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Shower cable 6mm or 10mm

Please advise, I have just had my flat rewired, kitchen and bathroom installers say that the 6mm cable is not sufficient for the best modern showers, that a 10mm cable should have been installed.

Both the installer and the electrician who did the rewiring are registered.

What is currently considered best practice, and if a matter of opinion, should the customer be consulted during the rewiring?



A clear decision from experienced electricians, which is helpful.

All four electricians agree that 10 is the current industry standard, including G W Electrics, who carried out the rewiring of my flat, and provided only 6 for my shower, how arrogant!

Thanks to the other 3 electricians who kindly responded to my question, if only my rewiring had been carried out by one of them.

6 Answers from MyBuilder Electricians

Best Answer

normally a 10mm twin and earth would/could be installed
although 6mm twin & earth installed using reference method c has a current carrying capacity of 47amps which would be sufficient enough for most showers as calculations show below

10500watts divided by 230volts = 45.65amps
9500watts divided by 230volts = 41.30amps
8500watts divided by 230volts = 36.95amps

10.5kw shower is the highest that would probably be installed

the 10mm twin & earth has a current carrying capacity of 65amps using reference method c which would still do the job but a bit overkill

im sure your registered electrician has suficiently designed the circuit

****10mm is not the current industry standard
Each circuit is individually designed for it's purpose , you have a 6mm cable installed using reference method C the cable will take 47amps there is no problem here you are free to fit any shower 8.5kw +


Answered 18th May 2014

10mm cable should be used on showers of 8.5kw and above, so yes if your fitters are saying that the best showers are those with the highest rating (e.g.10.8Kw), then they are right in that the 6mm cable is inadequate.

This issue is that unless you had bought the shower already or were clear with your electrician, then its not really his fault, as he would not know what type or rating of shower you were going to get? . A standard 7.5 to 8Kw shower is fine on 6mm cable. Ideally the pro's and cons of both cables and costs should have been discussed, but the risk is that if you fit an 8.5kw or higher powered shower on the installed 6mm cable, the cable may be overloaded and get damaged.


Answered 18th May 2014

Hi there,

You can use 6mm cable for up 9.2 kW. Only condition is if you have a 9.2 kW shower on a 6 mm cable it should not be more then 6 meter away from the consumer unit. But 7.5 kW is ok. If you have a shower more then 9.2 kW, 10mm cable must be used.

I normally always use 10 mm cable if the shower is 7.5 kW or more. At least this way proper wiring is done and in future if the customer wants to change the shower to a more powerful one, cable is always there. Because it could be very costly to change the cable on a later stage. Specially after a whole new rewrite.

When I do a job I always ask customer which shower they are going for. This way I calculated the job based on cable size. If your installer has done a 10 mm cable, it is good. You don't have to worry.

I hope this helps,



Answered 17th May 2014

Now cable size is dependent on the KW of the shower itself and the way the cable has been installed. I would normally use a 10mm cable for most shower installations as this allows for future upgrades and has the current carrying capacity for most/all installation methods.


Answered 17th May 2014

10mm and Protected by an RCD will be sufficient for now and anything that you may require in the future.


Answered 30th Apr 2016

If Gw electrics have installed a 6mm, and signed it of then y are you even questioning it? Are you gw electrics QS? Have you had another company around who carried out a full EICR ?

They have signed a 5yr/10yr cert to say that the install complies .. well id hope they have .. so it's all good :)


Answered 24th Jun 2018

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