2.5 mm² socket circuit under insulation, bundled, with a new 10 A heater
You are asked to certify alterations at a three-year-old house where the owner has added a 2.4 kW (10 A) convection heater to an existing 20 A general-purpose circuit. The circuit is 2.5 mm² two-core plus earth TPS on a 20 A RCBO, feeding 11 double socket outlets through the kitchen and laundry. In the roof space the cable runs for about 6 m beneath 200 mm of blown-in insulation, and for 3 m it is clipped in a bundle with four other TPS circuits. On the day you attend, the roof space is 48 °C. The heater is plugged into one of the existing socket outlets. No test results exist from the original installation, and the owner does not want a new cable run if it can be avoided.
1. Assess whether the existing 2.5 mm² cable on a 20 A RCBO is adequate for these installation conditions. Show the derating approach rather than quoting a single number.
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Start from the tabulated value and apply every applicable factor - this is where the judgement lies, not in remembering a number.
2.5 mm² TPS (V-75) in free air is around 27 A in AS/NZS 3008.1.1. Now apply the conditions actually present:
- Ambient 48 °C: Ca ≈ 0.80–0.82 (40 °C is the reference; the factor falls steeply above it).
- Grouping of 5 circuits: Cg ≈ 0.75.
- Under 200 mm of blown-in insulation: the cable cannot dissipate heat as designed. Where a cable is surrounded by thermal insulation, AS/NZS 3000 requires it to be derated to suit the insulation's thermal resistance (or kept clear of it). Even a modest insulation factor of 0.7 is significant.
Iz ≈ 27 × 0.81 × 0.75 × 0.7 ≈ 11.5 A
Even ignoring the insulation entirely: 27 × 0.81 × 0.75 ≈ 16.4 A, which is already below the 20 A device rating.
Conclusion: the circuit does not comply at 20 A. The heater makes it clearly unsafe - a 10 A continuous appliance plus general-purpose socket load on a cable whose real capacity is around 11–16 A is a fire risk, and the 20 A RCBO will not protect the cable.
2. What options would you put to the owner, and which would you recommend?
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Options, in order of preference:
- New dedicated circuit for the heater (2.5 mm² on a 16 A MCB, or 20 A if the derating supports it), run clear of the insulation. This is the cleanest fix and removes the 10 A load from the socket circuit. Recommend this.
- Re-route the existing cable so it lies on top of the insulation or is clipped to the side of a joist rather than buried - restores most of the tabulated capacity, then re-assess the grouping factor.
- Reduce the RCBO to 16 A if the connected load and socket-outlet requirements allow - this may bring the device rating within the derated cable capacity, but it does not fix the heater load and may cause nuisance tripping.
- Increase cable size (4 mm²) for the section that must stay under the insulation.
- Split the socket circuit into two circuits to reduce the grouping factor.
Note also the plug: 2.4 kW at 240 V is 10 A, at the limit of a 10 A outlet for a continuous appliance. A fixed-wired connection or a 15 A circuit is better practice for a fixed heater.
Whatever is chosen, the installation must be re-assessed with the final factors and re-tested.
3. What testing and documentation must you complete before the heater circuit is left in service?
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Tests (after the alteration, before energising and before handover):
- Visual inspection - sheath condition, support, clearances, correct colour coding, cable not damaged by insulation.
- Continuity of protective earthing conductors and equipotential bonding.
- Insulation resistance at 500 V DC with loads disconnected (minimum 1 MΩ).
- Polarity and correct circuit connections, including at each socket outlet.
- Earth-fault loop impedance at the furthest point, checked against the device's maximum Zs.
- RCD operation: no trip at 50% residual current, trip between 15 mA and 30 mA, trip time under 300 ms, plus the integral test button.
- Functional check of the heater and its control.
Documentation: results recorded on the Certificate of Test and Compliance, the Electrical Work Request lodged with the Electrical Safety Office for the notifiable work, and a copy given to the owner. If the derated capacity cannot be made to suit, the circuit must be left de-energised or made safe and the owner advised in writing.