10.4 Trunking, Cable Tray & Cable Supports

10.4 Trunking, Cable Tray & Cable Supports

In commercial, educational, healthcare, and industrial buildings, electrical containment systems must support hundreds of diverse power, lighting, control, and data cables. Containment systems provide mechanical protection, allow convenient cable drawing and modification, and support cables across building structures. Electricians must select the appropriate containment medium—cable trunking, cable tray, cable basket, or cable ladder—while adhering to strict thermal space factor rules, circuit segregation mandates, and fire safety regulations.

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Cable Trunking Systems

A cable trunking system is an enclosure of rectangular or square cross-section comprising a trough with a removable lid. Unlike conduit, which requires cables to be pulled through from box to box, trunking allows cables to be laid in along its entire run once the lid is removed, significantly speeding up first-fix installation.

Trunking Materials and Profiles

• Pre-Galvanised Sheet Steel Trunking: Fabricated from mild steel sheet hot-dip galvanised to BS EN 10346. Features inward-turned return flanges along both top edges. Return flanges provide structural rigidity over long spans and prevent sharp metal sheet edges from cutting cable insulation. Lids are secured with turnbuckle fasteners or quick-release screw fixings.
• PVC Trunking: Extruded high-impact uPVC trunking: mini-trunking (e.g., 16 × 25 mm, 25 × 38 mm) for surface additions in domestic and light commercial retrofits; maxi-trunking (50 × 50 mm to 100 × 100 mm) for distribution routes; and dado / skirting trunking with multi-compartments for commercial offices.

Circuit Segregation (BS 7671 Regulation 528.1)

Modern commercial installations carry diverse electrical services. BS 7671 categorizes circuits into two distinct voltage bands:

• Band I: Extra-Low Voltage (ELV) circuits, including telecommunications, Cat5e/Cat6/Cat6A data networking, door access, CCTV, and addressable fire alarm signalling operating at ≤ 50 V AC or ≤ 120 V DC ripple-free.
• Band II: Low Voltage (LV) power and lighting circuits operating at standard supply voltages (230 V AC single-phase, 400 V AC three-phase).

  Three-Compartment Steel Dado Trunking Segregation:
  ┌─────────────────────────────────────────────────────────────┐
  │ Compartment 1: Band I Data / Telecoms (Cat6 Networking)     │
  ├─────────────────────────────────────────────────────────────┤ Continuous Metal
  │ Compartment 2: Band II Low Voltage Power (230 V Sockets)    │ Earthed Dividers
  ├─────────────────────────────────────────────────────────────┤
  │ Compartment 3: Band I Fire Alarm / ELV Control Circuits     │
  └─────────────────────────────────────────────────────────────┘

The Segregation Rule: Band I and Band II circuits must not be drawn into the same containment compartment unless:

1. Every cable is insulated to the highest voltage present (e.g., data cables insulated to 400 V dielectric rating), or
2. The circuits are separated by a continuous, earthed metallic divider or separate compartments within multi-compartment trunking.

Physical separation prevents electromagnetic interference (EMI) and noise induction from high-current power cables from corrupting sensitive high-speed digital data streams, while eliminating the catastrophic hazard of 230 V mains crossing into touch-accessible telephone or data accessories.

Site Fabrication of Sheet Steel Trunking Fittings

While manufactured prefabricated fittings (bends, tees, crossovers) are readily available, electricians regularly fabricate custom fittings on site using standard lengths:

• 90° Gusset Bends & Flat Bends: Formed by measuring the trunking width, scribing two 45° lines on the side walls to form a 90° V-notch, cutting the notch with a hacksaw and aviation tin snips, filing all burrs smooth, folding the trunking into a right angle, and securing the overlapping corner gussets.
• Bolting Rules: Fittings are secured using M6 mushroom-headed roofing bolts (gutter bolts). The smooth, rounded mushroom head must always be installed on the INSIDE of the trunking, with the threaded shank and nut positioned on the exterior. If bolt threads project into the interior trough, pulling cables into the trunking will drag insulation across the razor-sharp threads, causing line-to-earth faults.
• Earth Continuity Links: When steel trunking is cut and bolted on site, paint and zinc coatings may compromise electrical continuity. A flexible copper bonding braid or green-and-yellow earth link must be bolted across every joint and across removable trunking lids.

The 45% Space Factor Rule (On-Site Guide Appendix E)

Cables generate heat (I²R) during operation. If a trunking trough is packed solid with cables, heat cannot dissipate by convective air movement. Conductor temperatures rise rapidly, degrading insulation life, causing thermal runaway, and creating fire risks.

Using the IET On-Site Guide Appendix E method, the total cross-sectional area of the cables installed in trunking must not exceed 45% of the internal cross-sectional area of the trunking:

Space Factor = (Total External Area of All Cables / Internal Area of Trunking) × 100% ≤ 45%

To calculate capacity, the On-Site Guide provides the unit factor method. Each cable size has a cable factor, and each trunking size has a trunking factor equal to about 45% of its usable internal area (the tabulated factors allow for wall thickness, so they are slightly less than width × depth × 0.45). The rule is:

Sum of (Number of Cables × Cable Factor) ≤ Trunking Factor

Worked Example: An installer wishes to run twenty-five 2.5 mm² stranded cables (cable factor = 12.6) and twelve 6.0 mm² stranded cables (cable factor = 21.2) along a trunking run. What is the minimum standard trunking size required?

• Total Cable Factor: (25 × 12.6) + (12 × 21.2) = 315 + 254.4 = 569.4
• Checking standard trunking: A 50 mm × 50 mm trunking provides a factor of 1,037.
• Because 569.4 ≤ 1,037, a 50 mm × 50 mm trunking is fully compliant.

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Cable Tray, Cable Basket & Cable Ladder Racking

Where large volumes of multi-core, armoured, or heavy power cables are routed, enclosed trunking is replaced by open-support containment systems.

  ┌─────────────────────────────────────────────────────────────┐
  │                Open Cable Support Systems                   │
  │                                                             │
  │  1. Perforated Cable Tray ── Heavy multi-core & SWA cables  │
  │  2. Wire Mesh Cable Basket ── 100% open airflow, IT & data  │
  │  3. Cable Ladder Rack ── Heavy industrial risers & HV mains │
  └─────────────────────────────────────────────────────────────┘

1. Perforated Sheet Steel Cable Tray

Manufactured from pre-galvanised or post-galvanised steel sheet with perforations across its base and low-profile side return flanges:

• Perforations allow cables to be fastened securely using cable ties or cable cleats while permitting air circulation and dust egress.
• Classified into Light Duty, Medium Duty, and Heavy Duty Return Flange (HDRF).
• Ideal for industrial plant rooms, underground tunnels, and vertical service shafts carrying heavy sub-mains.

2. Wire Mesh Cable Basket

Constructed from welded high-tensile steel wire mesh (typically 50 × 100 mm grid pattern), electro-zinc plated or stainless steel:

• Rapid Site Fabrication: Electricians quickly cut individual wire spokes using angled bolt cutters to form radius bends, tees, and drops without needing manufactured fittings.
• Maximum Heat Dissipation: 100% open mesh structure allows free, uninhibited convective air cooling around cables, minimizing the severe thermal grouping derating factors (Cg) that penalize enclosed trunking.
• Data & Commercial Routing: Standard containment choice for Category 6A computer network cabling, fiber optic links, and commercial false-ceiling voids.

3. Cable Ladder Racking

Consists of two heavy-gauge structural side channels connected by cross rungs spaced at regular intervals (typically 250 mm or 300 mm):

• Maximum structural load-bearing capacity.
• Accommodates very large, heavy multi-core and single-core SWA power cables.
• Open rung design allows heavy-duty metallic cable cleats or trefoil cleats to be bolted securely across rungs on high-voltage vertical riser runs.

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Cable Supports & Prevention of Premature Collapse (Regulation 521.10.202)

One of the most critical life-safety regulations tested in electrotechnical examinations is BS 7671 Regulation 521.10.202.

The Historical Rationale: Firefighter Safety

This regulation was enacted following coroners' inquests into fatal residential and commercial tower block fires—most notably the Harrow Court fire (Stevenage, 2005) and the Shirley Towers fire (Southampton, 2010). During these fires, plastic cable clips melted, and plastic trunking lids softened and fell away. Heavy bundles of electrical cables cascaded down from ceilings, forming a web of hanging wires across corridors and stairwells. Entering firefighters, operating in zero visibility and thick black smoke, became hopelessly entangled in the fallen cables and died before they could escape.

The Mandatory Regulatory Requirement

Regulation 521.10.202 states:

"Wiring systems shall be supported such that they will not be liable to premature collapse in the event of a fire."

Key Scope and Compliance Rules:

1. Applies to the ENTIRE Installation: The regulation applies throughout the entire building—not just designated escape routes. Cables in ceiling voids, service shafts, offices, and domestic dwellings must all comply.
2. Prohibition of Plastic as Sole Support: Non-metallic cable clips (standard plastic masonry clips), plastic cable ties, plastic conduit saddles, and PVC mini-trunking with snap-on plastic lids are strictly forbidden as the sole means of cable support.
3. Fire-Resistant Metallic Fixings Mandatory: Installers must fit metallic supports at specified intervals to carry the entire weight of the cable run during a fire:
   • Fire-rated all-metal cable clips and metal banding.
   • Stainless steel or zinc-plated steel cable ties on cable tray and basket.
   • Internal metal fire clips fitted inside PVC mini-trunking: these U-shaped spring-steel clips hold the cables inside the trunking base even after the plastic casing and lid completely melt away.
4. All-Metal Anchors: A metal clip screwed into a plastic wall plug does not comply! Plastic wall plugs soften and fail in a fire, letting the screw pull out of the ceiling or wall. Installers must use all-metal masonry anchors, concrete self-tapping screws, or all-metal drop-in wedge anchors.

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Substrate Fixing Methods & Anchoring Technologies

Electrical containment must be anchored securely to withstand both mechanical dead loads and dynamic cable-pulling forces: