Possible but existing wiring has to be able to take the load, 8.5 kW with 230 volt = 37 amp and 9.5 kW = 41 amp a MCB or fuse will allow more than rated valve for a short time, and since it is there to stop cables over heating which takes time we can get away with a 8.5 kW heater on a 32 amp supply because it does not run for long enough to trip it.
As we start to go higher the time before tripping reduces and it starts to degrade the trip because it gets so hot on a regular basis, so likely would get away with it for some years, but in time it will start tripping more and more.
6 mm cable with installation 100 is rated 34 amp, so to be able to fit a 40 amp trip the cable needs upgrading to 10 mm, the problem with cable is the current rating changes depending how installed, so same cable can be rated between 23.5 amp and 47 amp depending how installed, so in the main electricians will not fit larger trips as we have no way to know how it has been installed, rarely does the home owner have the installation certificate.
So the 8 kW shower using 35 amp was very common using 6 mm cable and a 32A MCB plus of course a RCD, and it works very well, but once you go over 8 kW then you start to look at heavier cable and larger trips, even 7.5 kW is slightly over the 32A limit, but in the main we don't shower for long enough for it to become a problem. But as we try to sneak the extra few amps so it gets worse and worse, main thing is don't change a trip, the trip protects the cable, so what ever is fitted now needs to remain, assuming correct size to start with.
So you are taking a chance, likely it will work for some years, but assuming using a 32A trip I would look for 8 kW model as shower going cold when washing hair is not nice.
Thank you, I already have 10mm cable and a 47amp rdc installed. Was going to up grade the whole shower but found out they do exact same shower in a 9.5kw. So was just going to order the spare parts to upgrade the original one.
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