A homeowner upgraded from a 7kW shower to a 10.5kW model and had the new unit connected to the existing 6mm cable, on the assumption that a cable is a cable. Within weeks there was a faint burning smell near the isolator switch. The cable wasn't faulty, it simply hadn't been sized for the extra load the new shower was drawing.

Why "6mm Is Usually Fine" Isn't a Safe Rule

A shower's kW rating determines its current draw, and that current has to travel through the cable without the cable overheating. A 9.5kW shower draws roughly 41A, close to the upper limit of what 6mm² twin and earth can carry even under good conditions, and that limit drops further if the cable is run through loft insulation, bundled with other cables, or covers a long distance from the consumer unit.

The risk with an undersized cable isn't necessarily an immediate fault, it's a cable running hotter than it should under normal use, degrading insulation over time in a way that isn't visible until it's already a problem. The IET's own guidance on BS 7671 Appendix 4 uses a shower circuit as its worked cable-sizing example, precisely because getting this calculation right matters most on exactly this kind of fixed, high-current load.

Why Upgrading the Shower Doesn't Always Mean Upgrading the Cable

Moving from a 7kW to an 8.5kW shower might stay within an existing 6mm² cable's capacity, depending on the run. Moving from 7kW to 9.5kW or higher is a different situation entirely, current draw increases enough that a cable sized correctly for the old shower can be genuinely undersized for the new one, even though nothing about the cable itself has changed.

This is exactly the gap that catches people out on a straightforward-looking shower swap: the assumption that if the old shower worked fine, any similar-looking replacement will too. The isolator switch and MCB rating need checking alongside the cable, since all three have to match the new shower's actual draw, not just the cable in isolation.