How the cable size is chosen
The calculator then picks the smallest standard size that is both above this area and has a guide current rating above your current. On short, high-current runs (like inverter cables) the current rating usually decides; on long, low-current runs (like lighting or solar cable) voltage drop decides.
Worked example
A 30 A DC-DC charger, 3 m from the starter battery, 3% maximum drop at 12 V: 2 × 3 × 30 × 0.01724 ÷ 0.36 = 8.6 mm², so 10 mm² (or 6 AWG).
Common sizes at 12 V, 3% drop (standard PVC cable)
| Current | 1 m (3 ft) | 3 m (10 ft) | 5 m (16 ft) |
|---|---|---|---|
| 10 A | 1 mm² | 4 mm² | 6 mm² |
| 20 A | 4 mm² | 6 mm² | 10 mm² |
| 50 A | 16 mm² | 16 mm² | 25 mm² |
| 100 A | 35 mm² | 35 mm² | 50 mm² |
| 200 A | 120 mm² | 120 mm² | 120 mm² |
On short, high-current runs the conservative PVC current rating decides the size. With 105 °C cable run on its own, those sizes can usually drop by one or two steps; choose that cable type above to see.
Current ratings are guide figures for single-core copper cable. 'Standard PVC' uses conservative values for 70 °C insulation, bundled or enclosed; '105 °C' uses typical marine-table values for high-temperature cable run on its own. Always check the cable maker's rating, and derate for hot spaces such as engine bays.
Good practice
- Use fine-stranded (flexible) copper cable in vehicles and boats; solid core can crack with vibration.
- Use proper crimp lugs sized to the cable, and heat-shrink them.
- Size the cable to the fuse, not just the load, so the fuse protects the cable.
Frequently asked questions
What size cable for a 2000W inverter at 12V?
It draws about 190 A at full load. On a short run (1–1.5 m) with 105 °C cable, about 35–50 mm² (2 to 1/0 AWG); with standard PVC cable, considerably larger. The inverter manual usually states a minimum, so use whichever is bigger.
What size cable from solar panel to controller?
Solar current is low, so voltage drop decides. A 10 A string over 10 m at 40 V needs about 2.5–4 mm² for under 3% drop; 4 mm² and 6 mm² solar cable are standard.
Is mm² the same as AWG?
No. AWG numbers get smaller as cable gets thicker. 10 mm² is close to 8 AWG (8.4 mm²) and 6 AWG (13.3 mm²) sits between 10 and 16 mm². Use the AWG to mm² converter.