If you are fitting a new electric shower, or upgrading to a more powerful one, the cable feeding it matters as much as the unit on the wall. An electric shower heats water instantly, so it draws more current than almost anything else in a home. Get the cable or breaker wrong and you risk tripping, poor performance or overheating connections.
This guide explains how electric shower cable size is worked out, what is typical for 7.5 kW to 10.8 kW showers, and why the honest answer is "it depends on your circuit". RIGS Electrical is a NICEIC registered domestic electrician based in Tring, working across Hertfordshire, Buckinghamshire and nearby border areas.
Why shower cable size matters
The current a shower draws is roughly its power divided by the supply voltage. Using the 240 V figure most shower ratings are quoted at:
- 7.5 kW draws about 31 amps
- 8.5 kW draws about 35 amps
- 9.5 kW draws about 40 amps
- 10.8 kW draws about 45 amps
That is as much as, or more than, a whole ring circuit of sockets, drawn through one dedicated circuit for as long as the shower runs. The cable has to carry it without overheating, and the breaker or RCBO at the consumer unit has to be matched to both the shower and the cable.
Typical cable sizes for electric showers
Manufacturer guidance is the best starting point, and it is fairly consistent:
- Recommended breaker ratings. Mira's installation guide for its Jump and Sprint models lists a 32 A MCB for 7.5 kW, 40 A for 8.5 kW and 9.5 kW, and 45 A for 10.8 kW. Triton gives the same pattern for its 7.5 kW to 10.5 kW models.
- Cable size. Triton's guidance is that 6 mm² cable is typically suitable for showers up to about 8 kW in many standard installations, while showers above 8 kW typically need 10 mm². Triton also strongly recommends a minimum of 10 mm² throughout a shower installation, because most real-world cable runs are affected by insulation, bunching or heat.
- The largest showers. For a 10.8 kW shower, 10 mm² is the usual starting point, but some installations need 16 mm². Mira's guide states its showers accept a supply cable of up to 16 mm².
So for a 10.8 kW shower, expect 10 mm² cable at minimum and possibly 16 mm². For a 7.5 kW shower, an existing 6 mm² cable may be fine on a short, uninsulated run, but it still needs checking rather than assuming.
These are typical figures, not a sizing chart for your home.
What decides the right cable for your home
An electrician sizes a shower cable using BS 7671, the IET Wiring Regulations. The main factors are:
How the cable is installed. A cable clipped to a joist or buried in a plain wall can carry more current than the same cable run through loft insulation or an insulated stud wall, because insulation traps heat. The same applies when cables are bunched together. Shower cables often cross lofts, which is why older 6 mm² circuits can be marginal today.
Length of the run. Longer cables lose more voltage. BS 7671 sets voltage-drop limits, so the further the bathroom is from the consumer unit, the more likely a larger cable is needed. Lower voltage also means less heat: Mira's guide shows its 10.8 kW unit delivering about 9.9 kW at 230 V.
The protective device. The breaker or RCBO must suit the shower's current without exceeding what the cable can carry where it is installed.
RCD protection. BS 7671 requires 30 mA RCD protection for circuits serving a room containing a bath or shower. If your consumer unit has no RCD, or uses old rewireable fuses, the shower work may be the point where a consumer unit replacement becomes the sensible option.
The isolator. Electric showers need a double-pole isolating switch, usually a ceiling pull-cord or a wall switch outside the bathroom. Mira recommends a 45 A double-pole isolator with 3 mm contact separation. Undersized or worn isolators are one of the most common hot spots on shower circuits.
Can you upgrade to a more powerful shower on your existing cable?
Sometimes, but this is where most problems start. Moving from a 7.5 kW or 8.5 kW unit to a 9.5 kW or 10.8 kW model raises the current significantly. An existing 6 mm² cable, especially through a loft, is unlikely to suit the larger shower, and the breaker usually needs changing too.
Before you buy, have the existing circuit checked so you know which ratings it can support. Our guide to replacing an electric shower covers the rest of the replacement process.
Signs your shower cable or circuit may be undersized
Call an electrician if you notice:
- The shower trips the breaker or RCD when it is running, rather than at switch-on
- The pull-cord or isolator feels warm, or has discoloured or melted plastic
- A burning smell or scorch marks around the shower or isolator
- The shower struggles to heat, particularly after a more powerful unit was fitted
Discolouration, scorching or a burning smell mean the shower should be switched off and not used until it is checked. Our guide on whether you need an emergency electrician can help you judge the urgency. Repeated tripping without obvious damage is usually a job for electrical fault finding.
Is installing a shower cable a DIY job?
In England, a new shower circuit is notifiable under Part P of the Building Regulations, so it should be installed by a registered electrician who can self-certify it, or signed off through building control. Our Part P guide explains which work is notifiable. A new circuit should come with an Electrical Installation Certificate and a Building Regulations compliance certificate.
Getting your shower circuit checked in Tring and nearby
If you are planning a new electric shower, upgrading to a higher kW model, or dealing with a shower that trips or runs warm, RIGS Electrical can check whether your circuit is suitable and install the right cable and protection where needed. See our electric shower installation service or electric shower installation in Tring.
To get a clear answer quickly, send us the rating of your current shower, the model you are considering, and a photo of your consumer unit if you can. We will tell you what your circuit can support and provide a clear quote for any work needed.
*Typical cable and breaker figures are taken from published manufacturer guidance and are for general information only. The correct cable size for any installation must be determined by a competent electrician under BS 7671. Always follow the shower manufacturer's installation instructions.*