What cable size do I need for a 9.5kW shower?

A 9.5kW shower draws approximately 41A (9500W ÷ 230V). You need 10mm² cable for most installations. 6mm² is only suitable for very short runs under 10m with Method C installation. Always calculate voltage drop for your specific cable length.

Can I use 6mm² cable for an electric shower?

6mm² can handle up to 32A in Method C, which covers 7.5kW showers (33A). For 8.5kW+ showers, 6mm² is only suitable for very short runs where voltage drop permits. Most shower installations should use 10mm² to be safe and allow for future upgrades.

What MCB rating do I need for an electric shower?

MCB rating depends on shower power: 8.5kW needs 40A, 9.5kW needs 45A, 10.5kW+ needs 45A or 50A. The MCB must be Type B for domestic, combined with 30mA RCD protection. Never use a fuse - MCBs provide better protection for shower circuits.

Does an electric shower need RCD protection?

Yes, mandatory. All shower circuits require 30mA RCD protection under BS 7671. This can be via an RCBO (combined MCB/RCD), a separate RCD protecting the circuit, or a main switch RCD if it covers all circuits. RCD protection is essential for bathroom electrical safety.

Can I connect a shower to a ring main?

No, never. Electric showers must have a dedicated radial circuit from the consumer unit. They draw too much current for a ring main or spur. The circuit must be protected by its own MCB and RCD, with cable sized for the shower rating.

Why is my shower running lukewarm?

Lukewarm water often indicates voltage drop. If the cable is undersized or the run is too long, voltage at the shower drops below optimal, reducing heating element performance. Check cable size and length against shower rating. Upgrading cable size often solves this.

Can I upgrade from 8.5kW to 10.5kW shower?

Only if your existing cable and MCB can handle the higher load. 8.5kW uses 6mm²/40A, while 10.5kW needs 10mm²/45A minimum. If you have 6mm² cable, you cannot safely upgrade without recabling. Always check before upgrading shower units.

What is the maximum shower power for a UK home?

Most UK homes can support up to 10.8kW showers on a 100A supply. Beyond this, you may need DNO supply upgrade. Check your main fuse rating and existing load. Some older properties with 60A supplies may be limited to 8.5kW showers.

How do I calculate shower current from kW?

Current (amps) = Power (watts) ÷ Voltage. For UK single-phase: Amps = kW × 1000 ÷ 230. Examples: 8.5kW = 37A, 9.5kW = 41A, 10.5kW = 46A, 10.8kW = 47A. Our calculator does this automatically.

Do I need Part P notification for shower installation?

Yes, installing a new shower circuit is notifiable work under Part P of Building Regulations. It must be done by a registered competent person (Part P scheme member) or inspected by Building Control. This ensures the work meets electrical safety standards.

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Electric Shower Cable Sizing Calculator

Calculate the correct cable size for electric shower installations - 8.5kW to 10.8kW units

Electric Shower Cable Quick Facts

8.5kW shower = 37A, needs 6mm² (short runs) or 10mm² (longer)
9.5kW shower = 41A, typically needs 10mm² cable
10.5kW shower = 46A, requires 10mm² minimum
Always run dedicated radial circuit from consumer unit
45A or 50A MCB with 30mA RCD protection required

Cable Size Calculator

BS 7671:2018+A2:2022 compliant

1. Load Current

1kW ≈ 4.35A at 230V

2. Cable Length (meters)

3. Installation Method

Lighting circuit (3% voltage drop limit)

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Common Electric Shower Installations

8.5kW electric shower

40A MCB, 6mm² up to 14m or 10mm² for longer runs

9.5kW electric shower

45A MCB, 10mm² cable for most installations

10.5kW electric shower

45A MCB, 10mm² cable, check voltage drop on runs over 20m

10.8kW electric shower

50A MCB, 10mm² minimum, may need 16mm² for long runs

Replacing existing shower (like for like)

Check existing cable size supports new shower rating

Related Bathroom & Water Heating

Bathroom projects often include electric underfloor heating for added comfort. If you're also upgrading hot water, see our immersion heater cable calculator. For whole-house water heating upgrades, consider an air source heat pump.

Electric Shower Cable Sizing

Electric shower cable sizing determines the conductor cross-sectional area (mm²) needed to safely supply an instantaneous electric shower. UK showers typically range from 7.5kW to 10.8kW, drawing 33-47A. Most installations require 10mm² cable to handle the load and keep voltage drop under 5%. A dedicated radial circuit with appropriate MCB and RCD protection is mandatory.

Electric Shower Installation Costs (2024)

Typical UK installation costs including cable, labour, and consumer unit work. Excludes shower unit cost (£80-300).

Installation Scenario	Materials	Labour	Total
New 9.5kW shower, short run (<10m)	£60-100	£150-250	£210-350
New 9.5kW shower, medium run (10-18m)	£100-160	£200-300	£300-460
New 10.5kW shower, long run (18-25m)	£150-220	£250-350	£400-570
Consumer unit upgrade required	£150-300	£200-350	£350-650 (additional)
Full bathroom rewire with shower	£200-350	£400-600	£600-950

Prices as of 2024. Actual costs vary by region, complexity, and supplier.

Instantaneous vs Storage Showers

Electric showers are instantaneous - they heat water on demand as it flows through the unit. Higher kW ratings provide more hot water flow. Unlike storage water heaters, they draw maximum current continuously while in use, making correct cable sizing critical. The cable must handle full load current for extended periods without overheating.

Voltage Drop in Shower Circuits

Voltage drop is the reduction in voltage along the cable due to resistance. BS 7671 limits this to 5% (11.5V at 230V). Long shower cable runs with high current cause significant voltage drop, reducing shower performance. If voltage drop exceeds limits, you must use larger cable to compensate.

IP Rating for Bathroom Electrics

IP (Ingress Protection) ratings define protection against water. Shower isolator switches need minimum IP44 (splash-proof) if in Zone 2, or IP65 if in Zone 1. The shower unit itself is designed for wet conditions. All bathroom electrical work must comply with BS 7671 Section 701.

Frequently Asked Questions

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Voltage Drop Calculator

Check voltage drop for long shower cable runs

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Full BS 7671 cable sizing for all circuits

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