In uk.d-i-y, Michael Brewer wrote:

1) You and others have mentioned that the regs say that the control
switch for an appliance must be within 2m of that appliance. Do the
regs say whether the control switch must be above the counter (i.e.
instantly accessible) or is it OK to have them out of sight under the
counter, but still accessible through a cupboard?

I'm pretty sure, but don't claim to be authoritative, that the switches
are supposed to be accessible. The full-on answer would depend on the
function the switches are considered to be performing: if emergency
shutoff, then visible-and-accessible; if isolation, less accessible would
be permitted. Let's use common sense he definitely one of the reasons
you want the switches is to cut power to your cooking appliance if something
catches fire on it or in it. Then you want to be able to cut the power
PDQ alongside doing Other Sensible Things (covering with well-dampened
towel, for example) - and the "you" might just be Auntie Mabel cooking
lunch for all of you. Hence the requirement for the switch to be in plain
view and close by - i.e. I'd think long and hard about pretending a
cooking appliance isolator does *not* have any emergency switching function.

2) A couple of remaining questions with domestic wiring that are
troubling me are as follows. A ring circuit is usually wired with
2.5mmsq cable (rating 30A), but is apparently able to provide up to
30A... is this because it's a ring not a radial and so current to any
point can go both directions and is therefore split in half, so that a
ring supplying 30A to a particular point would actually only be
carrying 15A in the supply cables?

That's the general idea - the load is shared among the two paths to
any place on the ring. But it's not shared half-and-half other than at
the midpoint of the ring - it's in proportion to the resistance of
each path, which is mainly determined by the length of each cable path
back to the CU, along with how well each joint along the way has been
made. That's why you can't just double the current-carrying capacity of
the 2.5mmsq cable in designing the ring: and also why it's poor practice
to design a ring layout in which heavy loads are concentrated at one end
of the ring - e.g. a ring to serve all downstairs which starts off in the
kitchen, where the heavy loads are, and then snakes lazilly all round the
house before coming back to the CU.

................................... This brings me to the question:
why are cooker circuits provided as 6mmsq radials and not (e.g.)
2.5mmsq rings, if the latter will take just as much loading as the
former?

Wiring practice for cooker circuits hasn't fully caught up with the fashion
for split-level hob-n-oven setups and similar separate-cooking-appliances
setups: by and large the conventional circuit design assumes there'll be
one hulking great standalone cooker. Such cookers have a *peak* current
draw of say 40A (turn on 4 rings - there's 7kW - and both top and bottom
ovens - another 3kW - look, ma, 10kW peak loading giving a current of
10,000/240 = 41.3A; or if you prefer to work with the nominal-voltage
fiction, 10,000/230 = 43.5A, though if the supply voltage did drop to
230 the power drawn through the cooker elements would also fall). Now, that
peak load won't be sustained for long, even if Auntie Mabel is cooking
Christmas dinner with all the trimmings and using all 4 rings and both
ovens, because the simmerstats/thermostats will be switching each bit of
the cooker on and off. That - and the fact we don't in practice turn on
all our cooker's component loads at once - is the thinking behind the
diversity rule which allows us to treat the worst-case-40A-plus load
of the cooker as less than that (10A + 30% of the rest in a domestic setting)
for purposes of counting up the total demand of the installation. *But*
we still size the wiring and switchgear for this appliance which on
occasion *will* draw its full 40A to suit that peak demand. (And voltage
drop along the length of the cable matters too). Hence, the Right Answer
is almost always 6mmsq for the dedicated cooker circuit feeding either
a single cooker, or split later to feed oven + rings as separate appliances.

So, 2.5mmsq rings do not 'take just as much loading as a 6mmsq radial' -
the dedicated circuit is much more appropriate to supply a single load,
and can safely meet a higher sustained load than a 2.5mmsq ring.

HTH - Stefek