Electric Shower Installation: 8.5kW to 10.8kW Circuit Sizing, RCD Protection, Minimum Water Pressure and Part P Notification

Quick Answer: An electric shower requires a dedicated radial circuit sized to the unit's wattage — 8.5kW needs a 40A circuit with 6mm² cable, 9.5kW and 10.8kW need a 45A or 50A circuit with 10mm² cable. All shower circuits must be RCD-protected, the shower unit and associated pipework require supplementary bonding, and the installation is notifiable under Part P of the Building Regulations unless carried out by a registered competent person.

Summary

Electric showers heat water instantaneously as it passes over a heating element, which means they draw very high currents during operation. A 10.8kW shower running on a 230V supply draws approximately 47A — close to the cable and overcurrent device limits. Getting the circuit sizing wrong doesn't just trip the breaker; undersized cable overheats inside the wall, creating a genuine fire risk behind surfaces where nobody can see it.

Unlike mixer showers and power showers, an electric shower draws from the cold mains supply and requires no hot water supply. This makes them especially common in en-suites, loft conversions, and properties with limited hot water capacity. The popularity of higher-wattage units (10.8kW is now standard) means many older 8.5kW installations are being upgraded, which almost always requires the circuit to be replaced entirely.

Part P of the Building Regulations requires that most electrical installation work in dwellings is either carried out by a registered competent person (who self-certifies) or notified to the local authority building control before work starts. Shower circuits fall squarely within this scope. Failing to notify or use a registered electrician means the installation has no certification, which causes problems when the property is sold.

Key Facts

Quick Reference Table

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Shower Wattage Current Draw (230V) Min Cable Size Min MCB/RCBO Rating Notes
7.5kW 32.6A 6mm² 32A Older units, rarely specified now
8.5kW 37A 6mm² 40A Check cable run length and derating
9.5kW 41.3A 10mm² 45A 6mm² marginal — use 10mm² to be safe
10.8kW 47A 10mm² 50A 6mm² not acceptable at this rating
12.5kW (commercial) 54.3A 10mm² 63A Commercial installations only

Detailed Guidance

Cable Sizing and Derating

The published current ratings for cable assume it is installed in a specific reference method (e.g., clipped to a surface, in conduit, buried in plaster). When cable passes through insulation — common in loft conversions or cavity walls — the rating drops significantly. A 6mm² cable clipped directly to a surface carries 47A, but the same cable running through 100mm of mineral wool insulation is derated to around 27A, which is totally inadequate for even an 8.5kW shower.

Before confirming cable size, establish the installation method along the entire cable route. If any part of the run passes through thermal insulation, upgrade to 10mm². The extra material cost is trivial compared to the liability of an undersized installation.

For long cable runs, also check the voltage drop. BS 7671 limits voltage drop on final circuits to 3% of the nominal voltage (6.9V on a 230V circuit). On a 10.8kW shower drawing 47A over a 15m run, 10mm² cable drops approximately 5.2V — acceptable. The same run in 6mm² would drop around 8.7V — not acceptable.

RCD Protection

All shower circuits must be protected by a 30mA RCD. In a modern consumer unit this is typically achieved with:

Type A RCDs detect both AC and pulsating DC fault currents. Type F or Type B are not required for electric showers specifically, but may be needed if the consumer unit also supplies an EV charger or inverter.

The RCD trip time at 30mA must not exceed 300ms under BS 7671; for additional protection, trip time at 150mA must not exceed 40ms. Standard 30mA RCDs meet this requirement.

Bathroom Zones Explained

The bathroom zone system under BS 7671 divides the bathroom into areas based on proximity to water:

Electric shower units are typically installed in Zone 1 and must have a minimum of IPX4 protection. No switches, socket outlets, or mains-voltage accessories can be within Zone 1. The cord pull switch or ceiling switch must be accessible from outside Zone 1. The DP isolator for the shower (45A) should be positioned outside Zone 1 where possible, or in Zone 2 with IP44 minimum rating.

Supplementary Bonding

Main protective bonding connects all extraneous conductive parts (gas, water, and oil pipes) at the point of entry to the property. Supplementary bonding within bathrooms is required where the main bonding cannot guarantee that any metalwork in the bathroom is at the same potential.

For an electric shower, supplementary bonding is needed between:

Bonding conductors must be minimum 4mm² copper. If all pipework entering the bathroom is plastic throughout, and is connected to the main supply by plastic throughout, supplementary bonding to pipework is not required — but the shower earth terminal still needs to be connected to the circuit protective conductor (CPC).

Part P Notification Process

Installing a new circuit (which a new electric shower always requires) is notifiable under Building Regulations Part P. There are two routes:

  1. Competent person self-certification — Use a registered electrician (NICEIC, NAPIT, ELECSA, STROMA, etc.). They inspect, test, certify, and notify the local authority on your behalf. The homeowner receives a BS 7671 Electrical Installation Certificate (EIC).

  2. Local authority building control (LABC) route — Submit a Building Regulations application before starting work. An LABC-approved inspector will inspect on completion. Higher cost and longer lead time, typically used only when an unregistered person is doing the work.

There is no 30-day grace period for notifiable electrical work in dwellings — notification must occur before completion of the installation. The EIC is a legally required document and should be retained by the homeowner; it will be requested by solicitors during property conveyancing.

Minimum Water Pressure Requirements

Electric showers are sensitive to low mains water pressure. Below the minimum dynamic pressure, the flow switch inside the unit fails to activate, or the unit cuts out under thermal overload because insufficient water flows over the element.

Typical minimum specifications:

Where mains pressure is borderline, use a flow meter to measure dynamic pressure at the shower supply point under flow conditions. Do not test with a Bourdon gauge at rest — this gives static pressure only and will overstate actual available pressure.

Properties on private water supplies, rural mains networks, or at the end of long supply runs often have low dynamic pressure. If pressure is insufficient, a whole-property pressure-boosting pump may be required, which adds complexity and cost that should be included in the quote.

Frequently Asked Questions

Can I reuse the old shower circuit when upgrading from 8.5kW to 10.8kW?

Almost certainly not. An 8.5kW shower circuit is typically wired in 6mm² cable with a 40A MCB. A 10.8kW shower draws 47A — more than the 40A overcurrent device and at the upper limit of 6mm² cable even before derating. You will need to replace the cable (usually 10mm²), the MCB (45–50A), and test the whole circuit. Budget for a full circuit replacement, not a like-for-like swap.

Does the shower need a separate earth rod?

No. The shower connects to the circuit protective conductor (CPC) in the circuit cable. Supplementary bonding to pipework is required (see above) but this connects to the circuit CPC, not to an earth rod. Earth rods are part of the earthing arrangement at the consumer unit level and are not fitted for individual appliances.

My shower cuts out after 5–10 minutes. Is it the circuit or the shower?

The most common cause is thermal cutout inside the unit triggered by low water flow or pressure. Check dynamic water pressure under flow conditions. Other causes include loose terminations causing volt drop, a failing heating element, or a marginal MCB that trips on thermal overload before the magnetic trip. Test the circuit voltage at the shower terminals under load to identify a volt-drop issue.

Is a pull-cord switch always required?

A double-pole isolating switch must be provided for safe isolation of the shower. In bathrooms and shower rooms, this must be outside Zone 1. Ceiling-mounted pull-cord switches (typically 45A DP) are the standard solution. If the shower room has a high enough ceiling that a wall-mounted switch outside Zone 1 is practical, a faceplate switch is permitted — but it must be at least 600mm from the shower. In practice, a pull-cord switch is almost always the simplest compliant solution.

Regulations & Standards