Cable Trunking: Materials, Sizing, and Installation Best Practices

Direct Answer: Match Material and Size to Load, Then Follow Fill Ratio Rules

Cable trunking is a rigid, channel-shaped enclosure with a removable lid used to route, organize, and protect electrical cables along walls, ceilings, or floors. Choosing the right system comes down to three decisions: select a material rated for the installation environment (PVC for general indoor use, galvanized steel for industrial or fire-rated needs, aluminum for lightweight or corrosion-prone settings), size the trunking so cable fill does not exceed 45% of the internal cross-sectional area per most electrical codes, and install with proper support spacing — typically every 1 to 1.5 meters for horizontal runs.

Getting these three factors right prevents the most common trunking failures: overheating from overfilled channels, sagging from inadequate support, and premature material degradation in the wrong environment. The sections below cover each factor in detail, along with sizing tables and installation steps.

Common Cable Trunking Materials

Material choice affects fire performance, durability, cost, and suitability for the installation environment. Four materials cover the vast majority of commercial and industrial applications.

For most commercial buildings, PVC trunking accounts for the bulk of installations due to its low cost and ease of modification, while galvanized steel is the default choice wherever fire rating or mechanical protection is a code requirement, such as in plant rooms or escape routes.

How to Size Cable Trunking Correctly

Undersized trunking is one of the most common and costly mistakes in electrical installation, since it can cause cables to overheat and forces expensive rework. Sizing follows a straightforward fill-ratio rule.

The 45% Fill Rule

Most electrical codes, including BS 7671 in the UK and similar international standards, recommend that the total cross-sectional area of all cables inside the trunking should not exceed 45% of the trunking's internal cross-sectional area. This allows for heat dissipation and leaves room for future cable additions.

Standard Trunking Sizes

To calculate the required size: sum the cross-sectional area of all cables to be installed, divide by 0.45, and select the next standard trunking size that meets or exceeds that result. Always size for at least 20-30% future capacity if additional circuits are anticipated, since retrofitting trunking after walls or ceilings are finished is significantly more expensive than installing larger trunking initially.

Installation Best Practices

Proper installation affects both code compliance and long-term system reliability. The following practices apply across most material types.

1. Plan the route before cutting. Map the full cable route first, accounting for bends, junctions, and access points for future maintenance, to minimize waste and avoid undersized sections mid-run.
2. Support at proper intervals. Horizontal runs typically require support brackets every 1 to 1.5 meters, while vertical runs may need supports every 0.75 to 1 meter, depending on trunking weight and local code requirements.
3. Maintain separation between power and data cables. Where power and low-voltage data cables share a route, use divided trunking or maintain a minimum separation (commonly 50-300mm depending on cable type and length of parallel run) to reduce electromagnetic interference.
4. Bond and ground metal trunking. Galvanized steel and aluminum trunking must be properly earthed/grounded at regular intervals along the run, per local electrical code, since the trunking itself can become an extended conductive path.
5. Seal penetrations through fire-rated walls. Where trunking passes through fire-rated walls or floors, use approved fire-stopping materials to maintain the wall's fire rating.
6. Leave the lid accessible. Avoid installing trunking in locations where the cover cannot be removed for future cable additions or maintenance, particularly behind fixed furniture or permanent fixtures.

Common Installation Mistakes to Avoid

• Overfilling beyond the 45% rule — this is the most frequent code violation found during electrical inspections and can lead to cable insulation breakdown from heat buildup.
• Mixing incompatible cable types without separation — running unshielded data cable directly alongside high-current power cable can introduce signal interference that's difficult to diagnose after the fact.
• Inadequate support spacing — under-supported runs sag over time, stressing cable connections and joints at termination points.
• Using indoor-rated PVC trunking outdoors — standard PVC degrades under prolonged UV exposure; outdoor installations need UV-stabilized PVC or a metal alternative.
• Skipping expansion joints on long runs — PVC trunking can expand and contract with temperature changes; runs longer than approximately 3 meters in environments with significant temperature swings should include expansion joints.

Choosing Between Trunking, Tray, and Conduit

Trunking is often confused with related cable management products. The right choice depends on cable volume, accessibility needs, and aesthetics.

Bottom Line

Successful cable trunking installation comes down to matching material to environment, sizing to the 45% fill rule with room for future expansion, and supporting runs at proper intervals. PVC trunking covers most general indoor commercial needs at the lowest cost, while galvanized steel handles industrial, fire-rated, or high-mechanical-stress environments. Getting the size right the first time — by calculating total cable cross-sectional area and building in 20-30% spare capacity — avoids the much higher cost of retrofitting undersized trunking later.