Why low-voltage cables need to be thick
Voltage drop is the current times the resistance of the whole loop, out and back. At 230 V, losing a volt doesn't matter. At 12 V and 20 A, a volt is 8% of your supply, and the 100 W radio that needs 13.8 V may only get 12.8 V when you transmit, which can cause distorted audio, lower power and even resets. Every metre of cable counts twice, because current flows down the positive and back along the negative.
How much drop is acceptable
- Radio transceivers: under 3%, and less for 100 W HF rigs, which are specified at 13.8 V ±15%. Run the shortest, thickest cable you can straight to the battery or supply.
- Lighting and general loads: 3 to 5% is fine.
- Solar panels to the charge controller: aim for 1 to 3%, because the loss is wasted energy all day.
Current rating and fusing
A cable also has a maximum current before it overheats, which depends on its insulation and how it's run. The cable that gives a low enough voltage drop over a long run is usually thicker than the current rating demands, but always check both. Fit a fuse in the positive lead close to the battery, rated to protect the cable. The calculation uses the resistivity of copper, corrected for temperature. For wire diameter and SWG conversions, see the wire gauge converter, and for battery life, the battery runtime calculator.
Questions
What size cable do I need for a 100 W radio?
A 100 W HF radio draws about 20 to 22 A. For a 3 m run, 6 mm², or 8 AWG, keeps the drop under 3%. Longer runs need 10 mm² or more.
Is the length one way or both ways?
Enter the one-way distance. The calculator doubles it for the return wire.
What is an acceptable voltage drop on 12 V?
Usually 3% or less, which is about 0.4 V. For transceivers and anything sensitive, aim lower.
Does temperature matter?
Copper's resistance rises about 0.4% per °C, so a hot engine bay or a cable in the sun has a little more drop.