Electric Shower Calculator — Cable Size & MCB Rating

Trade-standard sizing (the MCB is the next standard size above the full-load current, In ≥ Ib): up to 7kW needs 6mm² cable with a 32A MCB, 7.5-9kW needs 10mm² cable with a 40A MCB, 9.5-10kW needs 10mm² cable with a 45A MCB, and 10.8kW needs 16mm² cable with a 50A MCB. Larger units (11.6kW+, over 50A) step up to a 63A MCB on 16mm². The 40A and 45A bands split at 9.2kW (40.0A) — a 9kW shower draws 39.1A so it sits on a 40A MCB, while 9.5kW draws 41.3A and needs 45A. Although 6mm² is rated 47A clipped direct (Reference Method C), showers of 8kW and above are standardly run in 10mm² because the cable usually passes through loft insulation or above a ceiling, where Reference Method 100 derates 6mm² to 27A (insulation up to 100mm) and Method 101 to 22A (insulation over 100mm) — leaving no overload margin against a 40A or 45A MCB. Always verify voltage drop with our Voltage Drop Calculator, and use our Cable Sizing Calculator for exact sizing.

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.

Excessive voltage drop reduces the power delivered to the shower. A shower element is a fixed resistance, so its power scales with the square of the supply voltage — a 5% voltage drop leaves the element at 95% of nominal voltage, giving 0.95² ≈ 90% of rated power, so a 9.5kW shower delivers about 8.6kW. That means a 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.

It depends on the existing cable size and length. A 40A MCB caps the load at 40A, which is about 9.2kW at 230V (9200 ÷ 230 = 40.0A) — that is the absolute ceiling with zero margin, so in practice keep below it and confirm the MCB is the next size above the new full-load current (BS 7671 Reg 433.1.1, In ≥ Ib). Upgrading from 8.5kW to 9.5kW on existing 10mm² cable is usually fine if the run is short. But going from 9.5kW (41A) to 10.8kW (47A) needs the MCB upgrading to 50A and a cable upgrade to 16mm². Always recalculate — never assume the existing cable is adequate for a higher-rated 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: 7.0kW (30A) = 32A MCB, 7.5kW (33A) = 40A, 8.5kW (37A) = 40A, 9.5kW (41A) = 45A, 10.8kW (47A) = 50A, 12.0kW (52A) = 63A. Note 7.5kW draws 32.6A, which exceeds a 32A device, so it needs a 40A MCB; and a 12kW shower draws 52.2A, which exceeds 50A, so it needs a 63A MCB (the next BS EN 60898 size). An RCBO (combined MCB + RCD) is preferred as it provides both overcurrent and earth fault protection in one device. See our RCBO Selection Calculator to choose the right type.