Worked Example: Cable Tray Fill and Fire Barrier Design for a 24-Storey Residential Tower — The Grenfell Tower Cable Riser Question

On 14 June 2017, the Grenfell Tower fire in North Kensington, London, killed 72 people. While the primary fire spread was through combustible ACM cladding on the building’s exterior, the internal investigation revealed that cable risers and cable trays were critical infrastructure that failed catastrophically during the fire. Cable trays in the vertical risers were overloaded beyond their design capacity, and inadequate fire stopping at floor penetrations allowed the riser shafts to act as chimneys — channelling heat and smoke vertically through the building at alarming speed.

The fire propagation through cable bundles is a well-documented phenomenon: when cables are packed tightly in a tray, the heat from burning insulation on one cable ignites adjacent cables, creating a self-sustaining fire front that travels along the tray. IEC 61537 limits cable tray fill to 50% for power cables specifically to maintain air gaps that slow fire propagation and allow adequate heat dissipation during normal operation. When trays are overloaded to 60–70% fill, those air gaps disappear — and so does the safety margin.

This worked example demonstrates the complete cable tray design process for a high-rise residential tower, covering fill ratio compliance, grouping derating, fire barrier spacing, and structural loading — every factor that, when neglected, turns a cable riser into a vertical fire highway.

Design the cable tray system for 3 cable risers serving 120 apartments (5 apartments per floor, 24 floors) in a residential tower.

Each riser serves approximately 8 floors (24 floors ÷ 3 risers). The cable inventory per floor for each riser:

Total cables per floor: 16 cables (10 power/fire, 6 data/comms)

At the base of the riser (ground floor), all 8 floors’ cables accumulate. The worst-case tray fill occurs at the bottom of the riser where cables from all floors converge:

Total cables at riser base = 16 × 8 = 128 cables — (Eq. 1)

The tray fill calculation uses the overall cable diameter (including insulation), not the conductor area. From manufacturer data:

Cable cross-sectional area uses the overall diameter:

A_cable = π × (OD/2)² — (Eq. 2)

Total cable area = 5,724 + 1,576 + 1,810 + 2,122 = 11,232 mm²

IEC 61537:2006, Clause 9.1 specifies maximum fill ratios for cable trays:

• Power cables: 50% maximum fill (to allow heat dissipation and air circulation)
• Data/comms cables: 50% maximum fill (separate tray recommended)

Per BS 7671, Regulation 521.5, power and fire alarm cables must be segregated from data cables. We therefore use two separate trays:

Tray 1 — Power + fire alarm:

Power + fire cable area = 5,724 + 1,576 + 1,810 = 9,110 mm²

Required tray area ≥ 9,110 / 0.50 = 18,220 mm² — (Eq. 3)

Standard perforated cable tray sizes (width × depth): 300×50 mm (15,000 mm²), 450×50 mm (22,500 mm²), 600×50 mm (30,000 mm²).

Selected: 450 × 50 mm tray (usable area = 22,500 mm²)

Fill ratio = 9,110 / 22,500 = 40.5% ≤ 50% — ✓ PASS

Tray 2 — Data/comms:

Data cable area = 2,122 mm²

Required tray area ≥ 2,122 / 0.50 = 4,244 mm²

Selected: 150 × 50 mm tray (usable area = 7,500 mm², fill = 28.3%) — ✓ PASS

The power cables on Tray 1 are grouped, requiring derating per BS 7671, Table C.7 (Installation Method E) for multicore cables on perforated cable tray:

At the base of the riser, there are 56 power circuits on the tray (40 apartment feeds + 16 common area circuits). For cables touching in a single layer on a perforated tray:

With 56 circuits in the tray, cables will be arranged in multiple layers. For multi-layer arrangements, the grouping factor is further reduced. Per BS 7671, Table C.7, Note 5, multiply by 0.80 for two layers and 0.73 for three layers.

Assuming 3 layers in the 450 mm tray:

C_g = 0.64 × 0.73 = 0.467 — (Eq. 4)

Ambient temperature derating (C_a):

From BS 7671 Table C.1, Row: 35°C, Column: 70°C rated PVC:

C_a = 0.94

C_total = C_g × C_a = 0.467 × 0.94 = 0.439

This means each 10 mm² apartment feed cable (base rating 57 A for Method E) is derated to:

I_z = 57 × 0.439 = 25.0 A

Each apartment has a 63 A main switch. The cable must carry at least 63 A after derating:

I_t ≥ I_n / C_total = 63 / 0.439 = 143.5 A — (Eq. 5)

From BS 7671 Table C.9, Installation Method E (perforated tray), single-phase 2C+E copper PVC:

The grouping penalty requires 50 mm² cables for apartment feeds — five times the size that a standalone calculation (10 mm²) would suggest. This dramatically increases cable cost and tray fill.

Design optimisation: Reduce the number of cables per tray by splitting the riser into sub-risers. If each sub-riser serves 4 floors instead of 8, the cable count halves, the grouping factor improves to approximately 0.55, and 25 mm² cables suffice. This is the standard approach in well-designed high-rise buildings.

BS 8519:2010, Clause 7.4 requires fire stopping (fire barriers) at every floor penetration in cable risers. The fire barriers must:

• Achieve the same fire rating as the floor slab (typically 120 minutes for residential towers per Approved Document B, Table A2)
• Be installed at every floor penetration without exception
• Maintain their fire rating after cables are added or removed (re-enterable system)

Additionally, BS 8519 Clause 7.5 recommends intermediate fire barriers within the riser shaft at maximum 3-floor intervals to prevent chimney effect:

Fire barrier spacing = 3 floors × 3.0 m floor-to-floor = 9 m maximum — (Eq. 6)

For a 24-storey tower:

Floor penetration barriers = 24 (one per floor)

Intermediate barriers = 24 / 3 = 8 additional mid-riser barriers

Total fire barriers per riser = 32

Each barrier must be tested and certified to the required fire rating with the specific cable types and fill density installed. A barrier rated for 50% tray fill may not achieve its fire rating at 70% fill — this was one of the specific failures identified in the Grenfell investigation.

The cable tray must support the weight of all cables plus a safety factor for installation loading per IEC 61537, Clause 8.3. Calculate the weight per metre at the riser base:

Total cable weight = 11.6 + 3.0 + 3.4 + 2.4 = 20.4 kg/m — (Eq. 7)

With safety factor of 1.5 (for installation loads and dynamic effects):

Design load = 20.4 × 1.5 = 30.6 kg/m

Standard 450 × 50 mm perforated cable tray (1.5 mm steel) is typically rated at 25–35 kg/m depending on support spacing. With support brackets at 1.5 m centres:

Tray rating at 1.5 m span = 32 kg/m (manufacturer data)

30.6 kg/m < 32 kg/m — ✓ PASS (but marginal — 1.2 m support spacing recommended)

BS 7671, Regulation 528.1 requires segregation of circuits that could cause mutual detrimental influence. For a residential tower, the critical segregation requirements are:

This means the riser shaft requires a minimum of 4 cable containment systems:

1. Power tray (450 × 50 mm perforated tray)
2. Fire alarm / emergency lighting tray (150 × 50 mm, fire-rated cables only)
3. Data / comms tray (150 × 50 mm, separate from power)
4. Lift supply conduit (dedicated steel conduit, fire-rated)

Minimum riser shaft internal width = 450 + 150 + 150 + 100 + gaps = 950 mm — (Eq. 8)

Result: 4-tray riser system with 32 fire barriers per riser, 950 mm minimum shaft width, and split sub-risers (4 floors each) recommended to manage grouping derating.

The Grenfell Tower fire was primarily caused by combustible cladding, not cable tray overloading. However, the building’s cable risers failed to contain fire spread, and the investigation identified systemic failures in cable management that apply to every high-rise building:

• Enforce the 50% fill limit strictly — cable trays at 60–70% fill eliminate the air gaps needed for heat dissipation and fire propagation resistance; there must be a documented “no more cables” policy once 50% is reached
• Install fire barriers at every floor penetration — missing or compromised fire barriers allow the riser to act as a chimney, drawing heat and smoke vertically through the building
• Size the riser shaft for segregation — a shaft designed for “one tray” will inevitably have mixed cable types crammed together, defeating both electrical segregation and fire containment
• Account for grouping derating in riser cables — the 50–60% derating penalty for densely packed risers means cables must be significantly upsized compared to standalone calculations
• Design for the building’s lifetime, not day one — over 60 years, cables will be added for telecoms upgrades, EV charging, and building management systems; build in 30% spare capacity from the outset