Quick answer
Cable size depends on two things: the current the accessory draws and the total length of the cable run (there and back). At 12V, even a small voltage loss matters, so long runs need much thicker cable than you’d expect. Always compare cable by its copper area in mm², not its “mm” label: a cable sold as “6mm auto cable” has roughly 4.6 mm² of copper, not 6 mm². Then fuse the cable close to the battery so the fuse protects the cable.
Is this a DIY job?
- Difficulty: Medium
- Time: 15 minutes to work out
- Tools: accessory’s current rating (from its label or manual), tape measure, the table below
- Skills: simple maths
Step by step
- Find the current. Check the accessory’s label or manual for amps (A). If it only gives watts, divide by 12 for a rough figure (for example, 60 W ÷ 12 V = 5 A).
- Measure the run. Measure the one-way distance from the battery (or fuse block) to the accessory, along the route the cable will actually take.
- Decide the acceptable voltage drop. Many installers aim to lose no more than about 3% (around 0.36 V on a 12V system). Charging circuits such as DC-DC chargers and fridges often perform better with less: check the maker’s instructions, which usually specify a cable size.
- Look up the table below. Find your current and pick a cable whose maximum length is longer than your one-way run.
- Check the cable’s current rating. The table only covers voltage drop. The cable must also be rated for the current: use the ratings table below or the maker’s figure, and reduce it for heat and bundling.
- Choose the fuse. Fit a fuse or circuit breaker close to the battery, rated to protect the cable (at or below the cable’s rating) and suitable for the accessory. (see Where should the fuse go when wiring an accessory?)
- Use twin-core or a proper return. The negative needs the same size as the positive if it runs back to the battery. A thin earth wire undoes a thick positive cable.
Specs table
Auto cable labels, copper area and current rating
The “mm” in an auto cable’s name is roughly its outside size, not its copper. What carries the current is the conductor cross-section in mm². Tycab, a major Australian cable maker, publishes these figures for its single-core auto cable:
| Sold as | Stranding | Copper area | Tycab current rating at 30°C |
|---|---|---|---|
| 3mm auto cable | 14/0.32 | 1.13 mm² | 20 A |
| 4mm auto cable | 23/0.32 | 1.84 mm² | 28 A |
| 5mm auto cable | 36/0.32 | 2.90 mm² | 37 A |
| 6mm auto cable | 57/0.32 | 4.59 mm² | 48 A |
Heavier cable (B&S)
B&S is the Australian name for the American wire gauge (AWG) sizes, but Australian battery cable is often made a little lighter than the true AWG area. Compare mm², not the gauge number.
| Size | True AWG area | Tycab battery cable area | Tycab current rating at 30°C |
|---|---|---|---|
| 8 B&S | 8.37 mm² | 7.71 mm² | 74 A |
| 6 B&S | 13.3 mm² | 13.5 mm² | 103 A |
| 4 B&S | 21.2 mm² | 20.26 mm² | 135 A |
| 2 B&S | 33.6 mm² | 32.07 mm² | 188 A |
| 0 B&S | 53.5 mm² | 49.0 mm² | 246 A |
Ratings drop with heat and bundling. Tycab’s ratings are for a single cable at 30°C. In hotter places, multiply by Tycab’s derating factor: 0.9 at 40°C, 0.76 at 50°C, 0.60 at 60°C and 0.45 at 70°C. A 6mm cable rated at 48 A carries only about 29 A in a 60°C engine bay. Cables bundled together in a loom lose more again. Twin-core cable is rated lower than single-core of the same size for the same reason; check the rating for the exact cable you buy.
Maximum one-way run for about 3% voltage drop at 12V (copper, 20°C)
| Cable | 5 A | 10 A | 20 A | 30 A | 50 A | 100 A |
|---|---|---|---|---|---|---|
| 3mm auto (1.13 mm²) | 2.4 m | 1.2 m | too small | too small | too small | too small |
| 4mm auto (1.84 mm²) | 3.9 m | 1.9 m | 1.0 m | too small | too small | too small |
| 5mm auto (2.90 mm²) | 6.1 m | 3.0 m | 1.5 m | too small | too small | too small |
| 6mm auto (4.59 mm²) | 9.6 m | 4.8 m | 2.4 m | 1.6 m | too small | too small |
| 8 B&S (8.37 mm²) | 17.5 m | 8.8 m | 4.4 m | 2.9 m | 1.8 m | too small |
| 6 B&S (13.3 mm²) | 27.8 m | 13.9 m | 7.0 m | 4.6 m | 2.8 m | too small |
| 4 B&S (21.2 mm²) | – | 22.2 m | 11.1 m | 7.4 m | 4.4 m | 2.2 m |
| 2 B&S (33.6 mm²) | – | – | 17.6 m | 11.7 m | 7.0 m | 3.5 m |
| 0 B&S (53.5 mm²) | – | – | 28.0 m | 18.7 m | 11.2 m | 5.6 m |
How the table is worked out: voltage drop = current × resistance of the full loop (out and back). Copper resistance is about 0.0172 ohm per metre for 1 mm² at 20°C. Real cable gets warmer and loses a little more. B&S rows use the true AWG area; Tycab’s slightly lighter battery cable gives runs up to about 8% shorter at 8 B&S and 0 B&S. “Too small” means the current is at or above the cable’s rating, or leaves no margin for heat (for example 3mm cable rated at 20 A carrying 20 A, 5mm rated at 37 A carrying 30 A in a warm engine bay, or 6 B&S rated at 103 A carrying 100 A). “–” means the cable is far bigger than needed.
What you’ll need
- twin core auto cable (size from the table)
- heavy duty single core auto cable (for high-current runs)
- cable lugs and crimp terminals
- inline fuse holder / circuit breaker / MIDI fuse holder
- split loom tube and firewall grommet
When to call an auto electrician
- The run is long and the current is high (DC-DC chargers, inverters, winches, caravan charging).
- You’re not sure what the accessory draws, or it has high start-up current (compressors, fridges, inverters).
- Cable or terminals are getting hot, or accessories cut out under load.
Need an auto electrician? Find local auto electricians and mobile services in your city.
Related questions
Q: Is “6mm” cable 6 mm²? A: No. In Australian auto cable, “6mm” usually describes the outside diameter including insulation. The copper area is about 4.6 mm². Always compare mm².
Q: Why does my fridge cut out when I’m driving or camping? A: Often because the cable is too thin for the length, so the voltage at the fridge drops below its cut-out setting. Thicker cable or a shorter run usually fixes it.
Q: Can I use thicker cable than I need? A: Yes. Thicker cable has less voltage drop and runs cooler. It costs more and is harder to route, but there’s no electrical downside.
Q: Does the negative cable need to be as thick as the positive? A: Yes, if it carries the same current back to the battery. The voltage drop happens on both sides of the circuit.
Sources
- Solar4RVs, “Understanding cable size types: mm, mm², B&S, AWG” – auto cable “mm” is outside diameter; 3mm/4mm/6mm copper areas; B&S to mm² conversions; heat derating note (AS/NZS 3008). https://www.solar4rvs.com.au/buying/buyer-guides/understanding-cable-size-types-mm-mm2-b_s-awg/ (checked October 2026)
- Tycab, automotive cable catalogue – single-core auto cable (ASC) and battery cable (ABC) stranding, mm², current ratings at 30°C and temperature derating factors. https://www.tyree.nz/wp-content/uploads/2019/09/Automotive-Comp.pdf (checked October 2026)
- HS Auto Parts, “Tycab cable – how to select the right gauge” – Tycab battery cable B&S sizes, mm² and rating guide. https://hsautoparts.com.au/tycab-cable-selection-guide/ (checked October 2026)
- Technical review, Ozautoelectrics (October 2026) – compare cable by conductor cross-section in mm²; “3mm” auto cable is 1.13 mm².
- Resistivity of copper (about 1.72 × 10⁻⁸ ohm-metre at 20°C), standard physical constant used to calculate the voltage-drop table.