Shower circuit design.

Why would an electrician install a 10 mm twin and earth circuit protected by a B32 MCB for a 8.5 kW shower?

Interesting thread, as you have “the shower circuit” with the OPD, the cable and how and what it is installed in, the size you choose, together with a isolator somewhere at 45 amps [or 50amps] rating, maybe, and you have the shower- being the fixed equipment. To match all aspects as a best fit “for fully complying with the regs” can be a struggle as Showers fail and get changed, perhaps as little as every five years. You could tie yourself in knots and, worst case, actually end up with something that does have potential for danger due to the cavorting with the intricacies of trying to be be prescriptive with individual requirements of BS7671. That is why the decision on these things is at install. You know what has been put where and how. There is no guess after the event, perhaps enagaging some kind of “regularisation” process, else you can end up chasing your  tail. Go for what is the least risky approach. I think somewhere in this thread, remedial work is mentioned. Was this upping the ante due to a “recommend improve”? Yes the busbar bodge is bad; that can be dealt with in isolation along with mixed manufacture at the same time, but there has to be 100% confidence in where, through what and how  the cable is installed when increasing the OPD . I am sure it has not been completely excavated from beginning to end to find out. 

So a one size fits all declaration of a 8.5Kw shower being on a 32 amp MCB is bad design cannot hold true. There may have been a very good reason why it is done so.

 It does not help that cable sizes for flat t/e are 6, 10 or 16mm and that most showers installed are 8.5kW +. 16mm cable for a shower circuit is madness and never done in a domestic for many obvious reasons. It was simpler when the only shower show in town was a 7 or 7.5kW. But when we are in the guess what may or may not happen territory in some endeavour to regularise some design after the event with only partial knowledge of what went on originally, you could foresee some unwanted consequence of changing to a 40 amp. 200mm of cable in a shower circuit with a ceiling switch can easily be fully enveloped. We accept this , 100mm in, 100mm out of the switch gives 200mm. Still fine on 40amp on 10mm. But what if its longer, say 400mm? So the original designer considered the cable rating a priority all things considered. There is nothing wrong with that. Next cable size up is not an option, nor is a ceiling isolator the size of a bread bin. It may be habit, to err on less risk approach of protect the cable or, knowing from experience, that when the shower fails the homeowner/ builder/ plumber sticks in a 9.5+ for bigger bang for your bucks. I am not saying that this is what happened in the OP , as we simply do not know all the facts, but simply the least risk, if the 32amp device is showing no signs of distress is too leave the rating as it was. Least risk for you and the cable. 

Why would an electrician install a 10 mm twin and earth circuit protected by a B32 MCB for a 8.5 kW shower?

Interesting thread, as you have “the shower circuit” with the OPD, the cable and how and what it is installed in, the size you choose, together with a isolator somewhere at 45 amps [or 50amps] rating, maybe, and you have the shower- being the fixed equipment. To match all aspects as a best fit “for fully complying with the regs” can be a struggle as Showers fail and get changed, perhaps as little as every five years. You could tie yourself in knots and, worst case, actually end up with something that does have potential for danger due to the cavorting with the intricacies of trying to be be prescriptive with individual requirements of BS7671. That is why the decision on these things is at install. You know what has been put where and how. There is no guess after the event, perhaps enagaging some kind of “regularisation” process, else you can end up chasing your  tail. Go for what is the least risky approach. I think somewhere in this thread, remedial work is mentioned. Was this upping the ante due to a “recommend improve”? Yes the busbar bodge is bad; that can be dealt with in isolation along with mixed manufacture at the same time, but there has to be 100% confidence in where, through what and how  the cable is installed when increasing the OPD . I am sure it has not been completely excavated from beginning to end to find out. 

So a one size fits all declaration of a 8.5Kw shower being on a 32 amp MCB is bad design cannot hold true. There may have been a very good reason why it is done so.

 It does not help that cable sizes for flat t/e are 6, 10 or 16mm and that most showers installed are 8.5kW +. 16mm cable for a shower circuit is madness and never done in a domestic for many obvious reasons. It was simpler when the only shower show in town was a 7 or 7.5kW. But when we are in the guess what may or may not happen territory in some endeavour to regularise some design after the event with only partial knowledge of what went on originally, you could foresee some unwanted consequence of changing to a 40 amp. 200mm of cable in a shower circuit with a ceiling switch can easily be fully enveloped. We accept this , 100mm in, 100mm out of the switch gives 200mm. Still fine on 40amp on 10mm. But what if its longer, say 400mm? So the original designer considered the cable rating a priority all things considered. There is nothing wrong with that. Next cable size up is not an option, nor is a ceiling isolator the size of a bread bin. It may be habit, to err on less risk approach of protect the cable or, knowing from experience, that when the shower fails the homeowner/ builder/ plumber sticks in a 9.5+ for bigger bang for your bucks. I am not saying that this is what happened in the OP , as we simply do not know all the facts, but simply the least risk, if the 32amp device is showing no signs of distress is too leave the rating as it was. Least risk for you and the cable.