Electric showers draw up to 52 amps — making the shower circuit one of the most demanding in a domestic installation. This guide covers cable sizing for every kW rating, MCB selection, pull cord vs ceiling switch options, RCD protection, bathroom zone requirements, Part P notification, and the installation mistakes that result in EICR defects.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
It depends on the shower rating and the install conditions. As a guide, an 8.5 to 9.5kW shower draws roughly 37 to 41A and is commonly wired in 10mm² twin and earth on a 40A or 45A breaker; a 10.5kW unit (around 46A) typically needs a 50A device. The correct size always depends on the cable route, grouping and insulation — size it from the design current and apply correction factors (BS 7671 Reg 433.1.1).
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An electric shower heats cold mains water instantaneously using a high-power heating element. Because of the power required, electric showers are among the highest-current fixed appliances in a domestic installation — a 9.5kW shower draws over 41 amps. This means the shower must be supplied by a dedicated radial circuit from the distribution board, with cable and protection sized specifically for the shower's kW rating.
The circuit comprises four main elements: the MCB or RCBO at the distribution board, the supply cable from the board to the bathroom, a double-pole isolator switch (typically a ceiling-mounted pull-cord), and the cable from the isolator to the shower unit itself. Each element must be correctly rated for the shower's full load current.
Electric shower installation in a bathroom is notifiable work under Part P of the Building Regulations. It must be carried out by an electrician registered with a competent person scheme or notified to Building Control before the work starts. An Electrical Installation Certificate must be issued on completion.
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Electric showers are available in a range of kW ratings. The higher the kW rating, the more water the shower can heat to a comfortable temperature — and the more current it draws. Here are the common ratings and their approximate current demand at 230V:
When advising customers, recommend sizing the cable for a potential future upgrade. If the customer installs an 8.5kW shower now, they may want to upgrade to a 9.5kW or 10.5kW model in a few years. Installing 10mm² cable from the start avoids having to replace the cable later.
The cable size depends on the shower rating, the installation method, and the applicable correction factors. The cable must carry the full load current continuously — diversity does not apply to electric shower circuits because the full load operates whenever the shower is in use.
7.5kW (32.6A): 6mm² on 32A MCB or 10mm² on 40A MCB
8.5kW (37A): 10mm² on 40A MCB
9.5kW (41.3A): 10mm² on 45A MCB
10.5kW (45.7A): 10mm² on 45A MCB or 16mm² on 50A MCB
10.8kW+ (47A+): 16mm² on 50A MCB
These sizes assume standard installation conditions with no adverse correction factors. If the cable is grouped with other cables, passes through thermal insulation, or is installed in a high ambient temperature, apply the correction factors from Appendix 4 of BS 7671. After applying correction factors, the cable's adjusted current-carrying capacity must still exceed the MCB rating.
Check voltage drop for the actual cable length and design current. Shower circuits often have longer cable runs because the distribution board may be on the ground floor and the shower on the first floor or in a loft conversion. A 20-metre run of 10mm² at 41.3A gives a voltage drop of approximately 6.1V — within the 11.5V limit, but check your specific run length.
Electric shower wiring guide: cable and MCB size by kW rating, RCD and bathroom zone requirements, and isolator placement to BS 7671.
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The MCB (or RCBO) at the distribution board must be rated to carry the full load current of the shower without tripping, while providing overcurrent protection for the cable.
In a modern RCBO consumer unit, the shower circuit gets a dedicated RCBO combining overcurrent and 30mA RCD protection in a single device. This is the simplest and most reliable arrangement.
BS 7671 requires a means of local isolation for the shower circuit — a double-pole switch that disconnects both the line and neutral conductors. This switch must be accessible to the person using the shower and must be clearly identifiable.
The pull-cord ceiling switch is the standard choice for most UK bathroom installations because it can be positioned inside the room at ceiling height, which is outside the defined bathroom zones. A wall-mounted switch must be at least 0.6 metres horizontally from the edge of the bath or shower tray (outside Zone 2) — in small bathrooms, this often means mounting it outside the room entirely.
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Start free trialElectric shower circuits require 30mA RCD protection under BS 7671. The specific regulations are:
When testing the completed installation, the RCD must be tested at rated residual operating current (30mA) and at 5x (150mA). The trip times must be within 300ms at 1x and 40ms at 5x.
A4:2026 — Reg 411.3.4 (lighting circuits): Within domestic premises, AC final circuits supplying luminaires must also have 30mA RCD additional protection. When installing a new shower circuit in an older home, check whether the existing bathroom lighting circuit is RCD-protected. An unprotected lighting circuit is a C2 observation on an EICR under current regulations and may require remedial work as part of associated works.
Installing a new electric shower circuit is notifiable work under Part P of the Building Regulations (England and Wales). This is because the work involves a new circuit in a special location (a room containing a bath or shower).
Simply replacing a shower unit on an existing circuit (like-for-like replacement) is not notifiable, provided no new wiring is installed and the circuit is not altered. However, if the new shower has a higher kW rating and the cable needs upgrading, that becomes a new circuit and is notifiable.
Bathroom zone requirements under BS 7671 Section 701 define where electrical equipment can be installed relative to baths and showers. The three zones are:
Where there is no shower basin or tray — for example a walk-in wet room with a floor drain — Zone 2 is absent. Instead, Zone 1 extends to a vertical surface at 1.20 m horizontally from the centre of the fixed water outlet on the wall or ceiling (Reg 701.32.3). Electricians sometimes apply bath-type zone dimensions to wet rooms; the correct reference point is the fixed outlet, not the room edge.
Socket outlets (Reg 701.512.3): Standard socket outlets (other than SELV sockets per Section 414 and BS EN 61558-2-5 shaver units) are prohibited within 2.50 m horizontally from the boundary of Zone 1. Measuring from the wrong reference point — particularly in wet rooms where Zone 1 is defined from the outlet centre rather than a tray edge — is a common EICR defect.
Outside all zones, standard accessories can be used. The distribution board and MCB are always outside the bathroom and outside all zones. Cable routes should avoid Zones 0 and 1 where possible, and cables in Zone 1 should be routed vertically to the shower unit from above or horizontally from behind the wall.
7.5kW shower needs 10mm² cable minimum. Calculate exact sizing, earthing, and MCB ratings for safe installation to regs.
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