Oversized Cable Run on a Workshop Extension
You must supply a new workshop extension with a 32 A three-phase socket and lighting. The feed runs 48 m from the existing distribution board. The electrician proposes a 5 × 2.5 mm² copper cable, arguing that 32 A is within the tabulated current-carrying capacity for that cross-section under the installation method used. The extension has steel shelving, a concrete floor, and a metal workshop door frame. Measured ambient temperature in the roof void where the cable is clipped is 42 °C. The client also wants a second 16 A circuit for a welder used intermittently.
1. Evaluate whether the proposed 2.5 mm² cable is acceptable, explaining the two independent checks that must both be satisfied.
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Two checks apply and both must pass: current-carrying capacity (with derating) and voltage drop.
Capacity: 2.5 mm² may carry close to 32 A only under favourable reference conditions (specific installation method, 30 °C ambient, no grouping). Here the ambient is 42 °C, which requires a derating factor, and grouping with the second circuit may require a further factor. Applying typical derating, the effective capacity falls below 32 A, so the cable would be overloaded at full load. Protection must in any case not exceed the derated capacity of the cable.
Voltage drop: for a 48 m run at 32 A, 2.5 mm² copper gives a drop that is likely to exceed the commonly applied 3–5% limit for combined installation; the exact figure must be calculated, not guessed. A larger cross-section, typically 6 mm², is the usual remedy at this length and load.
Conclusion: reject the 2.5 mm² proposal. Recalculate with the actual installation method, ambient temperature and grouping, and select the cross-section that satisfies both capacity and voltage drop, then verify the protective device rating against the result.
2. The metal shelving and door frame are within reach of the socket circuits. What protective measure issue arises, and how do you resolve it?
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The issue is protective equipotential bonding of extraneous conductive parts. Steel shelving, a metal door frame and similar structural metal that is not part of the electrical installation but can introduce earth potential are extraneous conductive parts. If they are simultaneously accessible with exposed conductive parts or with other extraneous parts, they must be included in the protective equipotential bonding, connected to the main earthing terminal by a suitably sized conductor.
Procedure: (1) identify all extraneous conductive parts in the extension, including pipework and structural steel; (2) verify whether they are already reliably connected, e.g. via the steel structure, and test continuity; (3) install main bonding conductors of the required minimum cross-section to the main earthing terminal; (4) where the welder circuit is concerned, consider supplementary bonding or a residual current device as additional protection, since welding equipment and metal surroundings raise the risk; (5) document the test results, including continuity and insulation resistance, in the handover report.
Do not rely on the metal parts being 'earthed by the concrete'. Bonding must be verified by measurement, not assumed.