Stefek Zaba wrote:

It's entirely reasonable to apply the "10A + 30%-of-the-rest" guideline
to say 'what realistic load will the cooking-appliance final circuit(s)
present to the whole installation under domestic usage".

OK so far...

It's entirely INAPPROPRIATE to use that guideline to say 'how should I
size the cables and overcurrent protection for the cooker final
circuit'!

.... No, for a *household* cooking appliance the "first 10 A plus 30% of
remainder" rule appears in both Tables 1A and 1B of the OSG. Therefore
it applies to *both* the design current of the cooker circuit itself and
to the loading presented to the installation.

Since the bleedin' thing's capable of drawing 5kW = 20A peak,
it should freakin' well be connected through an arrangement which can
safely provide 20 of the finest amperes in the land.

Well 5 kW is nearer to to 22 A. Applying the diversity rule gives you
13.6 A, so a 16 A circuit would be OK, but a 13 A 'spur' wouldn't. In
practice though I guess most designers would opt for a 20 A radial
circuit in 2.5 mm^2 giving a generous diversity factor of ~ 90%.

By your argument a typical free-standing cooker (12 kW, flat-out) would
need a 52 A circuit, meaning a 63 A fuse or breaker in practice. As
we all know though, a 32 A circuit (~60 % diversity) is fine [OSG p. 154].

--
Andy