Battery Cable Size Guide for Automotive Wiring

Common Battery Cable Sizes

The table below is a practical guide for common automotive battery cable sizes. Cable brands and strand counts can vary, so always check the actual conductor area, insulation rating and manufacturer data before ordering.

How to Choose Battery Cable Size

To choose the correct cable size, work through the circuit properly instead of guessing from the accessory or starter motor alone.

1. Work out the current draw

Find the expected current draw of the circuit. For example, a PDM feed, fuel pump, thermo fan, alternator charge cable and starter motor cable all have very different load profiles.

2. Measure the cable length

Long cable runs need larger cable. A short engine bay cable may work with a smaller size, while the same current draw from a boot-mounted battery may need a much larger cable.

3. Include the return path

The circuit needs a complete path back to the battery. If the vehicle uses chassis earth, the engine earth strap, battery earth and body connections must be large enough and in good condition.

4. Allow for voltage drop

Voltage drop is a major issue in 12 volt vehicles. A cable that technically carries the current can still be too small if the voltage drop is excessive. Starter motors, ECUs, PDMs, fuel pumps and ignition systems all benefit from a stable voltage supply.

5. Fuse the cable correctly

The fuse or circuit protection must protect the cable. Do not choose a fuse only from the device current draw. The cable size, insulation, route and fault risk matter.

Battery Relocation Cable Size

Battery relocation usually needs larger cable than people expect. Moving the battery to the boot or rear of the vehicle increases cable length, which increases voltage drop.

For many performance and motorsport vehicles, 2 B&S, 1 B&S, 0 B&S or 00 B&S may be required depending on engine size, starter load, compression, cable route and distance.

Do not size a boot-mounted battery cable the same way you would size a short engine bay battery cable. The longer the run, the more important cable size becomes.

Starter Motor Cable Size

Starter motors draw high current for a short period. The cable must be large enough to supply the starter without excessive voltage drop during cranking.

Symptoms of undersized starter cable can include slow cranking, hot cables, voltage drop at the starter, poor hot-start performance and unreliable starting even with a good battery.

Starter cable tips:

• Keep the starter cable as short as practical.
• Use large cable for boot-mounted batteries.
• Make sure the engine block has a proper earth strap back to the chassis and battery.
• Protect the cable from sharp edges, heat and moving parts.
• Use quality lugs and correct crimp tooling.

Alternator Charge Cable Size

The alternator charge cable must be sized for the alternator output and cable length. Higher-output alternators need larger cable, especially if the alternator charge cable runs to a remote battery, kill switch, junction post or rear-mounted battery.

Voltage drop in the alternator charge cable can affect charging performance. The battery may not see the same voltage that the alternator is producing if the cable is too small or the connections are poor.

PDM Main Feed Cable Size

A PDM main feed should be sized for the total expected load and future expansion. Even if each output is protected electronically, the main feed and main protection still need to be sized correctly.

For a motorsport PDM setup, the main feed cable should be matched to the PDM current rating, cable length, fuse or breaker size, battery location and the expected combined current draw of the vehicle.

Positive Cable and Earth Cable

The earth side of the circuit is just as important as the positive side. A large positive cable with a poor or undersized earth path will still cause voltage drop and electrical issues.

For battery relocation, the main earth cable and engine earth straps should be treated as part of the main power system, not an afterthought.

Good earth practice:

• Use a proper battery earth cable.
• Use a proper engine-to-chassis earth strap.
• Clean paint, powder coat and corrosion from earth points.
• Use quality lugs, washers and secure fasteners.
• Check voltage drop under load, not only continuity with a multimeter.

Fuse and Circuit Protection

For main power feeds, the fuse or circuit protection should normally be mounted as close to the battery or power source as practical. This helps protect the cable if it shorts to chassis.

Starter motor cables are often treated differently from normal fused accessory feeds, depending on the vehicle and motorsport rules. If a cable is not fused, routing and protection become even more important.

The fuse protects the cable. The relay, PDM output or device protection does not automatically protect the main battery cable feeding it.

Common Mistakes

• Choosing battery cable size only by current draw and ignoring cable length.
• Using cable that is too small for a boot-mounted battery.
• Using a large positive cable but leaving the earth cable or engine strap undersized.
• Not fusing main feeds close to the battery.
• Running cable near exhaust, sharp edges or moving parts without protection.
• Using poor-quality lugs or hammer crimping large terminals.
• Forgetting voltage drop during cranking or under real load.

Practical Selection Tips

• For short engine bay starter runs, 4 B&S may be suitable depending on the vehicle.
• For battery relocation, start looking at 2 B&S, 1 B&S, 0 B&S or larger depending on length and load.
• For alternator charge cables, size the cable for alternator output and voltage drop, not only the original factory cable size.
• For PDM main feeds, size the cable and fuse for the total load, not just one output.
• Use the same level of care on earth cables as positive cables.
• Use proper crimp lugs, heatshrink, strain relief and abrasion protection.

FAQ

What size cable should I use for battery relocation?

Battery relocation usually needs larger cable than a short engine bay battery cable. Depending on the vehicle, cable length and starter load, 2 B&S, 1 B&S, 0 B&S or 00 B&S may be required.

Is 4 B&S cable enough for a starter motor?

4 B&S may be suitable for some short starter cable runs, but it may be too small for long battery relocation setups or high-current starter loads. Always check voltage drop, cable length and current draw.

Does the earth cable need to be the same size as the positive cable?

The earth path must be capable of carrying the same current as the positive path. In high-current automotive wiring, undersized earth cables and poor engine earth straps can cause major voltage drop and starting issues.

Where should the main fuse be mounted?

For main power feeds, the fuse or circuit protection should normally be mounted as close to the battery or power source as practical so the cable is protected if it shorts to chassis.

What causes voltage drop in battery cables?

Voltage drop is caused by resistance in the cable, terminals, joins, earth path and connections. Longer cable runs, smaller cable, poor crimps and bad earth points all increase voltage drop.

What cable size should I use for a PDM main feed?

A PDM main feed should be sized for the PDM current rating, total expected vehicle load, cable length and main fuse or breaker size. Do not size the main feed based on only one PDM output.

Last updated: June 2026