Cable Trunking and Conduit: PVC, Metal, and Flexible Systems for UK Electrical Installations

Selecting the right cable management system is a fundamental part of any electrical installation. The choice between PVC trunking, steel conduit, flexible conduit, and cable tray affects not only the aesthetics and ease of installation, but also the mechanical protection, thermal performance, and long-term compliance of the installation under BS 7671 (18th Edition).

This guide covers the full range of cable management options used in UK domestic, commercial, and light industrial work, with guidance on sizing, installation practice, and regulatory requirements.

Why Cable Management Matters

Cable management serves three core purposes:

• Mechanical protection — preventing damage to cable sheaths from impact, abrasion, or compression, especially in accessible areas
• Circuit identification and maintainability — enabling future cable replacement or addition without disturbing the structure
• Fire containment — particularly in commercial buildings, where trunking routes must be fire-stopped at penetrations and in fire-rated compartments

BS 7671 does not mandate a specific cable management system in most domestic situations, but it requires cables to be protected from mechanical damage where there is a risk of it (Regulation 522.6). The method of installation affects current-carrying capacity derating — cables installed in trunking use Reference Method B or C depending on the arrangement.

PVC Mini-Trunking

PVC mini-trunking is the most common surface-mounted cable management in domestic and light commercial work. It is inexpensive, quick to install, and available in white to blend with skirting boards and walls.

Standard Sizes

Mini-trunking is sized by width × depth (mm), with the most common sizes being:

• 16 × 16 mm — single T&E cable (1.0 mm² or 1.5 mm²)
• 25 × 16 mm — two T&E cables or one 4 mm² T&E
• 38 × 16 mm — three to four T&E cables
• 50 × 25 mm — six or more cables, lighting circuits combined with power
• 75 × 25 mm and 100 × 50 mm — heavier commercial runs

Never overfill trunking. BS 7671 Appendix 5 provides a space factor method: for circular cables, the sum of cable cross-sectional areas should not exceed 45% of the trunking internal cross-section when cables are all the same size, or 40% for mixed sizes. Overfilling causes heat build-up, reduces current-carrying capacity, and makes future cable pulling impossible without damage.

Fittings

A good trunking installation uses purpose-made fittings rather than mitred cuts wherever possible:

• Internal corners — for horizontal direction changes on the same wall plane
• External corners — for corners projecting outward
• Flat tee pieces — for T-junction branches
• End caps — to close open ends neatly
• Couplers — for joining trunking lengths (standard lengths are 2 m)
• Vertical internal/external bends — for rising or dropping off to an outlet

Adhesive-backed trunking is available but is not reliable as a permanent fixing on uneven or porous surfaces. Always use mechanical fixings (screws into plugged holes or self-adhesive clips for lighter runs) on solid walls, and use the appropriate trunking clips on dry lining.

Dado and Skirting Trunking

Dado trunking sits at desk height (approximately 900–1,000 mm from finished floor level) and is used in offices to carry power, data, and AV circuits separately in compartmented channels. It is typically aluminium-extruded with a clip-on face for easy access.

Skirting trunking replaces or sits above the decorative skirting board and is particularly popular in refurbished commercial spaces where chasing walls is undesirable. It usually provides two or three compartments: one for mains power, one for data, and one for AV/telephone — keeping them separated to reduce electromagnetic interference.

Key considerations for dado and skirting trunking:

• All joints, corners, and end caps must be mechanically secured and fire-stopped at compartment boundaries
• Socket outlets fitted into dado trunking are usually surface-mounted shallow modules designed for the specific trunking range — confirm compatibility before ordering
• Separation between mains and data cables should follow IEC 60950 guidance — most dado trunking has a physical separator between compartments

Galvanised Steel Conduit

Steel conduit is the cable management of choice for industrial and commercial installations requiring robust mechanical protection. Unlike trunking, conduit is circular and cables are drawn through after the conduit is installed — this means the conduit itself forms the cable route, and additional conductors can be added later without disturbing the installation.

Conduit Sizes

Steel conduit is specified by nominal outside diameter:

• 16 mm — up to four 1.5 mm² singles or two 2.5 mm² singles
• 20 mm — the most common domestic/light commercial size; up to six 2.5 mm² singles
• 25 mm — heavier circuits; up to eight 4 mm² singles
• 32 mm — sub-main runs and three-phase circuits
• 50 mm and 63 mm — heavy industrial sub-main and main cable

As with trunking, apply the 40–45% space factor rule when calculating the number of cables. For circular conduit, use the conduit factor tables in BS 7671 Appendix 5 or the Electrical Contractors Association (ECA) conduit capacity charts.

Heavy Gauge vs Light Gauge

Steel conduit is manufactured to BS EN 61386 in two wall thickness grades:

• Heavy gauge (Class 4) — the standard for surface and buried installation where mechanical impact is a risk
• Light gauge (Class 3) — for enclosed or embedded installation in walls or concrete (not suitable for exposed runs)

Avoid light gauge conduit for surface runs in warehouses, plant rooms, or any area where it may be physically damaged.

Types of Steel Conduit

• Rigid steel conduit (black enamel or galvanised) — for exposed surface or buried runs; galvanised is preferred for damp environments and exterior use
• Screwed conduit — the standard method; each conduit length is threaded and connects via couplings, boxes, and fittings with threaded entries
• Slip-over couplings — not accepted for installations where the conduit forms the CPC (circuit protective conductor); only use where a separate CPC is included

Using Conduit as a CPC

Steel conduit may be used as the circuit protective conductor provided all joints are mechanically and electrically sound, all boxes are properly bonded, and the cross-sectional area is verified by calculation or by using the adiabatic equation (Regulation 543.1). In practice, most modern installations include a separate green/yellow earth conductor even in steel conduit, to avoid dependence on the mechanical integrity of every joint.

Conduit Boxes and Fittings

Conduit is assembled using:

• Draw-in boxes — square or circular boxes with removable lids used at bends and at intervals to allow cable pulling; maximum draw length between boxes is typically 10 m for straight runs and less for runs with multiple bends
• Terminal boxes — for connecting outlets or lighting points
• Inspection elbows and bends — 90° fittings with removable inspection covers for tight corners
• Solid elbows — non-inspectable bends for embedded or behind-panel installation
• Locknuts and bushes — for securing conduit into adaptable boxes
• Male bushes — used with couplings for building up conduit entries into boxes

Threading and Cutting

Steel conduit must be cut square with a hacksaw or pipe cutter (pipe cutters can produce a burr inside the conduit that damages cables — always ream after cutting). Threads are cut with a conduit die set to produce a parallel (not tapered) thread to BS EN 60423. Apply conduit compound to threads on outdoor or damp location conduit systems to ensure water-tight joints and prevent corrosion at threaded connections.

PVC Rigid Conduit

PVC (rigid) conduit offers similar physical protection to steel conduit without the conductivity. It is used where:

• A separate CPC is always included (PVC conduit cannot act as a CPC)
• Corrosive environments make steel impractical
• Lighter weight is preferable (e.g., ceiling grid installations)

PVC conduit uses solvent-welded joints (not threaded) and is available in 20, 25, and 32 mm nominal sizes. It is not suitable for high-temperature locations (above 60°C) — in plant rooms and near heat sources, use steel or LSZH-sheathed conduit.

PVC conduit expands significantly with temperature changes — allow for thermal movement at long runs by using expansion couplings and not fixing conduit rigidly at closer than 500 mm intervals (use saddle clips that allow linear movement).

Flexible Conduit

Flexible conduit is used for the final connection to motors, compressors, and luminaires that vibrate, or to equipment that needs to be moved for maintenance. It is not a substitute for rigid conduit on fixed runs.

Types

• PVC-covered steel spiral (Anaconda/Flexicon type) — the most common; available in 12–50 mm diameter, rated for intermittent flexing, provides good mechanical protection
• Stainless steel braid-over-PVC — for food processing, chemical environments, or where hygiene cleaning is required
• Plain galvanised spiral without PVC cover — not suitable for damp areas; for dry indoor runs only
• Liquid-tight (IP68-rated) — with compression glands at each end, for outdoor luminaires, pump connections, or any outdoor motor termination

Installation Rules

• Maximum length should be as short as practical — typically 0.5–1.0 m
• Support the flexible conduit at each rigid conduit end with a gland or fitting; do not allow it to bear the cable weight
• Flexible conduit connections use compression-type fittings — ensure the correct fitting type is used (male thread entry to the rigid conduit box, and a separate female gland at the equipment end)
• Never use flexible conduit as a CPC — always include a separate bonding earth conductor from the rigid conduit earth point to the equipment earth terminal

Cable Tray and Cable Ladder

Cable tray is a ventilated support structure (typically perforated pressed steel or aluminium) used to carry multiple cables in larger commercial and industrial installations. It is not an enclosure — cables on tray are exposed, so they must be suitable for the environment (SWA, XLPE, or LSZH sheathing is typical).

When to Use Cable Tray

• Sub-main distribution to DBs across a building
• Multi-cable containment in plant rooms, server rooms, and factories
• Where future expansion is anticipated — tray allows cable additions without disturbing existing cables

Cable Tray Sizes and Loading

Perforated cable tray is available in widths from 50 mm to 600 mm and is rated by the manufacturer for distributed load (kg/m). Do not exceed the rated load. Standard lengths are 3 m.

Cable ladder (with longitudinal side rails and cross rungs rather than a continuous perforated base) is used for heavier cable loads and for better ventilation on large multicore cables where thermal build-up is a concern.

Earthing of Cable Tray

Steel cable tray systems must be earthed. The tray itself can form the CPC for the installation only if continuity is verified at every joint. In practice, run a separate earth conductor along the tray and bond all sections together — this is particularly important where tray is used in Zone 1 or 2 hazardous areas.

Cable Derating in Trunking and Conduit

Bundling cables together reduces their current-carrying capacity. BS 7671 Appendix 4 provides grouping factors (Cg) for cables grouped in trunking, conduit, and on tray:

• 2 circuits: Cg = 0.80
• 3 circuits: Cg = 0.70
• 4 circuits: Cg = 0.65
• 5 circuits: Cg = 0.60
• 6 circuits: Cg = 0.57

These factors apply to the reference current-carrying capacity (Iz) from Appendix 4 tables. A 2.5 mm² T&E cable has an Iz of 27 A (Reference Method B, clipped direct); with three circuits grouped in trunking, the derated capacity is 27 × 0.70 = 18.9 A — adequate for a standard 16 A ring final circuit but not for a 20 A dedicated circuit.

Always recalculate cable sizes when grouping multiple circuits. See our guide to UK electrical cable types for cable sizing reference. For full sizing calculation method, see our cable sizing and voltage drop guide.

Fire Barriers and Compartmentation

Where trunking or conduit passes through a fire-rated wall or floor, the penetration must be fire-stopped to restore the integrity of the compartment. This is a legal requirement under Building Regulations Part B and is particularly enforced in commercial premises.

Acceptable fire-stopping methods for cable management penetrations:

• Intumescent putty pads — wrapped around cables within the aperture and expanding on heat to seal the gap; suitable for trunking penetrations
• Intumescent sealant (mastic) — injected around conduit penetrations and into the annular gap between conduit and the structure
• Fire-rated conduit sleeves — for single conduit penetrations in fire-rated walls; rated to EI120 or higher
• Fire barrier boards (mineral fibre) — for multi-cable tray penetrations; boards cut to fit the tray section with intumescent material around individual cables

See our guide to passive fire protection for plumbers and electricians for full compartmentation requirements under Part B.

IP Ratings for Cable Management in Wet Areas

In bathrooms and other wet areas, cable management must be rated for the zone in which it is installed:

• Zone 1 (above and around the bath/shower) — minimum IP44 required; use IP65-rated adaptable boxes and galvanised conduit with sealed fittings
• Zone 2 (within 600 mm beyond zone 1) — minimum IP44; standard PVC trunking is not suitable unless IP-rated
• Outside zones — standard IP20 cable management is acceptable where no water spray is present

In domestic bathrooms, avoid surface-run trunking where possible — chase the wall and use galvanised conduit for the circuit feeding bathroom outlets, or use MICC cable. See our bathroom renovation electrical requirements guide for full zone rules.

Outdoor and Underground Cable Management

For outdoor overhead runs, use galvanised steel conduit on properly spaced brackets, or SWA cable without conduit where it can be adequately supported. PVC conduit is acceptable for underground runs where it will not be disturbed, but it must be protected with cable covers or used in duct (HDPE or clay duct) if there is any possibility of future excavation.

For underground runs:

• Minimum cover depth is 450 mm under paved areas and 600 mm under unpaved areas (BS 7671 Regulation 522.8.10)
• Use HDPE or PVC underground duct for all new buried cable routes — this allows future cable replacement by draw-through without excavation
• Mark the route with cable marker tape laid 150 mm above the cable
• Seal conduit ends where they enter buildings to prevent water ingress and rodent entry

See our outdoor electrical installations guide for full requirements for garden and external circuits.

Marking and Identification in Trunking

In any installation where multiple circuits share a trunking run, cables must be identifiable throughout. Acceptable methods:

• Coloured cable ties or adhesive labels at regular intervals and at each end
• Cable numbering sleeves at junction and draw-in boxes
• Circuit schedule posted inside each distribution board identifying the trunking runs

BS 7671 Regulation 514.3 requires all circuits to be identifiable at every access point. For long trunking runs with multiple circuits, add identification labels every 2–3 m or at each draw-in box.

Practical Tips for a Professional Installation

• Plan the route first — measure and mark before cutting trunking; off-square cuts on mini-trunking look unprofessional and compromise lid fitting
• Use a mitre box for 45° external corners — purpose-made corner pieces are neater but a well-cut mitre on mini-trunking is acceptable in awkward spaces
• Do not mix cable types in the same compartment — keep mains separate from data/communications in compartmented trunking
• Chase behind trunking for concealed runs to sockets — a short buried tail from the trunking to the socket backbox is neater than running trunking down to floor level
• Support conduit at the correct intervals — BS EN 61386 requires supports at maximum 500 mm from each box, fitting, or bend, and at maximum 900 mm (20 mm conduit) to 1,200 mm (25–32 mm conduit) along straight runs
• Thread cables before bending conduit where possible — pulling cable through conduit with multiple bends increases friction dramatically; add draw-in boxes to keep pull runs manageable

Summary: Choosing the Right Cable Management

APM Electricals stocks a full range of PVC mini-trunking, dado trunking, galvanised steel conduit, PVC rigid conduit, flexible conduit, conduit fittings, cable tray, and fire-stopping accessories. Search by size or type to find the right product for your installation.

Visit us at APM Electricals, 24 Western Avenue, Acton, London, W3 7TZ or call 020 8896 0800 for trade and retail electrical and plumbing supplies.