Supply Cable Cross-Section
The cross-sectional area of the current-carrying conductor in mm². In 12V systems the real criterion determining the choice of cross-section is not heating but voltage drop. A 3% limit leaves only 0.36 V of margin at 12V. For a 10 A load and a 3 m one-way distance a 4 mm² cross-section is recommended; 2.5 mm² only stays within the limit up to about 2.5 m at this load.
The logic of the calculation is simple: the voltage lost is the current multiplied by the cable resistance, and the whole out-and-back path counts in the resistance calculation. That is, for a distance of 3 metres, 6 metres of conductor enter the calculation.
- Temperature: a temperature of 90–105 °C in the engine area increases the resistance of copper by 27–33%; a cross-section chosen at the limit becomes insufficient when hot.
- Distance: when the length doubles, the voltage drop also doubles. On a long run, increasing the cross-section is the only solution.
- Quality: use automotive-type cable with many fine strands; single-core rigid cable breaks under vibration.
You see the result in the field like this: on an undersized run the motor turns slowly, the illuminated switch goes dim and the system becomes unstable under load. These are often reported as "the kit is faulty"; yet the difference between the two voltages measured at the battery terminals and at the device input shows the truth immediately.
Context in which this term is used: Converting a tractor hydraulic lever to electronic control