Calculate cable size, MCB rating, and full circuit specification for electric shower installations to BS 7671
Calculate cable size, MCB rating, and full circuit specification for electric shower installations per BS 7671
An electric shower is typically the single highest-demand appliance in a UK home. It requires a dedicated radial circuit from the consumer unit with correctly sized cable, appropriate overcurrent protection, and mandatory RCD protection. Getting this design right is critical for both safety and performance.
The design current for an electric shower is calculated using the simple resistive load formula at the UK nominal voltage of 230V:
| Shower Rating | Design Current | Minimum MCB | Typical Cable |
|---|---|---|---|
| 7.2kW | 31.3A | 32A | 6mm² T&E |
| 8.5kW | 37.0A | 40A | 6mm² T&E |
| 9.5kW | 41.3A | 45A | 10mm² T&E |
| 10.5kW | 45.7A | 45A | 10mm² T&E |
| 10.8kW | 47.0A | 50A | 10mm² T&E |
Current ratings based on BS 7671 Table 4D5, Method C (clipped direct). Always verify for your specific installation method.
Check your main fuse rating before installing a high-power shower. A 10.5kW shower draws 45.7A, which is a significant portion of a 60A main fuse. If the shower runs simultaneously with a cooker and kettle, the main fuse may blow. Properties with a 60A supply should typically limit shower rating to 8.5kW, or upgrade the main fuse to 80A/100A via the DNO.
Cable selection for shower circuits depends on the shower power rating, cable length, and installation method. The cable must satisfy both current-carrying capacity and voltage drop requirements.
Marginally adequate for 9.5kW (41.3A vs 47A capacity) - leaves little safety margin especially if cable passes through any insulation.
Recommended as standard for all new shower installations above 8.5kW. Provides good margin for future-proofing.
BS 7671 Regulation 525 limits voltage drop to 5% of nominal voltage for non-lighting circuits. At 230V, this means a maximum of 11.5V drop. Excessive voltage drop results in reduced power at the shower element and poor water temperature.
| Shower | Max Run (6mm²) | Max Run (10mm²) |
|---|---|---|
| 7.5kW (32.6A) | 48m | 80m |
| 8.5kW (37.0A) | 43m | 71m |
| 9.5kW (41.3A) | 38m | 63m |
| 10.5kW (45.7A) | 34m | 57m |
| 10.8kW (47.0A) | 34m | 56m |
Maximum cable lengths for voltage drop compliance only. Earth fault loop impedance (Zs) may limit actual length further.
Many properties have existing 6mm² shower circuits installed when 7kW showers were standard. While 6mm² cable is rated at 47A (clipped direct), this is marginal for 10.5kW showers (45.7A) with zero safety margin. If the cable passes through any thermal insulation, it becomes immediately non-compliant. Professional guidance is to re-wire in 10mm² when upgrading to 8.5kW or higher.
Bathrooms are classified as "Special Locations" in BS 7671 because the presence of water significantly increases the risk of electric shock. Section 701 sets specific requirements for all electrical installations in these locations.
Regulation 701.411.3.3 requires all circuits in a bathroom to be protected by a 30mA RCD. For shower circuits, the best practice is:
Amendment 3 note: BS 7671:2018+A3:2024 requires verification that RCBOs are compatible with the consumer unit's power flow direction (unidirectional vs bidirectional) - particularly important in properties with solar PV or battery storage.
Supplementary bonding in a bathroom connects together all metallic pipework and earth conductors to prevent dangerous touch voltages. It may be omitted if all three conditions are met:
If any condition is not met, 4mm² supplementary bonding conductors must be installed between all metallic pipework, bath/shower tray (if metallic), radiators, and circuit CPCs within the bathroom.
Every shower circuit requires a local means of isolation. The following options are standard in UK installations:
Most common in UK bathrooms. Must be 45A or 50A rated, double-pole (isolates both live and neutral), with neon indicator. Positioned outside Zone 1 at ceiling height. The pull cord can extend into the shower area.
Must be located outside the bathroom entirely or in the Outside Zone (beyond Zone 2), at least 0.6m from the bath or shower tray edge. Less common but acceptable.
Common fault: Using a 6A or 10A lighting-style pull cord for a shower circuit. This is dangerous - a dedicated 45A or 50A rated shower pull cord switch is required.
BS 7671 Section 701 defines zones in bathrooms based on water exposure risk. Each zone has minimum IP (Ingress Protection) rating requirements for electrical equipment.
Inside the bath tub or shower tray. Only SELV (Safety Extra-Low Voltage) equipment rated IPX7 (immersion-proof) permitted. Maximum 12V AC or 30V DC.
Above the bath/shower tray to 2.25m from floor. This is where the shower unit is located. Minimum IPX4 required (splash-proof). Equipment must be suitable for the zone - the shower unit itself is designed for this.
Extends 0.6m horizontally beyond Zone 1, up to 2.25m from floor. Minimum IPX4 required. Shaver sockets (with isolation transformer) are permitted here.
Beyond Zone 2. No specific IP rating required by BS 7671, but good practice suggests IPX4 or IP20 minimum due to bathroom humidity. Standard accessories and switches can be located here.
A 9.5kW shower draws 41.3A at 230V. While 6mm² cable is rated at 47A (clipped direct), the preferred choice is 10mm² twin & earth with a 45A Type B MCB/RCBO. This provides adequate safety margin, allows for longer cable runs without voltage drop issues, and future-proofs the circuit if the homeowner later upgrades to a 10.5kW unit.
Technically, 6mm² cable has a 47A rating when clipped direct, which covers the 45.7A demand of a 10.5kW shower. However, this leaves only 1.3A margin. If the cable passes through any thermal insulation, even briefly, the capacity drops significantly and becomes non-compliant. Professional practice strongly recommends 10mm² cable as minimum for 10.5kW showers.
Yes, 30mA RCD protection is mandatory for all circuits in a bathroom under BS 7671 Regulation 701.411.3.3. The absence of RCD protection on a shower circuit is classified as a C2 code (Potentially Dangerous) during an EICR inspection, requiring urgent remedial action. A Type A RCBO is the recommended solution.
The MCB (or RCBO) must be rated at or above the design current of the shower. Common pairings: 7.5kW = 32A, 8.5kW = 40A, 9.5-10.5kW = 45A, 10.8kW = 50A. Always use Type B characteristic for resistive loads like showers.
Yes. Under Part P of the Building Regulations (England and Wales), all electrical work in a bathroom is notifiable. This includes new shower installations, circuit modifications, and even upgrading a shower pull cord switch. Work must be carried out by a Competent Person Scheme member (NICEIC, NAPIT, etc.) or notified to Local Authority Building Control before starting.
Maximum cable length is limited by two factors: voltage drop (max 5%) and earth fault loop impedance (Zs). For a typical 10.5kW shower using 10mm² cable with TN-C-S earthing, the practical maximum is approximately 40-55m depending on Ze and MCB rating. Use the calculator above to determine the exact limit for your specific installation.
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