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.
The short answer
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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01 · Installation Guide
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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02 · Installation Guide
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.
03 · Installation Guide
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): 10mm² on 40A MCB — 32.6A is above a 32A device, so Ib ≤ In fails on a 32A MCB at 230V
8.5kW (37A): 10mm² on 40A MCB
9.5kW (41.3A): 10mm² on 45A MCB
10.5kW (45.7A): 50A device (45.7A is above a 45A MCB) — 10mm² or 16mm² depending on the reference method and correction factors
10.8kW (47A): 16mm² on 50A MCB. A 12kW unit (52.2A) needs a 63A device
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.
In the app
Electric shower wiring guide: cable and MCB size by kW rating, RCD and bathroom zone requirements, and isolator placement to BS 7671.
04 · Installation Guide
Free to use, no sign-up and no email required. Enter the shower’s design current, the run length and the installation method, and the calculator returns the cable size, the corrected current-carrying capacity and the voltage drop.
For the design current, divide the shower rating by the voltage: a 9.5kW shower is 9500 ÷ 230 = 41.3A. Use the manufacturer’s declared current where it differs — many units are rated at 240V. The quick-reference sizes above assume no adverse correction factors; use the calculator where the cable is grouped, runs through thermal insulation, or sits in a warm loft.
Professional cable sizing with BS 7671 compliance validation
The cable is sized on In, not on Ib (Reg 433.1.1: Ib ≤ In ≤ Iz). A BS 3036 semi-enclosed fuse also applies Cf = 0.725 per Appendix 4 §5.1.1(c)(i).
Standard: 30°C
Affects current rating
Lighting: 3%, Power: 5%
1.0 = 100% simultaneous load
Voltage drop, App 4 §6.2. Resistive 1.0, inductive 0.8-0.9
Empty: Design Current (A), Cable Length (m).
Cable sizing depends on multiple factors beyond current rating alone:
Always consult relevant electrical codes and standards for your specific application.
05 · Installation Guide
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.
06 · Installation Guide
Reg 462.2 requires every circuit to be provided with a means of isolation for all live conductors, and Reg 537.2.7 requires each isolating device to be clearly identified by position or durable marking. Reg 462.3(c) gives location adjacent to the associated equipment as one way of guarding against inadvertent closure, which is why a local double-pole switch is the normal arrangement for a shower. Note the exception in Reg 461.2: on a TN-S or TN-C-S system, isolation of the neutral is not required where protective equipotential bonding is installed and the neutral is reliably connected to Earth — but double-pole remains the UK norm and is required on a TT system.
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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07 · Installation Guide
Electric shower circuits require 30mA RCD protection under BS 7671. The specific regulations are:
When testing the completed installation, the RCD must be tested with a single alternating current test at its rated residual operating current (IΔn = 30mA). BS 7671:2018+A4:2026 deleted Appendix 3 Table 3A, so the old ½x / 1x / 5x sequence is no longer the verification basis. Regardless of RCD Type, effectiveness is deemed verified where the device disconnects within 300ms at IΔn (Reg 643.8 for additional protection; Reg 643.7.1 for automatic disconnection). A delay ‘S’ type RCD must operate between 130ms and 500ms. The 40ms figure comes from BS EN 61008 product testing, not from BS 7671.
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.
08 · Installation Guide
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.
09 · Installation Guide
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.
10 · Installation Guide
In the app
7.5kW shower needs 10mm² cable minimum. Calculate exact sizing, earthing, and MCB ratings for safe installation to regs.
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