Whether the cable is 10mm² or 6mm² is irrelevant.
The current carrying capacity of either is well over what is needed.
In answer to the original question - increasing the shower from 7kW to 7.5kW will have no effect on the present fuse or wiring.
The manufacturer will likely these days not quote the requirements for a re-wirable fuse and in any case the size quoted will merely be the next size up with no other consideration.
To be pedantic we are supposed to design with the nominal voltage of 230V and therefore installations will supposedly be able to cope with any voltage which is actually higher than this.
At 230V the '7.5kW' shower in question will be 6.888kW and draw 29.95A.
So, a straight swap of the shower is perfectly acceptable.
The snag is the manufacturer's requirement for an RCD to be fitted which, for the safety of the user, is desirable.
As this requirement has nothing to do with the actual shower, some may think it merely a way for the manufacturer to cover their ***** by dictating to home owners what they must do.
Don't get me wrong; it is a good idea but whether you think it should be mandated by a manufacturer but not the actual regulations for the replacement of a shower or any other appliance is debatable.
A stand alone RCD could be fitted on the circuit anywhere between the consumer unit and the shower switch.
Obviously somewhere accessible would be best.
However, this would mean the circuit has been altered and added to which means the work may now be notifiable - depending on you interpretation of "any addition or alteration to existing circuits in a special location" - to the Building Control either by paying a large fee or employing a registered electrician.
The current carrying capacity of either is well over what is needed.
In answer to the original question - increasing the shower from 7kW to 7.5kW will have no effect on the present fuse or wiring.
The manufacturer will likely these days not quote the requirements for a re-wirable fuse and in any case the size quoted will merely be the next size up with no other consideration.
To be pedantic we are supposed to design with the nominal voltage of 230V and therefore installations will supposedly be able to cope with any voltage which is actually higher than this.
At 230V the '7.5kW' shower in question will be 6.888kW and draw 29.95A.
So, a straight swap of the shower is perfectly acceptable.
The snag is the manufacturer's requirement for an RCD to be fitted which, for the safety of the user, is desirable.
As this requirement has nothing to do with the actual shower, some may think it merely a way for the manufacturer to cover their ***** by dictating to home owners what they must do.
Don't get me wrong; it is a good idea but whether you think it should be mandated by a manufacturer but not the actual regulations for the replacement of a shower or any other appliance is debatable.
A stand alone RCD could be fitted on the circuit anywhere between the consumer unit and the shower switch.
Obviously somewhere accessible would be best.
However, this would mean the circuit has been altered and added to which means the work may now be notifiable - depending on you interpretation of "any addition or alteration to existing circuits in a special location" - to the Building Control either by paying a large fee or employing a registered electrician.