I’m okay with running 10 mm for a new shower circuit, it may be OTT, but not an issue as it’s rated at 52 amps.

Actually the circuit could possibly have a 45 or 50 amp MCB rather than a 40 amp, but the installer chose a 32 amp MCB.

I don’t have an issue with using 10 mm, it may be OTT as in this particular instance I would rate the cable at 52 amps.

So it could possibly be protected by a 45 or 50 amp device rather than a 40 amp, but the installer actually chose a 32 amp. 

OK.

But you do not know the “rationale” for the design. The regs are not prescriptive. Sometimes a balance is made between one need and another. The rationale can simply be a judgment on what is the most safe choice given a particular set of circumstances and requirements of the Client.  It is the original installers judgement; someone later will not likely have all the information, unless you fully dismantle everything.

If someone has selected and erected a cable and an OPD and have a particular rationale at install for doing so, any person coming along later and increasing that OPD would surely have to have 100% confidence in exactly how and where that cable has been installed. Do you excavate the lot and have a look?  If you you do not have such assurance, then it is no more than a punt to up the ante “to fully comply with the regs”

You can see this from both extremes if you unbox it. Your “solution” could also “not fully comply with the regs”, but is that really the point?

Sparkingchip: 
 

I have ordered a replacement MCB to be posted to the customers home from Screwfix, because in addition to being rated at 32 amps it was not a original equipment manufacturer device being   a MCG MCB in a Chint consumer unit with the busbar pin cut down to get it onto the busbar and the slots don’t really line up. So when other remedial work is done if I don’t get back the customer will have the correct device ready for whoever turns up to install on a spare untouched pin.

So in this particular instance the job just wasn’t good enough, but I see some really good installations installed in a really professional manner by electricians who use a 32 amp protective device for a 8.5 kW shower overloading it by around 20% which stresses the MCB and overheats the MCB and the adjoining devices.

I cannot get a grip on their rationale.

Chint. Oh 'ek.

Z.

I see no part of BS7671 that says a 32A breaker may not be fitted, although it may be slightly unwise from a convenience point of view. Andy, you are putting forward slightly odd views, the case size of a 63A breaker is the same as a 32A one, so why do you think it may “overheat”? That is exactly what its thermal section is supposed to do to switch off small overloads, and the actual heat load for all sizes is exactly the same! You are falling into the “I don't like the look of that” trap, you may not but it is not dangerous in any way, and if the breaker trips, that is exactly what it is designed to do. I suspect too that Andy J is missing a point, the faster trip time is fairly unlikely as the thermal section is pretty accurate, because it also needs to be very repeatable, and there is no advantage this being quick, although there may be for the magnetic part as contact wear will be reduced. As usual, the question brings up many other pieces of knowledge and information that are largely unconsidered.

From experience I can tell you that the idea that a 8.5 kW shower will never blow a 30 amp fuse is a complete and utter load of tosh.

I had to install a new consumer unit because the customer had a 8.5 kW shower supplied from a 30 amp Wylex fused switch with a BS1361 cartridge fuse. The fuse blew a few times and was replaced, then the entire switch fuse unit went up in flames, luckily it was on a bare brick wall in his garage so other than soot marks on the wall there wasn’t any other damage.

You cannot have a circuit or protective device which is permanently overloaded by design.

Any way, my question is answered, electricians consider that protecting a 8.5 kW with a 32 amp protective device is acceptable based on custom and practice rather than because they have considered the design requirements.

davezawadi (David Stone): 
 

I see no part of BS7671 that says a 32A breaker may not be fitted, although it may be slightly unwise from a convenience point of view. Andy, you are putting forward slightly odd views, the case size of a 63A breaker is the same as a 32A one, so why do you think it may “overheat”? That is exactly what its thermal section is supposed to do to switch off small overloads, and the actual heat load for all sizes is exactly the same! You are falling into the “I don't like the look of that” trap, you may not but it is not dangerous in any way, and if the breaker trips, that is exactly what it is designed to do. I suspect too that Andy J is missing a point, the faster trip time is fairly unlikely as the thermal section is pretty accurate, because it also needs to be very repeatable, and there is no advantage this being quick, although there may be for the magnetic part as contact wear will be reduced. As usual, the question brings up many other pieces of knowledge and information that are largely unconsidered.

It is a good thing you did not have this as a question in your Wiring Regulations exam.

433.1.1 (i) The rated current setting of the protective device (In) is not less than the design current (Ib) of the circuit

But it is not necessarily co-ordination between shower and OPD?

You could elect that the cable determines your Ib.   ?

And the design current of a dedicated appliance circuit is going to be based on what?