Glossary

Voltage drop

Unit V · Also Voltage loss

What is Voltage drop?

The loss of voltage across a cable and its connections. It only occurs when current flows, and on board it is usually the value that determines the required cable cross-section — not heating.

U = (2 × L × I × 0.0175) ÷ A

What matters about Voltage drop in practice?

The calculation uses twice the cable length, because the current flows there and back. Three percent of the nominal voltage is the planning value — at 12 volts that is 0.36 volts; ten percent is an upper limit for non-critical loads, not a target.

The formula only describes the cable. Contact resistance at terminals, busbars and main switches is not included and can double the measured value. That is why every design needs a check measurement under load.

How do you calculate voltage drop?

Voltage drop = 2 × cable length × current ÷ (56 × cross-section). The 2 stands for the positive and return conductor, 56 is the conductivity of copper in m/(Ω·mm²). Example: 10 m one-way length, 20 A and 6 mm² give 1.19 V, almost 10 percent at 12 V. With 16 mm² it is 0.45 V, with 25 mm² 0.29 V and thus below three percent.

How do you notice too much voltage drop?

  • Pumps and fans run slower, LED lights flicker.
  • Chargers and DC-DC chargers see the wrong battery voltage, and the battery never gets full.
  • Cables and terminals get warm under load.

How can it be reduced?

Increase the cross-section, keep cables short, crimp connections properly and avoid contact resistance at terminals and switches. The cable configurator calculates the right cross-section for length and current.

Related terms

All terms from A to Z

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