What Size Twin Core Cable Do I Need? OEX Size Guide

The Short Answer: Which Twin Core Cable Size Do You Need?

As a general starting point:

However, cable size cannot be selected from the accessory type alone. A long run to the rear of a caravan may require larger cable than the same load installed close to the battery.

For the OEX red-and-black sheathed twin core cable discussed in this guide, the advertised ratings at 30°C are:

These ratings are not universal recommendations. Actual suitability depends on cable length, ambient temperature, bundling, installation method, terminals, voltage drop and the connected equipment's requirements.

What Is Twin Core Automotive Cable?

Twin core automotive cable contains two insulated conductors inside a protective outer sheath. In common red-and-black cable, the red conductor is typically used for positive and the black conductor for negative or earth.

Keeping both conductors together provides a tidy wiring run and gives the circuit a dedicated return path. The outer sheath also adds protection against abrasion and the conditions commonly encountered in automotive installations.

Twin core cable is often used for:

• 12V lighting
• Trailer and caravan wiring
• Pumps and small motors
• Fans
• Sockets and accessory outlets
• Brake-control and auxiliary circuits
• Monitoring equipment
• Low-current 4WD accessories

It should not be assumed that one twin core cable size suits every product in these categories. Always check the actual current draw and installation length.

Understanding 3mm, 4mm and 6mm Automotive Cable

One of the most confusing parts of buying automotive cable in Australia is the size description.

Automotive cable may be sold using a nominal or trade size such as 3mm, 4mm or 6mm. That label should not automatically be treated as the conductor's cross-sectional area in square millimetres. It may also differ from American Wire Gauge, commonly written as AWG.

When comparing products, check the manufacturer's specifications for:

• Conductor cross-sectional area
• Number and diameter of conductor strands
• Advertised current rating
• Insulation and temperature rating
• Cable length
• Overall dimensions

Do not select cable solely because another brand uses the same size in its product name. Compare the actual specifications.

OEX 3mm Twin Core Automotive Cable

OEX 4mm Twin Core Automotive Cable

OEX 6mm Twin Core Automotive Cable

3mm vs 4mm vs 6mm Twin Core Cable

This comparison is a selection guide, not permission to load each cable continuously to its advertised rating. Heat, cable bundling and other installation factors may reduce safe current capacity.

How to Calculate Your Accessory's Current Draw

If the accessory lists watts rather than amps, estimate its current using:

For example, a 60W accessory operating at 12V has a theoretical current draw of:

This calculation is only a starting point. Real current draw can change with battery voltage, product efficiency and operating conditions. Motors, compressors and pumps may also draw significantly more current during startup.

Use the equipment manufacturer's maximum-current figure and recommended cable and fuse sizes whenever they are available.

Cable Length and Voltage Drop

Current capacity is only one part of cable selection. You also need to consider voltage drop.

Every cable has resistance. As current flows through it, some voltage is lost before it reaches the accessory. The loss increases as:

In a 12V system, even a relatively small loss can affect lighting output, pump operation, charging performance and other voltage-sensitive equipment.

When measuring a cable run, calculate the complete circuit length: from the power source to the accessory and back to the source. For example, an accessory mounted five metres from the battery may have approximately ten metres of conductor in the complete positive-and-negative circuit.

If the voltage drop is too high, move up to a larger cable size—even when the smaller cable can technically carry the current.

A Practical Cable-Sizing Example

Imagine you are installing a 12V accessory that draws 8A and will be mounted several metres from the battery.

The OEX 3mm cable has an advertised 10A rating at 30°C, but that alone does not confirm it is the right choice. You must also consider:

After accounting for those factors, 4mm or 6mm cable may be more suitable. The final choice should be based on the equipment manufacturer's requirements and a proper voltage-drop calculation.

Why Choose OEX Twin Core Cable?

OEX twin core automotive cable is designed for demanding vehicle wiring applications. Depending on the specific product, key features include:

Fine-strand construction makes the cable easier to route through tight vehicle and engine-bay spaces, while the sheath helps protect both conductors along the run.

Always check the individual product listing because specifications, lengths and packaging may vary between OEX cable sizes.

Choosing the Correct Fuse or Circuit Breaker

Every positive supply cable connected to a battery requires suitable circuit protection unless the equipment manufacturer specifies a different arrangement.

The fuse or circuit breaker should generally be mounted as close as practical to the power source so that only a minimal length of positive cable remains unprotected.

The protection device must be large enough to avoid opening during normal operation but small enough to protect the cable under the actual installation conditions. Never install a larger fuse merely because the original one keeps blowing; identify and fix the cause first.

Common Twin Core Cable Sizing Mistakes

Frequently Asked Questions

Final Checklist: Choosing OEX Twin Core Cable

Before ordering your cable, confirm:

For small loads and short runs, OEX 3mm twin core cable may be suitable. The 4mm option offers additional capacity for moderate-current circuits, while 6mm twin core cable provides greater capacity and lower voltage drop for more demanding or longer runs.