What cable size do I need for an electric shower?

For most electric showers: up to 7kW needs 6mm² cable with 32A MCB, 7.5-8.5kW needs 6mm² cable with 40A MCB, 9-9.5kW needs 10mm² cable with 40A or 45A MCB, 10-10.5kW needs 10mm² cable with 45A MCB, and 10.8kW+ needs 16mm² cable with 50A MCB. These assume PVC twin and earth cable clipped direct (Reference Method C) at 30°C ambient. Always verify voltage drop for your cable run length.

Does an electric shower need a dedicated circuit?

Yes — BS 7671 requires electric showers to be on a dedicated radial circuit from the consumer unit. The circuit must have its own MCB (not shared with any other load) and 30mA RCD protection. The shower must also be controlled by a double-pole isolating switch — typically a 45A or 50A ceiling-mounted pull cord switch located outside the bathroom zones but within reach from inside the bathroom.

How does voltage drop affect shower performance?

Excessive voltage drop reduces the power delivered to the shower — a 5% voltage drop on a 9.5kW shower reduces actual power to about 9kW, which means lower water temperature at the same flow rate. BS 7671 limits voltage drop to 5% for power circuits (11.5V on 230V). For long cable runs (15m+), you may need to increase cable size beyond the minimum for current capacity to stay within the voltage drop limit.

Can I upgrade my shower kW rating without re-cabling?

It depends on the existing cable size and length. If you have 10mm² cable on a 40A MCB, you can go up to about 9.2kW. Upgrading from 8.5kW to 9.5kW on the same cable is usually fine if the run is short. But going from 9.5kW (41A) to 10.8kW (47A) would need the MCB upgrading to 50A and likely a cable upgrade to 16mm². Always recalculate — never assume the existing cable is adequate for a higher-rated shower.

What type of MCB should I use for an electric shower?

Use a Type B MCB for electric showers — they trip at 3-5 times rated current. Type B is standard for resistive loads like showers. The MCB rating must be the next standard size above the shower's full load current: 7.5kW = 32A MCB, 8.5kW = 40A, 9.5kW = 45A, 10.8kW = 50A. An RCBO (combined MCB + RCD) is preferred as it provides both overcurrent and earth fault protection in one device.

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Electric Shower Calculator — Cable Size & MCB Rating

Calculate the correct cable size and MCB rating for an electric shower installation, including voltage drop check per BS 7671. Covers 7kW to 12kW showers.

Shower Power Rating

Check shower nameplate (7-12kW typical)

Cable Run Length

Consumer unit to shower unit

How We Calculate This

This calculator determines the MCB rating, cable size, and voltage drop for an electric shower circuit based on BS 7671 18th Edition requirements.

Formulas

Current (A) = Power (W) ÷ Voltage (V)

Voltage drop (V) = (mV/A/m × I × L) ÷ 1000

Typical Shower Circuits

7.5kW: 33A — 6mm² cable, 32A MCB
8.5kW: 37A — 10mm² cable, 40A MCB
9.5kW: 41A — 10mm² cable, 45A MCB
10.8kW: 47A — 16mm² cable, 50A MCB
Voltage drop limit: 5% (11.5V on 230V supply)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.