Electric Shower Upgrades: Cable Size, Isolators and the Consumer Unit

A more powerful electric shower is one of the most common upgrades in London flats, especially where there is no gas boiler or the water pressure is low. But a shower is the single biggest load in most homes, and fitting a bigger unit on the old cable is one of the faults we find most often on EICRs. This guide explains what has to be checked before the new shower goes in.

Why the cable size matters

Electric showers range from around 7.5kW to 10.5kW or more. The higher the rating, the more current the shower draws, and the thicker the cable and the bigger the protective device it needs. An old 7kW shower may be on cable that is too small for a 9.5kW replacement. Fitting the new unit on the old cable can cause the cable to overheat, especially where it runs through insulation in the loft or under floors.

The isolator

Every electric shower needs a double pole isolator so it can be switched off completely for maintenance. It is usually a ceiling pull cord switch in the bathroom or a wall switch just outside the door. The isolator must be rated for the shower's current. Undersized or scorched pull cord switches are a common finding.

The consumer unit

• The shower needs its own circuit, protected by a device matched to the cable and the shower rating.
• The circuit needs 30mA RCD protection, as it serves a bathroom.
• An old board may have no suitable way spare, or no RCD at all, which means an upgrade first.

Our guide to fuse board replacement in London explains what affects the cost if the board needs changing.

Supply and main fuse

In smaller flats with storage heaters, an immersion heater and a car charger, adding a bigger shower can push the total demand close to the limit of the main fuse. A quick load check before choosing the shower avoids nuisance problems later.

Bathroom rules still apply

The shower itself sits in zone 1, so it must have a suitable water protection rating and be installed to the maker's instructions. Our guide to bathroom electrics and zones covers the rest of the room.

Book a shower circuit check or upgrade

Energy North Ltd upgrades shower circuits, isolators and consumer units, and carries out EICR inspections across North, East and West London. Our City & Guilds certified electricians can usually attend within 48 hours, and certificates are emailed the same day. Planning a home charger too? See our EV charger installation page.