Electric shower installation cost and the circuit it needs.

Plumber or electrician for an electric shower

Usually both. The water side is plumbing, covered by the Water Supply (Water Fittings) Regulations 1999, and the electrical side is a circuit running from your board to the shower, which is installation work under BS 7671 and Part P.

I do the electrics up to the shower and make the connection at the unit. It helps if your plumber and I agree the order of work before the day.

An electric shower takes its water straight from the cold mains, so the plumber also checks that the pressure and flow suit the model you've picked.

If one tradesperson offers to do the lot, ask who will test the electrical side and who signs the certificate for it.

Sizing the circuit to the shower's kW rating

Electric showers are sold by power, and the higher the kW figure, the more current the circuit has to carry. A 10.5 kW shower draws about 45 amps at full heat.

So the cable is chosen for the rating and for the way it runs. Cable buried in loft insulation cannot shed heat like cable clipped to a joist, and the current-carrying tables in BS 7671 lower its rating to match. A cable that suited an old 8.5 kW shower can be too small for a new 10.5 kW one.

The isolator has to carry the shower's full current as well, which is why a shower pull-cord is a heavier switch than the one for a bathroom light.

The circuit price covers showers up to 10.5 kW. A bigger unit, or a run that's longer or harder than usual, gets a written quote after I've had a look, and that's the price you pay.

The incoming supply has to take the load too. Approved Document P expects the rating of the existing equipment to be checked before a large new load goes on, the network operator's equipment included. If the main fuse is too small, upgrading it is a job for the network operator, since everything on their side of the meter is theirs. If a car charger is likely as well, mention it, because a shower and a charger running at once are a heavy load for one main fuse.

Bathroom rules for an electric shower

A bathroom is a special location in BS 7671 (Section 701), mapped out in zones around the bath and the shower. An electric shower is one of the few things allowed inside zone 1, provided it's fixed, permanently connected and suitable for that zone under the maker's instructions.

Every circuit serving the room needs RCD protection, the shower circuit included. Where all of them have it and the main bonding is sound, the extra bonding clamps that older bathrooms carry on their pipes can be left out.

The isolator is normally a ceiling pull-cord inside the room or a switch on the wall just outside it, so the shower can be made completely dead for maintenance. Ordinary sockets are kept well back from the bath and shower, and a shaver socket is the usual exception.

A like-for-like extractor fan swap is one of the small jobs on the price list, and it can be done while I'm there.

Replacing an old electric shower

Swapping a shower for one of the same rating can be straightforward, if the cable, isolator and protection are in good order.

A pull-cord switch that has turned brown, or a shower that cuts out at full heat, is worth looking at before the new one goes on. Both can point to a connection running hot.

An old board with no RCDs, or no spare way, may mean a new consumer unit before the shower circuit can join it. If so, we quote both together, so you see the whole cost before deciding.

What to expect while the shower circuit goes in

Connecting a new circuit into the board means switching off at the main switch for part of the job, so expect the house to be without power for a while. The rest of the time only the shower circuit is off.

The cable route is agreed at the survey, whether it runs through the loft or down a wall. If it has to be chased into plaster or behind tiles, putting the wall back is a separate item, quoted on request.

Part P and the shower certificate

In England a new shower circuit is notifiable work (Building Regulations 2010, regulation 12(6A)), and so is any addition or alteration to circuits within the space around a bath or a shower that regulation 12(9) calls a special location.

A like-for-like swap on the existing circuit is not notifiable. Moving the shower, or extending its circuit within that space, is.

When the circuit is wired and tested, I write up its Electrical Installation Certificate.