12V Wire Gauge Guide: AWG to mm2 for 4x4 and Caravans

Getting the 12V wire gauge right is the difference between a fridge that keeps running on day three of a trip and one that cuts out every time the compressor kicks in. Undersized cable gets warm, drops voltage and starves whatever is on the end of it. Oversized cable costs more and is harder to route, but it never causes a fault.

The trouble is that wire size is quoted three different ways in Australia: AWG (American Wire Gauge), B&S (the older name for the same scale) and mm2 (cross-sectional area). On top of that, local "auto cable" is sold under labels like 3mm and 6mm that are not the copper area at all. This guide puts all of it in one place for 12V and 24V work in a 4x4, ute, caravan or boat.

You will find an AWG to mm2 chart, the Australian auto cable equivalents, which Solderstick connector colour suits each wire, and a simple way to pick a size from the current and the length of the run.

Quick answer:

• Lower AWG numbers are thicker wire. B&S sizes are the same scale as AWG, so 8 B&S is 8 AWG, about 8.4mm2.
• Pick the wire gauge from two checks: the current it must carry, and the voltage drop over the full run (positive and negative). Use whichever check gives the thicker cable.
• Read the mm2 figure on auto cable, not the label. Australian "3mm" auto cable is about 1.13mm2 of copper and "6mm" is about 4.6mm2.
• Solderstick connectors: white for 26-24 AWG, red for 22-18 AWG, blue for 16-14 AWG, yellow for 12-10 AWG. Anything heavier than 6mm2 gets crimped or soldered lugs.
• Winches and battery cables need far heavier cable than any splice connector covers. Follow the winch maker's cable spec.

What wire gauge means: AWG, B&S and mm2

AWG is a numbered scale where each step down in number gives a thicker wire. Every three steps roughly doubles the copper area, so 10 AWG has about twice the copper of 13 AWG and four times the copper of 16 AWG.

B&S stands for Brown and Sharpe, the company that set up the scale. In Australia you will see heavy cable sold as 8 B&S, 6 B&S, 4 B&S and 2 B&S. These are the same numbers as AWG, so 6 B&S and 6 AWG describe the same nominal size.

mm2 is the metric measure: the cross-sectional area of the copper in square millimetres. It is the number that actually decides how much current a cable can carry and how much voltage it loses. When two labels disagree, trust the mm2 figure on the spec sheet or the cable printing.

One more thing to keep straight: mm (diameter) and mm2 (area) are different measurements. A cable that is 3mm across its insulation might only have 1mm2 of copper inside it.

AWG to mm2 chart for 12V wiring

The table below gives the true copper area for each AWG size, calculated from the standard AWG formula, with the nearest Australian auto cable label and the Solderstick connector that matches the AWG range. Real stranded cable varies a little from these figures depending on the maker and the strand count.

The auto cable figures come from Australian makers' published data and are typical rather than exact. Two brands' 8 B&S, for example, may differ by a few tenths of a square millimetre, and both can sit a little under the true 8 AWG area.

Australian auto cable names: why 3mm is not 3mm2

Australian auto cable has long been sold by an old size label: 2mm, 3mm, 4mm, 5mm, 6mm. That number is not the copper area. It is closer to an overall size name that stuck. A 3mm single core auto cable from a major Australian maker is about 1.13mm2 of copper (14 strands of 0.32mm), and a 6mm auto cable is about 4.59mm2.

This is where a lot of 12V wire gauge mistakes start. Someone reads "6mm" on a roll, assumes it is the same as a 6mm2 cable from an overseas spec sheet, and ends up with about three quarters of the copper they planned for. If an installation manual asks for 6mm2, a 6mm auto cable is not enough.

Our separate guide to auto cable sizes in Australia lists the common labels with their real mm2 and typical ratings. The short version: always check the mm2 printed on the cable or the product page.

Which Solderstick connector suits each wire gauge

Solderstick connectors are sized by colour. Each one is a clear heat shrink tube with a low-temperature solder ring in the middle and coloured sealing rings at each end. Pick the colour that matches the wire you are joining.

Two common Australian sizes sit between ranges on paper: 3mm auto cable (about 1.13mm2) falls between red and blue, and 5mm auto cable (about 2.9mm2) falls between blue and yellow. Before you heat one of these, do a dry fit. The stripped ends should sit under the solder ring with the sealing rings over the insulation on both sides. If you are unsure, get in touch before ordering and we will help you pick.

For battery leads, winch cables and anything heavier than 6mm2, use crimped or soldered lugs instead of a splice connector. A connector seals and joins a wire; it does not make an undersized cable safe. For a full walk-through of the colours, see the Solderstick colour chart.

How to choose 12V wire gauge by current

Start with the current. Find the amps on the device label or in its manual. If only watts are given, divide by the voltage: a 120W light bar on 12V draws about 10A.

Next, look up the cable's current rating from its maker. Ratings depend on the insulation, the air temperature and whether the cable is bundled with others, so they are a ceiling, not a target. As an example, one Australian maker rates its single core auto cable at 30 degrees C ambient as follows:

Those ratings drop for twin core cable, for bundled looms and in a hot engine bay, so leave a margin. Then fit a fuse that is above the device's normal draw and at or below the cable's rating. The fuse protects the cable, so it goes as close to the power source as practical.

Current alone is only half the job. On anything longer than a metre or two, the voltage drop check usually asks for a thicker 12V wire gauge than the current rating does.

Run length and voltage drop: the second wire gauge check

Every cable has resistance, and current through resistance costs voltage. On a 12V system a small loss is a big percentage: losing 1.2V is 10 percent of your supply. Fridges cut out early, DC-DC chargers drop their output, and lights dim.

A widely used marine guideline (from the American Boat and Yacht Council) allows 3 percent drop for critical circuits and up to 10 percent for non-critical ones. On 12V, 3 percent is 0.36V. It is a sound target for chargers, fridges and anything that cuts out on low voltage.

The formula

Voltage drop = 2 x one-way length (m) x current (A) x 0.0172 / area (mm2)

The 2 accounts for the return path, and 0.0172 is the resistance of copper in ohms per metre for a 1mm2 conductor. If the negative returns through the chassis, the real drop is a little lower, but counting the full loop gives a safe answer.

Worked example

A 10A load 5 metres from the battery: on 2.5mm2 cable the drop is about 0.69V (5.7 percent). On 4mm2 it is about 0.43V (3.6 percent). On 6mm2 it is about 0.29V (2.4 percent). For a fridge, 6mm2 is the right call; for a light that is not sensitive to voltage, 4mm2 would do.

Maximum one-way run for 3 percent drop at 12V

For a non-critical load where 10 percent is acceptable, you can roughly triple these lengths. On a 24V system the same current gives half the percentage drop, which is one reason trucks run 24V.

Typical 4x4 circuits and the wire gauge they need

Every vehicle is different, so use these as starting points and run the numbers for your own load and length.

Signal and trigger wires

Relay coil triggers, switch backlights and sensor leads carry a fraction of an amp. 0.5-1.0mm2 is plenty, with red Solderstick connectors for the joins.

Lights, USB sockets and radios

Most single lights, USB outlets and UHF radios draw a few amps. Over a short run, 1.5-2.5mm2 handles them, joined with blue connectors. Check the radio's manual, as some specify a minimum supply cable.

Fridges and accessory feeds

Compressor fridges are voltage sensitive and often sit well back from the battery. A 4-6mm2 feed with yellow connectors is common, and a long run to a canopy may need 8 B&S, which is a lug job.

Light bars and driving lights

A light bar runs through a relay. The heavy feed from battery to relay to light is usually 2.5-6mm2 depending on the draw and length. Our step-by-step on how to wire a light bar on a 4x4 covers the relay, fuse and high beam trigger.

DC-DC chargers and dual batteries

A 25A DC-DC charger needs heavy cable: 8 B&S is common for short runs and 6 B&S for longer runs to a canopy or tray. The charger's install manual has the final say. Our guide to dual battery wiring cable sizes goes through it in detail.

Winches, battery cables and anything over 6mm2

The old version of this article said 10-12 AWG suits a winch. That was the wrong wire gauge by a long way. An electric winch under load draws hundreds of amps: published specs for 9,500lb-class 12V winches list around 400A at full rated pull. 10-12 AWG cable is a fraction of what it needs.

Winches are normally supplied with their own heavy power and earth leads, commonly around 2 AWG (about 33mm2) or heavier, and the maker specifies the size and maximum length. Use what the manual calls for. If you need a longer run, use the manufacturer's extension kit or cable of the same size or heavier, with properly crimped lugs and a suitable isolator or circuit breaker.

The same goes for starter, battery and inverter cables. Anything heavier than 6mm2 is outside the range of splice connectors: terminate it with crimped or soldered lugs, then seal the lug barrel with adhesive-lined heat shrink. Our post on 4x4 winching in Australia covers safe winch use on the track.

Which should you use?

Match the 12V wire gauge to the load and the run first, then match the connector to the wire.

• If you're joining sensor, data or small LED wires (26-24 AWG), use white connectors.
• If you're wiring relay triggers, switch lights or trailer lamp tails (0.5-1.0mm2), use red connectors.
• If you're running lights, USB sockets or a radio over a short run (1.5-2.5mm2), use blue connectors.
• If you're feeding a fridge, light bar or accessory panel (4-6mm2), use yellow connectors and check voltage drop first.
• If you're using 3mm or 5mm auto cable, dry fit the two neighbouring colours before you heat.
• If you're wiring a winch, starter, battery link or inverter, use the cable size the manual specifies with crimped lugs, not splice connectors.

Wiring tips that make the right gauge work

• Use stranded automotive cable. Solid core household cable does not cope with vibration, and 230/240V mains cable has no place in a 12V loom.
• Fuse every positive feed as close to the battery as practical, rated to protect the cable.
• Disconnect the battery negative before you start, and keep it off until every join is finished.
• Run a dedicated negative back to the battery or a solid earth point on long runs instead of relying on a rusty body earth.
• Protect cable from heat, sharp edges and chafe with conduit, grommets and P-clips.
• Seal every join. In mud, dust and water crossings, an unsealed join is where corrosion starts. IP67-rated connectors keep out dust and handle temporary immersion.
• Use a heat gun to shrink Solderstick connectors. A lighter can scorch the tube and is a last resort. Let the join cool before you move it.

Solderstick connectors are for low-voltage work only: 12V and 24V automotive, marine, caravan, solar DC and electronics. Never use them on 230/240V mains wiring. In Australia, fixed mains wiring must be done by a licensed electrician.

Frequently asked questions

Is B&S the same as AWG?

Yes. B&S (Brown and Sharpe) is the original name of the AWG scale, so 8 B&S and 8 AWG are the same nominal size. Real cable from different makers can vary slightly in copper area, so check the mm2 figure.

What size is 6mm auto cable in mm2?

Australian 6mm auto cable is about 4.6mm2 of copper, roughly 11 AWG. It is not the same as 6mm2 cable, which has about 30 percent more copper.

What wire gauge do I need for 12V lights?

A single light or small light bar over a short run usually needs 1.5-2.5mm2. For longer runs or bigger light bars, work out the current and use the voltage drop table above, which often points to 4mm2 or more.

Is 10 AWG enough for a winch?

No. Winches draw hundreds of amps under load and are usually supplied with leads around 2 AWG or heavier. Use the size and length in the winch manual and terminate with crimped lugs.

What gauge wire for a 12V fridge?

It depends on the run length. Fridges are sensitive to low voltage, so size for 3 percent drop or less: a 5A draw over 6 metres needs about 4mm2, and longer runs to a canopy may need 6mm2 or 8 B&S.

Can I use thicker wire than I need?

Yes. Thicker cable only reduces voltage drop and heat. The limits are cost, weight, routing and whether your terminals and connectors fit it.

Which Solderstick colour fits 2.5mm2 wire?

Blue connectors suit 16-14 AWG, listed as 1.5-2.5mm2. For 4-6mm2 wire, use yellow.

What to use

For a 4x4 or caravan loom, an assorted kit covers every colour and every wire gauge from 26 to 10 AWG. The 100PCS kit (25 white, 35 red, 30 blue, 10 yellow) suits most installs, the 50PCS kit is a good glovebox spare, and the 250PCS kit or 500PCS kit suit full rewires and workshops.

If you go through one size faster than the others, the bulk 100-pack bags come in a single colour. For lugs and heavy cable, our adhesive-lined heat shrink includes 4:1 dual wall and 6:1 heavy wall sizes. Orders placed by 1pm ship the same day with free express shipping Australia wide.