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6mm cable joined to 10mm cable

I am getting a new bathroom fixed. The current one has a 8.5kW electric shower which is connected via 6mm cable all the way to consumer unit , Now I am thinking of upgrading the wires in the bath to 10mm now , and change the wiring from bath to consumer unit later when I upgrade the electric shower ( hence won't require to redo the cabling in the bath later) Could someone kindly tell me if that is safe and legal?
Having 6mm cable run from CU to bathroom , 10mm running to a 8.5kW electric shower.

4 Answers from MyBuilder Electricians

Best Answer

What your proposing is fine, provided that the work is notified to building control under part P of the building regulations and a Minor Electrical Works Certificate issued. Also it will be vitally important that the remaining section of 6mm cable is also upgraded in the future BEFORE a higher powered shower is fitted.


Answered 4th Aug 2015

work must be done by a self certified electrician - this work as stated in earlier post will come under special locations and is notifiable under part p of the building regs.

as it stands the cross sectional area of the cable (6mm) is insufficient for the intended current carrying capacity of the current shower (8.5kw).

8500Watt/230Volts = 36.95Amps (this is what the shower will pull)

Now many my tell you that a 6mm twin and earth cable can pull 47A (this is only if it is clipped directly to a wall).

As soon as that cable runs through loft insulation (which 9 out of 10 times it does) not exceeding 100mm that figure will drop to 34A. And if the insulation is exceeding 100mm it will further reduce to 27A

Even if the cable is not covered by insulation, there is a variety of other de-rating factors that may potentially have to be applied thus rendering the 6mm cable too small for an 8.5kw shower.

In short my advise would be to get a qualified, self certified electrician to carry out the works and get the right sized cable installed from the word go.

Hope this helps,


Answered 7th Aug 2015

No problem with that if you've done cable calc and registered with building control if not make sure your well covered on isurancnce for fire not that they will pat out if it's wrong

Or call out a local spark and do it right !!!


Answered 7th Aug 2015

Any work in a bathroom needs notifying to building control as it is in a special location, best get this work done by a registered electrician. Who will have the necessary knowledge and equipment to do it all correct. Don't mess with electric in bathrooms, it could end in disaster tbh.


Answered 3rd Aug 2015

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