An electric shower is plumbed into the mains water supply and it is the mains water pressure that causes a switch inside the unit to start a heater, which heats up the water as it flows to the shower heat. Because there is little time to heat the flowing water instantaneously, electric showers require a heavy load - from 6kW to 8kW of power, using 2 units of electricity every 15 minutes.

Because they have this heavy load, like electric cookers, electric showers must have there own separate radial circuit. The circuit cable must be 6mm sq. two-core and earth cable and be protected by a 30-amp fuse in a spare fuse way in the consumer unit or in its own separate switch fuse unit. The cable runs directly to the shower unit and must be wired exactly according to manufacturer's instructions.

Electric showers must also have their own on-off switch - just like an electric cooker. But because the combination of water and electricity is lethal, the shower unit, metal pipes and the fittings must be bonded to the earth. If these came into contact with live electrical conductors they would become extremely dangerous. IEE Wiring Regulations require all metal components in bathrooms, including central heating pipework, to be connected to each other by an earth conductor, which itself is connected to the terminal on the earthing block in the consumer unit. This is known as 'supplementary bonding' and it is now required for all new bathrooms - even if there is no electrical equipment installed in the room and even though the water and gas pipes are already bonded to the consumer's earth terminal near the consumer unit. This bonding should be tested by a qualified electrician.

With electric showers there must also be an isolating switch in the circuit that is out of reach of anyone in the shower. These isolating switches are ceiling mounted 30 amp double-pole switches operated by a pull cord. Inside the switch are the two cables: one is the radial circuit cable, the other the shower cable.

If you are uncertain about undertaking any electrical installation, always hire a professional qualified electrician to carry out the work for you. Don't take risks.

Regulations

• See our installing an electric shower article in our plumbing section for advice on the different types of shower available, along with the plumbing regulations regarding showers and the construction of shower cabinets.
• If you want to install a shower in a bedroom, it must be more than 3m away from any socket outlets, and they must be protected by a 30 milliamp RCD.