What Cable Size for an Electric Shower?

Worked Example: Design Current for a 9.5kW Shower

The design current is the load divided by the supply voltage. On the UK’s nominal 230V supply, the arithmetic is:

• Load: 9.5kW = 9,500W
• 9,500W ÷ 230V = 41.3A

Some older tables and installers still work from the pre-harmonisation 240V figure. At that voltage the same shower draws slightly less:

• 9,500W ÷ 240V ≈ 39.6A

Either way the design current lands at roughly 40A, which is why 9.5kW showers sit right on the boundary of the 40A and 45A MCB bands. Using the 230V figure of 41.3A, which is just over 40A, the circuit needs the next standard breaker size up: 45A. A 9.0kW shower, by contrast, draws only 39.1A at 230V and stays within a 40A MCB. That 0.5kW gap is the whole reason the two ratings end up on different breakers.

Why 10mm², Not 6mm²

It looks odd at first: a bare 6mm² twin and earth cable is rated 47A clipped direct (BS 7671 Table 4D5, Reference Method C), comfortably above the 41.3A design current of a 9.5kW shower. So why does trade practice move to 10mm² from 7.5kW upward rather than staying on 6mm² all the way to 47A?

The answer is the route, not the load. Shower cable runs typically pass through a loft, above a ceiling, or inside a stud wall, in contact with thermal insulation for at least part of the run. BS 7671 Table 4D5 rates cable much lower once insulation is involved: Reference Method 100 (touching insulation on one side, up to 100mm) derates 6mm² to just 27A, and Reference Method 101 (surrounded by more than 100mm of insulation) derates it further to 22A. Both figures sit well below a 40A or 45A MCB, so a 6mm² cable that looks fine on the bare Table 4D5 column 6 figure can be badly undersized once it is actually installed. Moving to 10mm² (64A clipped direct) keeps enough margin that a typical insulated run still clears the breaker rating. This is also the logic our own electric shower calculator applies when it recommends a minimum cable size for a given kW rating.

The RCD: 30mA Is Not Optional

A shower is in a bathroom, one of the special locations covered by BS 7671 Section 701. Regulation 701.411.3.3 requires every circuit that serves a room containing a bath or shower, and every circuit that merely passes through zones 1 or 2 of that room, to have additional protection from one or more 30mA RCDs. There is no smaller-load exception: the shower circuit, and the lighting circuit in the same room, both need it. In a modern consumer unit this is usually a single RCBO per circuit, combining overcurrent and 30mA earth-fault protection in one device. The circuit also needs its own double-pole isolating switch, typically a 45A or 50A ceiling-mounted pull-cord switch, sited outside the bathroom zones but reachable from inside. See our RCD selection guide for how trip ratings and RCD types are chosen more generally.

Why Voltage Drop Can Push You Up a Size Anyway

A shower element is a fixed resistance, so the power it delivers falls with thesquare of the supply voltage at the element, not in a straight line. BS 7671 limits voltage drop to 5% for power circuits, which is 11.5V on a 230V supply. If a long or thin cable run uses up the full 5%, the element only ever sees 95% of nominal voltage:

• 0.95² ≈ 0.9025, so about 90% of rated power reaches the water
• 9.5kW × 0.90 ≈ 8.6kW delivered, at a lower water temperature for the same flow rate

That is why a cable that already clears the current-carrying check can still fail on voltage drop over a long run, and why a longer or more awkward route can force a larger conductor even at the same kW rating. Our voltage drop calculator and cable sizing calculator both check this against your actual cable length.

Why This Page Cannot Give You a Final Answer

The bands above assume Reference Method C conditions and a run short enough to stay inside the 5% voltage-drop limit. Real installations vary: the cable may be bundled with others, buried in insulation for more of its length, run further than a typical installation, or sit in a warmer ambient space such as a loft in summer. Each of those factors can push the minimum cable size up a step from what the table above shows for a given kW rating. A registered electrician applies the actual derating factors for your specific route, checks the earth-fault loop impedance, and certifies the finished circuit, none of which a table on a website can do. See our explainer on how cable size is calculated for the full set of factors, and our UK cable sizes reference for current ratings across other common circuits.

Cable current ratings from BS 7671:2018+A2:2022 (18th Edition), Table 4D5. RCD requirement from BS 7671 Regulation 701.411.3.3 (locations containing a bath or shower). Shower cable and MCB bands cross-checked against this site’s electric shower calculator and independent UK trade guidance (elec-mate.com, electraguard.uk). This page is guidance only and does not replace a design carried out by a qualified electrician; all domestic shower circuit work must comply with Part P of the Building Regulations. Last updated: July 2026.