The installation is as 100A installation in cable sizes, circuits, safety, etc. no cable will overheat. A 100A fuse could be fitted not compromising any safety whatsoever. But the problem is, in the three phase MEM fuse cupboard it states "63A only". The only problem is that a 63A fuse is there with a sticker saying only use a 63A. The responses here are generally, treat the installation as any other, there is not a problem. The diversity calcs say 63A is too small while 100A is fine.
Yes, we understand that. As we (including yourself) have all said, the simplest way of addressing nearly all of your concerns would be (
IF the supply is adequate to provide 100A) to replace the fuse with a 100A one. If the "63A only" labelling concerns you, the alternative, as suggested by Andy, would be (again,
IF the supply is adequate to provide 100A) to run an additional feed, with an additional separate 63A (or maybe even 40A) switch-fuse for, say, the UFH.
However, as I've said, that all assumes that the supply is adequate to provide 100A. I presume that the "Three phase MEM fuse cupboard" is downstream of the DNO fuses, and I wonder what rating those fuses are (and whether anything other than 'your' flat is supplied by the phase concerned)?
If it were possible to move to a 100A fuse (or equivalent with two fuses), then all you would really gain would be a degree of reduced heating of the fuse during periods of high demand. The actual demand (hence currents in any cables) would not have changed and, as you've said, the cables are already more than adequately protected.
My work around is to have all the rooms with UFH on timers and stats (which is the norm anyhow), with UFH coming in, in staggered times. Then a E7 tariff, so the place is heated up cheaper when other appliances are off during the night, except the odd kick in of the fridge. Also have combined appliances like a washer/dryer.
As John/flameport has pointed out in the video, official guidance on diversity/'maximum demand' calculations (both in BS7671 and the OSG) is next-to-useless, leaving us reliant on common sense (which people possess in varying degrees).
Some people would undoubtedly argue that one can/should not use time switches etc. to overcome what would otherwise be 'maximum demand' problems with the design of an installation, on the basis that someone could easily change the settings of those time switches and therefore invoke the 'maximum demand problem'.
However, if one does
not take that view, then time-switching of large loads provides an excellent way of achieving considerable reduction of estimated 'maximum demand' on the basis of 'time diversity'. In other words, if the time switching were such that, say, the UFH was
never powered at the same time as other large loads (cookers, washers, dryers etc.), then (provided no-one interfered with the time settings) that would result in a dramatic reduction in the common-sense after-diversity 'maximum demand'.
Indeed, if one had a heating load of, say, 35A and other large potential loads totalling another 35A, then, if time-switching 'guaranteed' that the two were never on together, then the estimated 'maximum demand' would be 35A, not 70A. However, as above, views about the acceptability of that approach to design would depend yupon the extent to which is was felt one could rely on their being no 'interference' with the time switching.
