Cable Sizing Guide — 24V Systems

24V cable sizing guide for plant, agricultural machinery, LGV and PCV applications — cable sizes, current ratings and volt drop guidance

Before you start

This 24V cable sizing guide covers DC electrical systems as used in plant and agricultural machinery, light goods vehicles, coaches and buses, and other commercial and off-highway applications. For 12V leisure vehicle and campervan installations see our dedicated 12V Cable Sizing Guide. For emerging 48V architectures see our 48V Cable Sizing Guide.

Picking the right cable comes down to two things: how much current it needs to carry, and where it’s going to run. Get those right and the rest follows.

Current carrying capacity — ampacity — is a thermal limit. Too much current through too small a cable generates heat. Enough heat and the insulation fails. In plant and agricultural applications, ambient temperatures are often higher than in a leisure vehicle — engine bays, machinery cabs and enclosed compartments can easily exceed 40°C. Use the appropriate derating figures from our Ampacity Ready Reckoner for high ambient temperature applications.

When in doubt, go up a size. A cable running slightly under its rating runs cooler, lasts longer, and costs almost nothing extra.

The figures in this guide are for a single cable run in open air at normal temperature. If your cable runs through conduit, behind panels, or is bundled with other cables, go up a size as a minimum.

A word on stranding

Cable cross-sectional area determines current capacity — but how the conductor is constructed determines how pleasant it is to work with and how long it lasts. Automotive cable uses fine-stranded flex: many thin strands rather than a few thick ones. It routes more easily, terminates more cleanly, and holds up better to the vibration and flexing of any vehicle or machinery installation.

It costs a little more than generic cable. It is worth it.

📖 For a full explanation of cable construction, conductor classes and why it matters in practice, see our Cable Construction Know How guide at voltforge.co.uk

A word on insulation

For most commercial and off-highway 24V work, specify quality automotive PVC (105°C rated) as a minimum. For under-bonnet runs, engine bay proximity or high ambient temperature environments, specify 125°C XLPE or XLPO insulated cable. The current figures in this guide are based on quality 105°C cable.

📖 For full ampacity data across all insulation types and ambient temperatures, see our Ampacity Ready Reckoner at voltforge.co.uk

The 24V advantage

At 24V, the current for a given power load is half that of an equivalent 12V system. Half the current means smaller cable, less volt drop, lower losses and less heat generated in the wiring. For larger machinery and vehicles with significant electrical loads, 24V architecture makes very good sense — and it is the reason heavy commercial vehicles, most plant machinery and agricultural equipment have used 24V as standard for decades.

Common applications — what size do I need?

Use this table as a starting point. Always check the current draw of your specific device on its datasheet or label.

All cable sizes are minimum recommendations for free air runs at normal ambient temperature using quality 105°C cable. Go up a size for runs in conduit, behind panels, bundles, or in high ambient temperature environments typical of plant and agricultural machinery.

Quick current rating reference

Current carrying capacity is the same regardless of system voltage — it is determined by the conductor and insulation, not the voltage. The figures below apply at 24V. For 12V or 48V systems see our dedicated guides at voltforge.co.uk.

Figures for quality 105°C PVC insulated stranded copper flex, single conductor, free air, 25°C ambient. Real-world capacity will be lower in hot environments or bundled runs. For under-bonnet or high-ambient plant applications consider 125°C rated XLPE/XLPO cable — see our Ampacity Ready Reckoner.

Volt drop — why cable length matters

Every cable has resistance. Current flowing through resistance causes a voltage drop along the run. On a 24V system volt drop has less impact than on 12V for the same power delivery — the same volt drop is a smaller percentage of your supply voltage. But it still matters, particularly on long runs to remote loads on large machinery.

A rough rule of thumb: keep volt drop to 5% or less of your supply voltage on any circuit. On a 24V system that’s 1.2V maximum. Note that 24V systems have a significant advantage over 12V here — the same cable carries twice the power for the same volt drop percentage.

Longer runs need bigger cable — not just to carry the current safely, but to keep the voltage drop within acceptable limits. If your run is more than 3–4 metres, check the volt drop, don’t just check the current rating.

Volt drop = (cable resistance in mΩ/m) × current (A) × run length (m) ÷ 1000. Your cable datasheet will give you the resistance figure.

📖 For full resistance data (mV/A/m) for all cable sizes, see our Ampacity Ready Reckoner at voltforge.co.uk

Standards note

BS EN 1648 applies to leisure vehicles only and is not the relevant standard for plant, agricultural, LGV or PCV electrical installations. Applicable standards for these applications include BS EN ISO 11684, BS EN 13309, and vehicle or equipment manufacturer specifications. Always verify the applicable standards for your specific application.

Disclaimer

This document is provided for general reference and educational purposes only. The figures and guidance contained in it are derived from published industry standards and are intended to support informed decision-making — they are not a substitute for a full engineering assessment of a specific installation. It is the responsibility of the installer to carry out an appropriate assessment for their specific application and to ensure that any installation complies with applicable standards, regulations, and any vehicle or equipment manufacturer requirements. Voltforge and Zeromachine Ltd accept no liability for loss, damage, injury or consequential loss arising from the use of or reliance on the information in this document. If you are in any doubt about the suitability of a cable size or installation method for your application, stop and get proper advice before proceeding.