Upgrading shower circuit from 6mm to 10mm

14 Oct 2012
Reaction score
United Kingdom

Currently my shower is - from looking at the cable externally - on a 6mm cable circuit over about 5 metres connected to a 30 amp fuse. No RCD.

Shower is 8.5kw and I am limited to around 10 minutes of use before the fuse trips.

If I get an electrician to upgrade the cable to 10mm and fuse to 45amp will that work be classed as 'new' and need an RCD?

If so it may be time to replace the CU.
Sponsored Links
Common sense is that an electric shower should have an RCD in the supply and preferable NOT the one that the lighting is supplied from. If it trips due to a shock to the person in the shower the last thing that person needs is to be plunged into sudden darkness.

If the MCB has been tripped many times on over current then it should be replaced as frequent tripping can affect the reliability. An over current trip of a few times the rated current causes the inside of the MCB to heat up until a thermal trip operates. That also affects the operation of adjacent MCBs by heating them as well.

Best option in the short term is to have a separate mini CU added for the shower until a new CU can be fitted. You need to have this mini CU fitted as soon as possible before the present over worked MCB fails to operate on a real fault in the shower and the shower unit melts or ignites.
before the fuse trips
which I took to mean a "fuse" that is an MCB and not a fuse that blows.

Either way it is not good. Blow a wire fuse and vapourised copper and tin condenses on the fuse holder. Blow it many times and the layer of metals deposited on the fuse holder will make it more conductive than it should be.
Sponsored Links
Regardless of the detail in the original post. In my opinion the shower needs an RCD fitted as soon as possible. At present, if you get a leakage to earth just below 30A you have no protection.
Unless, with the main RCD in the CU tripped will that kill the shower? You will possible be in the dark, but alive.
Hi, If so it may be time to replace the CU.

Not necessarily

If its a Wylex wired board you can get a 40A plug in mcb and fit an inline RCD in the cable leading to the shower, as long as the cable does not lead through insulation and it can freely lose heat to air or surrounding masonry then it should be ok for 8.5 kw, there are other things to be considered and a qualified electrician would need to confirm the site condidtions are ok for the above solution.
The majority of wylex rewireable consumer units are not rated to accept a 40A device.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links