12V cable size and voltage drop calculator

At 12 V, small losses matter. Half a volt lost in the cable is about 4% of your system voltage, and a DC-DC charger or fridge may cut out or under-perform if its input voltage sags too far.

How it works

Voltage drop = current × resistance. Resistance = ρ × (2 × length) ÷ area, where ρ for copper is about 0.0172 Ω·mm²/m at 20 °C.^[1] The length is doubled because current flows out on the positive and back on the negative. If your return path is the chassis, the real resistance depends on the earth connections, so this calculator is conservative.

Watch out for cable size labels

Automotive cable in Australia is often sold by "B&S" or "AWG" gauge, or by a label like "6 mm", and these don't always match the actual copper cross-section in mm². Before buying, check the seller's stated copper area in mm² and enter that figure here.

Other limits besides voltage drop

• Current rating: a cable can have acceptable voltage drop over a short run but still be too small to carry the current safely. Check the cable maker's current rating.
• Manufacturer tables: charger makers publish recommended cable sizes and fuses for each model and length. The Enerdrive DC2DC+ manual has a table, for example.^[2] Follow the manual for your charger.
• Temperature: copper resistance rises with temperature, and engine bays get hot.

Sources

1. Wikipedia: Electrical resistivity and conductivity (copper ≈ 1.72 × 10⁻⁸ Ω·m at 20 °C)
2. Enerdrive ePOWER DC2DC+ user manual (PDF)

This is general information, not a substitute for the manuals for your specific charger, battery and vehicle. Fixed wiring and anything near the starter battery should be installed by a qualified auto electrician.

Last updated: 15 October 2026