Fitting a higher-rated shower onto existing cable Any assumption that a figure from a website applies to your installation Uncertified work in a special location, which is notifiable almost everywhere

Which cable size does an electric shower need?

Commonly 6 or 10 mm² depending on the shower rating, the run length and the installation method — and it is notifiable work in most jurisdictions.

An electric shower is one of the heaviest fixed loads in a domestic property — a 10.5 kW unit draws around 45 A — and it is installed in the wettest room in the house, which is why it is treated more strictly than almost any other circuit. Sizing must account for the run length, the route through insulation and the ambient temperature, and it also needs the correct isolation, supplementary bonding where required, and residual current protection. Undersized cable does not fail dramatically; it runs hot for years inside a wall, degrading its insulation. This is emphatically an entry that ends with the same answer whatever the numbers: have it designed and installed by someone qualified and certified.

The options, ranked

Ranked best-first for the stated job. The third column is why the others exist.
Option	Good at	Bad at
6.0 mm² twin and earth	Often sufficient for lower-rated showers on short, well-ventilated runs.	Frequently undersized once the cable passes through insulation or the run is long.
10.0 mm² twin and earth	The common specification for higher-rated showers and for realistic domestic installation conditions.	Stiff, awkward to terminate, and needs larger accessories and glands.
Dedicated circuit with its own protective device	A shower is one of the largest single loads in a house and always needs its own way from the board.	Often means a consumer unit with a spare way, or an upgrade.
A qualified electrician	This is high-current work in a wet location and is notifiable in most jurisdictions.	Costs money; costs far less than the alternative.

Technique that changes the outcome

Have the circuit designed for the specific shower rating and route
Fit the pull-cord or isolator the regulations require, sited outside the shower zone
Ensure RCD protection and any required supplementary bonding
Keep the certificate; it is needed when the property is sold

What goes wrong

Fitting a higher-rated shower onto existing cable
Any assumption that a figure from a website applies to your installation
Uncertified work in a special location, which is notifiable almost everywhere

Compiled practical guidance rather than a published standard. Product formulations vary by brand and market, so read the label — and where the work is structural, electrical or gas, the regulations that apply where you are govern rather than anything here.

For	Use
Extension leads	One with a conductor rated for the full load, and always fully unwound — a coiled cable reel can be derated to a fraction of its rating.
A socket circuit	Typically 2.5 mm² for a UK ring final on a 32 A device, or 12 AWG on a US 20 A branch circuit — subject to the code and the installation method.
12 V DC and automotive	Size it for voltage drop, not current — at 12 V a small drop is a large percentage, so the cable is far thicker than the amps suggest.
A lighting circuit	Typically 1.0 or 1.5 mm² twin and earth in the UK, or 14 AWG on a 15 A circuit in the US — but the code and the installation method decide.
Outdoor and buried runs	Steel wire armoured cable, or a direct-burial type, at the required depth with RCD or GFCI protection — never ordinary indoor cable.
Speakers	16 AWG for short runs and 14 AWG beyond about 15 metres or into low-impedance speakers — and make sure it is copper, not copper-clad aluminium.

Common questions

Which cable size does an electric shower need?: Commonly 6 or 10 mm² depending on the shower rating, the run length and the installation method — and it is notifiable work in most jurisdictions.
What should I avoid?: Fitting a higher-rated shower onto existing cable Any assumption that a figure from a website applies to your installation Uncertified work in a special location, which is notifiable almost everywhere

Which extension lead should I use for a heavy load? One with a conductor rated for the full load, and always fully unwound — a coiled cable reel can be derated to a fraction of its rating.
Which cable size is used for socket circuits? Typically 2.5 mm² for a UK ring final on a 32 A device, or 12 AWG on a US 20 A branch circuit — subject to the code and the installation method.
Which wire size should I use for a 12 V circuit? Size it for voltage drop, not current — at 12 V a small drop is a large percentage, so the cable is far thicker than the amps suggest.
Which cable size is used for a lighting circuit? Typically 1.0 or 1.5 mm² twin and earth in the UK, or 14 AWG on a 15 A circuit in the US — but the code and the installation method decide.
Which cable should I use outdoors or underground? Steel wire armoured cable, or a direct-burial type, at the required depth with RCD or GFCI protection — never ordinary indoor cable.
Which speaker cable should I use? 16 AWG for short runs and 14 AWG beyond about 15 metres or into low-impedance speakers — and make sure it is copper, not copper-clad aluminium.

Sources

Typical range compiled from manufacturer specifications — At One Place

How these figures are compiled and checked