Adding a Shower Circuit to an Older Dwelling
A 1970s semi-detached house has a consumer unit with rewireable fuses, a 30 mA RCD added only to the socket circuits, and a TN-C-S supply with a declared external loop impedance (Ze) of 0.35 Ω. The owner wants a 9.5 kW electric shower installed in an upstairs bathroom. The existing main protective bonding to the water and gas services is 6 mm² copper, run in a single length of about 12 m. The proposed shower circuit is 10 mm² twin-and-earth with a 45 m route, clipped direct through a cold loft and buried in insulated plasterboard for the last 4 m. The installer proposes a 40 A MCB and local isolation by a pull-cord switch.
1. Identify the design and compliance issues the installer must resolve before this shower circuit can be certified, and state what information is still missing.
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Load and cable sizing. 9.5 kW at 230 V is about 41.3 A, so a 40 A MCB is undersized for the appliance's rating; a 45 A or 50 A device is needed. 10 mm² clipped direct has a tabulated capacity around 63–65 A, but the 4 m buried in insulated plasterboard must be derated (method A or B, with derating for thermal insulation), and grouping/ambient in the loft must be checked. The design current, protective device rating, and corrected cable capacity must satisfy Ib ≤ In ≤ Iz.
Protection against electric shock. The bathroom is a special location. All circuits in the location, and the protective conductors, need 30 mA RCD protection (or supplementary equipotential bonding as an alternative for the shower itself, but RCD is the practical route). The existing RCD covers only sockets, so the shower must be placed on an RCD-protected way, or a new 30 mA RCBO used.
Earthing and bonding. With TN-C-S and Ze 0.35 Ω, the earth fault loop impedance must be verified, including the circuit's R1+R2, against the maximum permitted Zs for the chosen device. The 6 mm² main bonding is likely inadequate: for TN-C-S, main protective bonding conductors should be at least 10 mm² copper (or equivalent), and the gas/water intake sizes must be confirmed. Supplementary bonding in the bathroom may be required depending on whether all circuits are RCD-protected and whether extraneous-conductive-parts are reliably connected.
Isolation and switching. A pull-cord switch is acceptable as local isolation, but it must be double-pole and correctly rated and located outside zone 1/2 reach where required.
Missing information: earthing system confirmation, actual Ze measurement, R1+R2 of the proposed route, the cable's installation method and grouping, the shower manufacturer's supply requirements, and the condition/adequacy of the existing consumer unit and main switch.
2. The installer argues that because the shower is a fixed appliance and the bathroom has no socket outlet, no RCD is needed. Evaluate this argument.
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The argument is wrong for a modern installation. In a bathroom, BS 7671 requires that all low-voltage circuits serving the location are protected by 30 mA RCDs, and this includes the shower circuit, not just socket outlets. The presence or absence of socket outlets is irrelevant to the requirement for the shower itself. Additionally, cables buried in walls at a depth less than 50 mm without earthed metallic covering or mechanical protection require 30 mA RCD protection - the 4 m run in insulated plasterboard is a candidate. The pull-cord switch does not remove the need for RCD protection. The correct approach is to provide 30 mA RCD protection for the shower circuit, either by a dedicated RCBO or by relocating the circuit onto the existing RCD-protected section, and to verify disconnection times after the change.