Voltage drop calculator for 12 V and 24 V vehicle wiring
Every cable has resistance. The longer and thinner it is, and the more current it carries, the more voltage is lost before the load. On a 12 V system a few tenths of a volt can stop a fuel pump, a fan or a starter from working properly.
- Cable resistance
- 34.4 mΩ
- Voltage drop
- 0.34 V (2.9 %)
- Heat in the cable
- 3.4 W
- Smallest cable for ≤ 3 %
- 1.5 mm²
How to use it in a diagnosis
- Measure the cable length from the source to the load (one way); the calculator counts the return path too unless the return is through the body.
- Enter the cross-section in mm² and the current the load draws.
- Aim for under 3 % drop on most circuits and under 0.5 V on starter and charging cables.
Questions mechanics ask
What voltage drop is acceptable in car wiring?
A common guide is under 3 % (about 0.36 V at 12 V) for general circuits, and under about 0.5 V on the starter cable while cranking. Grounds should usually drop less than 0.1–0.2 V.
How is voltage drop calculated?
Voltage drop = current × resistance. The resistance of a copper cable is about 0.0172 Ω × length (m) ÷ cross-section (mm²) at 20 °C, and rises with temperature.
How do I measure voltage drop on a real car?
With the circuit working and under load, put one meter lead at each end of the cable or connection (for example battery post to starter terminal). The meter shows the voltage lost along that path.