I have a 40amp fuse and 10mm cable and want to replace my electric shower!

a 9.8kw load is around amps (9800watts divided by 230volts = 42.6 amps), so ideally a 45amp BSEN60898 MCB or a BSEN61009 RCBO (if your existing fuseboard does not have RCD protection), should be used. Using the existing 40amp MCB short term is not a major issue as most MCB's have a fusing factor of around 1.25 to 1.5 times the rated current, so it would not trip instantaneously on overload, as a current of between 50 and 60 amps is needed. Remember all electrical works in the bathroom and alterations carried out to the fuseboard are building regulation notifiable and must be registered with LABC to comply with Part P of the building regulations.

Hi there, a 9.8Kw shower will potentially overload your existing 40A MCB as it will draw 42A max load. Your existing 10mm cable can cope with 9.8Kw (dependant on installation method) in most cases, so I would advise you to tell your prospective electrician that he would need to source a 45A MCB for your brand of fuseboard to replace the existing 40A. Alternatively you could get an 8.5Kw shower and leave the existing 40A circuit breaker in place. All the best, Rob