ×2 covers the return conductor — with the substitution shown, not a black box
- ✓Built for van builders, overlanders and anyone adding 12 V accessories to a vehicle
- ✓Copy result and permalink buttons — share the exact calculation in a README, forum answer or design review
- ✓100% free, no sign-up, runs entirely in your browser (works offline once loaded)
Frequently asked questions
How do you calculate 12 v wire voltage drop?+
Voltage lost in car, camper and van 12 V wiring runs follows Vd = 2·I·(ρ/A)·L with copper at 1.724×10⁻⁸ Ω·m — the ×2 covers the return conductor. For example, 15 A over a 4 m route in 12 AWG drops ~0.31 V, already 2.6 % of a 12 V system. The calculator applies the same relation and shows the substituted arithmetic so you can verify every step.
Why is the 3 % rule so hard to meet at 12 V?+
Because 3 % is only 0.36 V. A few metres of modest wire plus two crimps and a fuse holder consume it easily — which is why 12 V design always specifies thicker copper than mains intuition suggests, and why upgrading to a 24 V system instantly relaxes everything by 4×.
Should I measure voltage with the load on or off?+
On — always. Voltage drop only exists while current flows: a corroded joint reads perfect at 0 A and loses half a volt at 15 A. Diagnose by measuring across each wire and connection with the load running; anything over a few tens of millivolts per joint is your culprit.
Is the 12 V Wire Voltage Drop Calculator free and private?+
Yes — completely free with no sign-up or usage limits, and it runs entirely in your browser: the values you enter are never uploaded or stored on a server.
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