Worked Example: Fire-Resistant Cable Sizing for a 67m Vertical Riser — The Grenfell Tower Cable Question

On 14 June 2017, the Grenfell Tower fire in London killed 72 people. While the primary cause was combustible ACM cladding that allowed external fire spread, the fire exposed critical questions about electrical infrastructure in high-rise buildings. The fire alarm system cables, emergency lighting circuits, and fire pump supply cables all needed to survive the fire long enough for evacuation — and many did not.

BS 7671:2018+A2, Regulation 422.2 requires fire-resistant cables for circuits that must function during a fire. BS 8519:2010 provides the code of practice for selecting and installing fire-resistant cable systems. But the engineering question that most designers get wrong is: how do you SIZE those cables?

Fire-resistant cables such as MICC (mineral insulated copper clad) and FP200 Gold have fundamentally different current ratings than standard PVC/XLPE cables. Their mineral or modified polymer insulation has different thermal properties, and vertical installation in a riser shaft introduces derating factors that most engineers overlook entirely. An engineer who simply replaces a PVC cable with the “same size” MICC cable may be undersizing the circuit by 16% or more — creating the very fire hazard the fire-resistant cable was supposed to prevent.

Size the fire-resistant feeder cable from the main switchboard at ground level to the fire pump on the roof of a 24-storey residential tower.

This circuit is classified as a fire safety circuit per BS 7671, Regulation 560.1. It must maintain circuit integrity for the duration of the fire rating, which means the cable itself must survive.

For a three-phase motor load with known power factor and efficiency:

I_b = P / (√3 × V × PF × η) — (Eq. 1)

I_b = 15,000 / (√3 × 415 × 0.85 × 0.90)

I_b = 15,000 / (718.6 × 0.85 × 0.90)

I_b = 15,000 / 549.7

I_b = 27.3 A

This is the continuous running current. The motor starting current will be significantly higher (see Step 8), but cable sizing is based on continuous current with derating factors.

The protective device must satisfy BS 7671, Regulation 433.1.1:

I_b ≤ I_n — (Eq. 2)

For a motor circuit, an MCCB with adjustable thermal-magnetic trip is typical. Standard ratings: 16, 20, 25, 32, 40, 50, 63 A.

I_n = 32 A MCCB

The thermal trip is set to 1.0× (32 A) for overload. The magnetic trip (short-circuit element) is set to 10× (320 A) to allow motor starting current without nuisance tripping, per BS 7671, Section 552.1.

This is where MICC cable sizing diverges critically from standard PVC/XLPE sizing. The derating factors must be taken from MICC-specific tables, not the general tables used for PVC cables.

Ambient temperature derating (C_a):

BS 7671 reference ambient is 30°C. Our riser shaft runs at 40°C due to heat stratification from heating pipes and building services.

From BS 7671 Table B.52.14 (MICC cables), Column: 70°C sheath operating temperature, Row: 40°C ambient:

C_a = 0.87

Grouping derating (C_g):

Seven circuits on the same cable tray. From BS 7671 Table C.3, Row: touching on single cable tray, 7 circuits:

C_g = 0.54

Combined derating factor:

C_total = C_a × C_g = 0.87 × 0.54 = 0.470 — (Eq. 3)

This combined factor means the cable’s effective current capacity is reduced to only 47% of its base rating — more than halved.

The cable must have a tabulated current rating (before derating) such that:

I_t ≥ I_n / C_total — (Eq. 4)

I_t ≥ 32 / 0.470

I_t ≥ 68.1 A

Now select from BS 7671 Table B.52.17 (MICC cables, 70°C sheath), for 3-core + earth, clipped to non-metallic surface:

The minimum cable size based on current capacity is 10 mm² MICC.

Check voltage drop for the 10 mm² MICC cable over the 85 m route. From manufacturer data for 3-core MICC cable (BS EN 60702-1):

mV/A/m = 4.4 (three-phase, at 70°C operating temperature)

Voltage drop calculation per BS 7671 Appendix 12:

ΔV = mV/A/m × I_b × L / 1000 — (Eq. 5)

ΔV = 4.4 × 27.3 × 85 / 1000

ΔV = 10.2 V

ΔV% = 10.2 / 415 × 100 = 2.46%

The BS 7671 Appendix 12 limit for power circuits from the supply terminals to the current-using equipment is 5%. At 2.46%, this passes.

However, for a fire pump motor, we should also check voltage drop at starting current to ensure the motor can start successfully (see Step 8).

For the 32 A MCCB to disconnect within the required time under earth fault conditions, the earth fault loop impedance must not exceed the value given in BS 7671 Table 41.3.

For a 32 A Type B device (5s disconnection for fixed equipment per Regulation 411.3.2.3):

Z_s (max) = 1.37 Ω

Calculated earth fault loop impedance for 10 mm² MICC cable:

Z_s = Z_e + (R₁ + R₂) — (Eq. 6)

Where Z_e = 0.35 Ω (external earth fault loop impedance, typical for TN-S supply), and R₁ + R₂ for 10 mm² phase + CPC in MICC cable at operating temperature:

R₁ + R₂ = (1.83 + 1.83) × 0.085 × 1.2 = 0.374 Ω

(Factor 1.2 accounts for conductor temperature rise to operating temperature per BS 7671 Table I.3)

Z_s = 0.35 + 0.374 = 0.724 Ω

0.724 Ω < 1.37 Ω — PASS

Verify the cable can withstand the prospective short circuit current for the duration of the protective device clearing time, per BS 7671, Regulation 434.5.2:

k²S² ≥ I²t — (Eq. 7)

For MICC cable, k = 135 (mineral insulation, copper conductors, per BS EN 60702-1 Table 5).

k²S² = 135² × 10² = 18,225 × 100 = 1,822,500 A²s

Prospective fault current at the fire pump location (85 m from switchboard):

I_f = U₀ / Z_s = 240 / 0.724 = 331 A

MCCB clearing time at 331 A (approximately 10× In): < 0.1 s (instantaneous magnetic trip).

I²t = 331² × 0.1 = 10,956 A²s

10,956 A²s < 1,822,500 A²s — PASS with large margin

A fire pump must start reliably under fire conditions. The starting current for a 15 kW DOL motor is typically 6× full load current:

I_start = 6 × I_b = 6 × 27.3 = 163.8 A — (Eq. 8)

Voltage drop at starting current:

ΔV_start = mV/A/m × I_start × L / 1000

ΔV_start = 4.4 × 163.8 × 85 / 1000

ΔV_start = 61.3 V

ΔV_start% = 61.3 / 415 × 100 = 14.8%

The motor terminal voltage during starting would be 415 − 61.3 = 353.7 V. Motor starting torque is proportional to voltage squared:

T_start / T_rated-start = (353.7 / 415)² = 0.727 — (Eq. 9)

The motor retains 72.7% of its rated starting torque. For a fire pump (centrifugal type, low starting torque requirement), this is adequate. IEEE 141 (Red Book) recommends a minimum of 80% terminal voltage for general motors, but centrifugal pumps can typically start at 70%.

To illustrate the danger, here is what happens if an engineer sizes this circuit using standard PVC cable data instead of MICC-specific data:

Wait — in this case the MICC cable actually has a higher base rating than PVC? Yes: for clipped direct installation, MICC cables benefit from the metallic sheath acting as a heat sink. The undersizing trap occurs with enclosed installations (conduit, trunking) where MICC has notably lower ratings.

Let’s recalculate for the alternative installation — cables in steel conduit in the riser (Installation Method B1):

For enclosed installations, an engineer substituting MICC for PVC at the same cross-section will overload the cable by up to 17%. Over months of continuous operation, this accelerated heating degrades the cable sheath and fixings — exactly the kind of slow-burn failure mode that the Grenfell investigation highlighted in other building systems.

Selected cable: 10 mm² MICC (mineral insulated copper clad), 3-core + CPC, protected by 32 A MCCB.

The governing factor is current-carrying capacity after the severe combined derating for ambient temperature and grouping in the riser shaft. The grouping factor alone (0.54 for 7 circuits) nearly halves the cable’s capacity, driving the cable two sizes above what running current alone would require.

The Grenfell Tower fire primarily demonstrated failures in building material regulation and fire safety management, not cable sizing. However, the principles this calculation illustrates are directly relevant to preventing electrical fire safety failures in high-rise buildings:

• Use MICC-specific current rating tables — never apply PVC/XLPE ratings to fire-resistant cables
• Account for riser shaft temperature — heat stratification in vertical shafts means ambient temperatures of 35–45°C, not the 30°C assumed by BS 7671 reference conditions
• Size for the grouping reality — riser shafts concentrate many circuits in close proximity; a grouping factor of 0.54 or lower is common
• Verify motor starting voltage drop separately — fire pump circuits must start reliably under fire conditions when the building electrical system may already be stressed
• Specify the fire rating — BS 8519 Category 3 (120 minutes) is the minimum for life-safety systems in high-rise residential buildings; Category 1 (30 minutes) is insufficient

Every worked calculation should be verified independently. Use the ECalPro cable sizing calculator to cross-check your results with automated derating factor lookup and multi-standard verification.