Typical domestic values — the final cable size must be confirmed for the actual installation method (thermal insulation, grouping) with the cable sizing calculator. Guidance only — always verify against BS 7671 and the On-Site Guide for the actual installation method.
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This calculator works out the design current, protective device rating and a typical cable size for an electric shower circuit. Enter the shower's kW rating and the supply voltage, and it returns the current the circuit will draw, the MCB or RCBO rating to protect it, and the cable size normally used for that load.
Electric showers are one of the heaviest single loads in a domestic installation, so the circuit has to be sized for the shower's actual rating at nominal voltage — not rounded down. A 9.5 kW shower pulls over 41 A, which is why showers get their own dedicated circuit, a 40–50 A device and a 6 mm² or 10 mm² cable rather than sharing a ring final.
The cable size shown is the size normally installed for that load run clipped direct. The final size must always be confirmed for the real installation conditions — cable buried in loft insulation or bunched with other circuits derates and can need to go up a size — which is what the cable sizing calculator is for.
Design current Ib = (kW × 1000) / voltage
The shower's power in kilowatts is converted to watts and divided by the supply voltage (230 V nominal in the UK) to give the design current in amps. An electric shower is a purely resistive load, so no power factor correction is needed. The protective device is then the next standard rating at or above that current — usually 40, 45 or 50 A for a shower — and the cable is selected to carry that current under the actual installation conditions.
Type the shower's power rating in kW from the unit or its data plate — common ratings are 7.5, 8.5, 9.5 and 10.5 kW. Size the circuit for the shower actually being fitted.
230 V is preset for a standard UK single-phase supply. Leave it unless you have a specific reason to design at a different voltage.
The calculator shows the design current in amps and the next standard MCB or RCBO rating that covers it — the device that protects the shower circuit at the consumer unit.
It suggests the cable size normally used for that load, and reminds you the circuit needs 30 mA RCD protection as a bathroom circuit. Confirm the final cable size for your installation method before you run it.
Electric showers must be on their own dedicated radial circuit from the consumer unit — never spurred off a ring or shared with other loads. The shower switch (a ceiling-mounted pull-cord or an out-of-zone wall switch) must be double-pole so it isolates both line and neutral.
As a common domestic guide, showers up to around 8.5 kW are usually run in 6 mm² twin and earth on a 40 A device, and 9.5–10.5 kW showers in 10 mm² on a 45 or 50 A device. These are typical clipped-direct figures — a cable run through thermal insulation or grouped with other circuits derates and may need to be a size larger, so verify with the cable sizing calculator for the actual route.
The circuit requires 30 mA RCD protection because it serves a location containing a bath or shower, and the design must also satisfy the maximum Zs for the device so it disconnects within the required time on an earth fault. As always, guidance only — verify the full design against BS 7671 and the On-Site Guide.
Typical domestic values — the final cable size must be confirmed for the actual installation method (thermal insulation, grouping) with the cable sizing calculator. Guidance only — always verify against BS 7671 and the On-Site Guide for the actual installation method.
A 9.5 kW shower draws about 41 A at 230 V, which sits on a 45 or 50 A device. That load is normally run in 10 mm² twin and earth clipped direct. If the cable passes through loft insulation or is bunched with other circuits it derates, so confirm the size for the actual installation method rather than assuming 10 mm² is always enough.
Take the shower's kW rating, divide by 230 to get the current, and use the next standard device rating above it. An 8.5 kW shower (about 37 A) sits on a 40 A device; a 9.5 kW shower (about 41 A) on a 45 A; a 10.5 kW shower (about 46 A) on a 50 A. The device also has to have a breaking capacity for the prospective fault current and meet the circuit's max Zs.
Yes. An electric shower is a large, sustained load and must be supplied by its own dedicated radial circuit direct from the consumer unit, with a double-pole isolation switch. It cannot be taken as a spur from a ring final or shared with other loads.
Yes. A shower circuit serves a location containing a bath or shower, so it requires 30 mA RCD protection under BS 7671 — commonly provided by an RCBO on the shower circuit or an RCD covering it. The RCD protects against electric shock in a high-risk wet location.
It depends on the shower rating and how the cable is installed. 6 mm² twin and earth clipped direct comfortably handles showers up to around 8.5 kW on a 40 A device, but a 9.5 kW or larger shower, or a cable derated by insulation or grouping, usually needs 10 mm². Work out the design current and confirm the cable against the installation method.
Required tabulated capacity It ≥ In / (Ca·Cg·Ci·Cf), with a T&E suggestion.
Convert between power and current for single or three phase.
Volt drop for twin & earth over a run, checked against BS 7671 limits.
Maximum earth fault loop impedance for BS EN 60898/61009 devices.