Shower Cable Size Calculator — Size the Circuit for 8.5kW to 10.5kW Electric Showers

Instantaneous electric showers heat water as it flows, which means the full electrical load is drawn the whole time the shower runs. There is no diversity to apply — the circuit must carry the full rated current continuously. That makes the shower circuit one of the most heavily loaded circuits in a domestic installation, and correct cable sizing is essential.

The design current is simply the shower power divided by the nominal voltage (I = P / V at 230V single-phase):

Because the load is continuous, the cable must have a current-carrying capacity ( after any derating for installation method, insulation, grouping, and ambient temperature) at least equal to the protective device rating. The cable sizing calculator embedded above applies these correction factors for you.

Here is the full sizing calculation for the most common scenario — a 9.5kW shower fed from a consumer unit 18 metres away in 10mm² twin and earth cable:

If the same shower were installed where the cable runs through insulation, the capacity of 10mm² can drop below the device rating and a larger cable or a different route becomes necessary. That is exactly the kind of case where the calculator earns its keep — change the installation method and watch the recommendation update.

For most domestic shower circuits, 10mm² twin and earth is the default choice. It comfortably carries 40-50A in the common installation methods, it covers voltage drop on typical domestic run lengths, and it means the circuit will not need rewiring if the customer later upgrades from an 8.5kW to a 10.5kW shower.

Cases where 10mm² is not automatically the answer:

• Cable surrounded by thermal insulation — capacity falls substantially and a larger cable may be required. Check the derated capacity in the calculator.
• Short runs with an 8.5kW shower — 6mm² can be adequate for a 40A circuit in favourable installation methods, but it leaves no headroom for a future shower upgrade and fails quickly when derated. Many electricians fit 10mm² regardless for exactly that reason.
• Long runs — beyond roughly 30-35 metres, voltage drop starts to govern even for 10mm², and 16mm² may be needed. The voltage drop calculator shows the maximum run length for each size.
• Grouped circuits — where the shower cable is bunched with other loaded cables, grouping factors reduce its capacity.

The embedded calculator applies the correction factors and voltage drop check in one pass, so the recommendation is based on your actual installation conditions rather than a rule of thumb.

The protective device for a shower circuit must be rated at or above the design current, and the cable must be sized to carry at least the device rating. Typical pairings are:

RCD protection is not optional. BS 7671 Section 701 (locations containing a bath or shower) requires additional protection by 30mA RCD for low voltage circuits serving the location — which includes the shower circuit itself. In a modern installation the cleanest solution is a dedicated 30mA RCBO for the shower, which combines overcurrent and RCD protection in one device and avoids the shower tripping a shared RCD that also feeds other circuits.

Disconnection times also need verifying — the earth fault loop impedance at the shower must be low enough for the device to disconnect within the required time. The disconnection time calculator covers that check.

BS 7671 limits voltage drop to 3% for lighting and 5% for other circuits (Appendix 4), measured from the origin of the installation to the load. For a shower circuit at 230V, 5% is 11.5V.

Voltage drop is calculated as the tabulated mV/A/m value for the cable, multiplied by the design current and the run length. Because shower currents are high, voltage drop builds quickly with distance:

The figures of 7.3 mV/A/m (6mm²) and 4.4 mV/A/m (10mm²) are the widely published values for copper twin and earth — the calculator uses the tabulated values for your selected cable type automatically. A shower running at reduced voltage heats water noticeably less well, so voltage drop is a performance issue as well as a compliance one.

Rooms containing a bath or shower are covered by Section 701 of BS 7671, which adds requirements on top of the general rules. The ones that matter most for a shower installation:

• 30mA RCD additional protection for low voltage circuits serving the location — the shower circuit must have it.
• Zoning — the space around the bath or shower is divided into zones that restrict what equipment may be installed and what IP rating it needs. The shower unit itself is designed for this, but pull-cord switches and isolators must be positioned appropriately.
• Local isolation — accepted practice is a double-pole isolating switch (typically a ceiling-mounted pull cord) so the shower can be isolated for maintenance without going to the consumer unit.

Once the installation is complete, the circuit details, test results, and RCD operation are recorded on the certificate. Elec-Mate handles the whole flow — size the circuit here, then complete the certification in the app. If you are adding a shower circuit to an existing board, check the effect on maximum demand with the maximum demand calculator.