Cable sizes are normally 4mm², 6mm², 10mm², 16mm² not sure how you get 6.5mm²? But 6mm² can be anywhere between 23.5 to 47 amp rated according to how installed. If we consider Reference Method 100# (above a plasterboard ceiling covered by thermal insulation not exceeding 100 mm in thickness) then 34 amp is limit so 32 amp MCB is correct for cable.
10mm² = 45 amp
16mm² = 57 amp
Moving to showers 7Kw = 30.43 Amp and 9.5Kw = 41.3 Amp
How ever if one looks at the chart for a 32A MCB at 41.3 Amp it should hold for 10,000 seconds in other words a couple of hours before tripping. In practice it's not so cut and dried but it would still work for a while before tripping. The MCB may get damaged over time and slowly you shower time would get reduced.
However most electricians would not fit a 9.5Kw shower on 6mm² cable and would insist on fitting 10mm² and the same with the LABC so since it's not like for like and you would need to register the change by using a register electrician or applying to the council under the Part P regulations in real terms you would need to up grade. Also in doing so you will need to comply with latest regulations and it will need RCD protection if not already installed.
By time you add all the costs together using a plumbed thermostatic valve may be cheaper? and will in the main be a better shower I would not use electric sprinkler I want a proper shower.
Yes I know many people will not comply with Part P but insurance will be void and house sale will be affected so you need to be aware.
In the main as long as the correct size MCB for cable is used overloading will only cause the MCB to trip and will not cause danger as the trip will open before the cable overheats but this would only be the case if the cable is installed correctly in first place. If installed originally using Reference Method 103# (in a stud wall with thermal insulation with cable not touching the inner wall surface) then it may already be overloaded [Rating for 6mm² would be 23.5 amp] in which case fitting a 9.5Kw shower could cause a fire. Since in many houses there is no installation certificate available to tell you how it was installed you may be introducing a real fire risk. It is because of these latent risks that Part P is required to protect you although many see it as a stealth tax.
I would not recommend DIY but I realise many will which is why I have tried to be as honest as I can and not just say you can't do it under Part P.
Do remember when electricians fit things we also test them with meters costing around £750 so most DIY with electrics is really taking a chance and hoping you make no mistakes.
Up to you!
Thank you for you comprehensive reply. I am most grateful. You have given me lots to think about. I could go with your mixer valve option, as we have a combi boiler, but my partner has already bought a bargain 9.5kw replacement shower. It looks like this "bargain" may not be such a bargain after all. Thanks anyway.
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