Electrical trunking provides a versatile, accessible cable containment system for commercial, industrial, and domestic installations. This guide covers PVC trunking, metal trunking, dado and skirting systems, mini trunking, compartmental trunking for cable segregation, sizing calculations, and installation best practice to BS 7671.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
Select trunking so the total cross-sectional area of all cables does not exceed 45% of the usable internal space (IET On-Site Guide space factor), then segregate voltage bands per BS 7671 Regulation 528.1 — Band I and Band II cables either share insulation rated for the highest voltage or run in separate compartments. Metal trunking may act as the circuit protective conductor under Section 543 where continuity is verified.
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Electrical trunking is a rectangular or square-section enclosure used to contain and protect electrical cables. Unlike conduit, which uses a round tube through which cables must be pulled or drawn, trunking has a removable lid or cover that allows cables to be laid in from the top or side. This makes trunking significantly easier to install cables into, and more importantly, easier to add, remove, or modify cables after the initial installation.
Trunking is classified as a wiring system under BS 7671 Chapter 52 and is widely used in commercial, industrial, and domestic installations. It is particularly common in office fit-outs, retail premises, schools, hospitals, and any environment where cable routes need to be accessible for future modifications.
The main types of trunking used in UK electrical installations are PVC trunking, galvanised steel (metal) trunking, dado (perimeter) trunking, skirting trunking, mini trunking, and compartmental trunking. Each type is designed for specific applications and environments.
Trunking is available in a wide range of sizes, from 16 mm by 16 mm mini trunking for a single data cable up to 300 mm by 300 mm heavy-duty metal trunking for main cable distribution routes in large commercial and industrial buildings.
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PVC trunking is the most common type for domestic and light commercial installations. It is lightweight, easy to cut and fit, requires no earthing, and is available in white, black, and woodgrain finishes. PVC trunking is non-conductive, so it cannot serve as a CPC, and all cables within it must include their own protective conductor.
PVC trunking fittings include internal and external angles, flat angles, tees, end caps, couplers, and accessory boxes (for mounting socket outlets and data points directly into the trunking). All major manufacturers produce comprehensive fitting ranges that allow the trunking to follow complex routes while maintaining a neat, professional appearance.
PVC trunking has similar temperature limitations to PVC conduit — it should not be used in environments where temperatures regularly exceed 60 degrees Celsius or fall below minus 5 degrees Celsius. For these environments, metal trunking or specialist high-temperature trunking should be specified.
Galvanised steel trunking is the standard containment system for commercial and industrial cable distribution. It provides superior mechanical protection, fire resistance, and can serve as a CPC when properly installed with continuity at all joints. Metal trunking is also required in certain applications where electromagnetic screening of cables is needed.
BS 7671 Regulation 543.2.1 lists a metallic cable management system or other enclosure among the permitted types of protective conductor, so metal trunking is allowed to act as the CPC. When you rely on it, every joint, bend, and tee must provide verified electrical continuity. Many manufacturers supply pre-galvanised trunking with integral earth bonding straps at joints; where these are not provided, separate earth continuity straps or bonding conductors must be installed across each joint. Regulation 543.2.7 also requires the earthing terminal of each accessory to be connected by a separate protective conductor to an earthing terminal in the associated box or enclosure — the trunking metalwork alone is not enough at the accessory. The continuity of the trunking CPC must be verified during initial verification. See the continuity testing (R1 + R2) guide for the test method.
Metal trunking with lid access from the top or front is used for cable distribution routes. Cable ladder and cable tray systems (open metalwork supports without covers) are used in plant rooms, risers, and industrial settings where cable access and ventilation are priorities over enclosure protection.
Beyond standard PVC and metal trunking, several specialised trunking types are designed for specific applications and environments.
Dado trunking is a multi-compartment system designed to run around the perimeter of a room at desk height (typically 800 to 1000 mm above floor level). It provides a surface-mounted distribution system for power, data, and telephone services without the need for floor boxes or under-floor wiring. Dado trunking typically has 2 or 3 compartments — one for mains power cables, one for data/network cables, and one for telephone or other low-voltage services. The compartments are physically separated by internal dividers that provide the cable segregation required by BS 7671.
Skirting trunking replaces the conventional timber skirting board with a trunking profile that looks like a skirting board but contains cable compartments. It runs at floor level around the room perimeter and is commonly used in domestic and residential refurbishments where a neat, unobtrusive cable distribution system is needed. Skirting trunking typically has 2 compartments — one for power cables and one for data or telephone cables.
Floor trunking is installed within or beneath the floor structure to provide cable distribution routes across open-plan floor areas. It feeds floor boxes (service outlets) that provide power, data, and telephone connections at individual desk positions. Floor trunking is cast into concrete floor slabs during construction or installed in raised access floor voids. It is the standard solution for modern open-plan offices.
Lighting trunking is a specialised system that combines cable containment with luminaire mounting. The trunking carries the lighting circuit cables and provides built-in adaptor plates for mounting fluorescent or LED luminaires directly onto the trunking. This eliminates the need for separate cable management and luminaire mounting points. Lighting trunking is widely used in commercial and industrial environments — warehouses, production facilities, retail spaces, and offices.
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Trunking sizing follows a similar principle to conduit fill calculation, but with a slightly different space factor. For trunking, the maximum cable fill is 45% of the usable internal cross-sectional area (compared to 40% for conduit). The higher fill percentage is acceptable because cables are laid into trunking from the open top rather than pulled through, reducing the risk of installation damage.
The total cross-sectional area of all cables (measured over the insulation) must not exceed 45% of the usable internal cross-sectional area of the trunking. This ensures cables can be laid in neatly, withdrawn for maintenance, and new cables added in future without excessive disturbance to existing cables.
The usable internal area is the actual space available after accounting for internal dividers, cable ties, and any other fittings that reduce the available space. For compartmental trunking, calculate the fill for each compartment separately.
| Trunking Size | Internal Area | 45% Fill Area | Approx. 2.5 mm sq Singles |
|---|---|---|---|
| 25 x 16 mm | 265 mm sq | 119 mm sq | 7 |
| 40 x 25 mm | 700 mm sq | 315 mm sq | 20 |
| 50 x 50 mm | 1,936 mm sq | 871 mm sq | 57 |
| 100 x 50 mm | 4,096 mm sq | 1,843 mm sq | 121 |
| 150 x 75 mm | 9,588 mm sq | 4,315 mm sq | 284 |
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Try it free for 7 daysBS 7671 Regulation 528.1 requires that cables of different voltage bands and cables from different systems be segregated to prevent interference and ensure safety. Within trunking, segregation is achieved using physical barriers (compartmental trunking) or by maintaining defined spacing between cable groups.
Regulation 528.1 prohibits a Band I and a Band II circuit sharing the same wiring system unless one of the following methods is adopted. Compartmental trunking satisfies the third method below.
For SELV and PELV systems the requirements of Regulation 414.4 also apply.
Compartmental trunking solves the segregation problem elegantly — a single trunking profile with 2 or 3 internal compartments provides separate cable channels with physical barriers meeting method (c) of Regulation 528.1. This is significantly more cost-effective than running separate trunking systems for each cable category.
BS 7671 18th Edition guide covers the full segregation requirements in detail.Professional trunking installation requires careful planning, accurate measurement, and attention to detail. A well-installed trunking system looks clean and professional, provides easy access for future modifications, and maintains cable protection throughout its life.
For commercial installations, provide clear labelling on the trunking system identifying the cable categories in each compartment and the circuit identification of each cable. This is essential for future maintenance and modifications — an unlabelled trunking system full of unidentified cables is extremely difficult to work with safely.
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