Bunched cables in thermal insulation
You are wiring a small extension containing a utility room and a shower room. Four final circuits leave a new 12-way consumer unit through a 300 mm section of wall that has been filled with mineral wool: a 2.5 mm² T&E 20 A radial socket circuit, a 1.5 mm² T&E 10 A lighting circuit, a 2.5 mm² T&E 16 A immersion heater circuit and a 1.5 mm² T&E 6 A extractor circuit. All four cables are bunched together where they pass through the insulation, and the ambient temperature in the loft space is 35°C. The manufacturer's data for the 2.5 mm² cable gives: tabulated current-carrying capacity clipped direct 27 A; Ca for 35°C 0.94; Ci for one side in thermal insulation 0.75; Cg for four bunched circuits 0.65. The socket circuit design current is 16 A and it is protected by a 20 A MCB. The client wants to know whether the existing 2.5 mm² cable can be reused for the socket circuit.
1. Calculate the effective current-carrying capacity of the 2.5 mm² cable and decide whether the 20 A MCB is acceptable.
Read the worked answer
Multiply the tabulated capacity by all applicable factors: It = 27 × 0.94 × 0.75 × 0.65. 27 × 0.94 = 25.38; × 0.75 = 19.035; × 0.65 = 12.37 A. For the circuit to be acceptable, the design current, protective device rating and cable capacity must satisfy Ib ≤ In ≤ Iz. Here Ib = 16 A and In = 20 A, but Iz = 12.37 A. The 20 A MCB is not acceptable, and neither is a 16 A MCB, because 16 A is greater than 12.37 A. The cable would be overloaded in the insulated and grouped section even though it is fine for the rest of the run. The 2.5 mm² cable cannot be reused for this 16 A socket circuit under these conditions.
2. If the cable is changed to 4 mm² with a tabulated capacity of 36 A, what protective device would you select and why?
Read the worked answer
Apply the same factors: 36 × 0.94 × 0.75 × 0.65 = 16.5 A. A 16 A MCB satisfies 16 A ≤ 16 A ≤ 16.5 A, so it is acceptable, but the margin is small. A 20 A MCB would not be acceptable because 20 A is greater than 16.5 A. I would confirm the actual installation conditions, because a slightly different Ci or Cg could reduce Iz below 16 A. If the client needs a 20 A circuit, the grouping or insulation must be improved, for example by spacing the cables, using a larger enclosure, or rerouting outside the insulation. The 4 mm² cable with a 16 A MCB is the better design.
3. Explain how grouping and thermal insulation affect cable temperature, and give two practical remedies.
Read the worked answer
Both reduce the cable's ability to dissipate heat. Thermal insulation surrounds the cable with air pockets that trap heat, so the conductor runs hotter for a given current. Grouping places several warm cables close together, so each cable receives heat from its neighbours and the surrounding air is hotter. The derating factors are applied together because the effects compound. Remedies: separate the cables so they are not bunched, or route them outside the insulation in a ventilated enclosure; use a cable with a higher tabulated capacity or a larger cross-section; reduce the number of circuits in the insulated section; or use a suitable conduit or trunking that maintains an air gap. The protective device must still satisfy Ib ≤ In ≤ Iz after the remedy.