Paul Lucas wrote:
Hi all,
This question has been asked in various forms,
It has, hasn't it? ;-)
The previous setup was thus:
30A CU fuse ---- Cooker switch ---- Outlet ==== oven + hob (two wires
from outlet, one to each appliance)
Now I have a new shiny oven which seems to have a much thinner wire
attached to it. The power rating on the back says 2400W and I've read
on numerous posts that these modern ovens are designed to be powered by
a normal 13A connection to the ring main. The hob is a bit of a
mystery, the instructions say it needs a 20A fused connection (with
2.5mmsq wire) but then goes on to say that the input power is 6500W. By
my calculations 230V x 20A = 4600W which means it'd have a PF 1 ??
Assuming it really is 20A is it safe/normal to connect the following up:
30A CU fuse --- Cooker switch --- Outlet --- (Spur) --- 13A FCU ---
Oven
and then wire the hob direct to the outlet??
Several points. In principle, it's going to be OK to run both appliances
from the existing cooker radial, and treat them as a single appliance
from the point of view of calculating the realistic load.
But... when we come to the detail... things get less than
straightforward. The oven's not too bad: it can be supplied through a
normal FCU, since its draw is well within the 13A design limit of such,
and the 13A cartridge fuse you'll fit will discriminate against the 30A
rewireable so providing closer short-circuit protection to the 'much
thinner' integral cable. No need to use a switched FCU unless the common
cooker switch would be more than 2m from the oven - though nothing to
stop you using a switched FCU either. *Don't* use a 13A socket, though:
you don't know what else might get plugged in, and the expectation is
that all 13A sockets will be on the kitchen ring.
The hob presents more of a PITA to comply with the manufacturer's
instructions. There's no such thing in the UK as a readily-available 20A
FCU. The hob's peak load (all-elements-just-switched-on) might well
reach the 6.5kW = 27A on its rating plate; their recommendation for a
20A protective device isn't silly, though, because this peak load won't
be drawn for long as the thermostats kick in and out of circuit. In
practice, wiring the final run from cooker-control-switch to the hob
with 4mmsq (or 6mmsq) will give a cable well-enough rated for the full
load of 27A even in the higher ambient temp, and protected against
short-circuit by the 30A rewireable - see Andy Wade's calculations in
http://groups-beta.google.com/group/...8?dmode=source
- though I'd feel better about it if the 30A protective device were an
MCB rather than a rewireable.
To comply exactly with the manufacturer's desire to see a 20A-rated
protective device would mean faffing about to find a way of putting a
20A MCB or fuseholder in the kitchen; and although a 'garage CU' (with 2
ways) would do the trick, and could in principle go in place of the
cooker switch, it's an ugly gert thing to have in your kitchen; you
mount it high up by the ceiling but then you'd need to retain the
existing cooker-control switch for ready isolation of the hob.
One final trick you *could* pull is to say "OK, hob and oven together I
can treat as one cooking appliance" (the Regs explicitly allow this),
with a Diversity-reduced load of (10A + 30%-of-the-rest). So, the hob's
peak draw is 27A, plus 10A for the oven, making a Diversified load of
(10 + 27*0.3) = 18.1A. "Therefore I could replace the 30A rewireable
with a 20A MCB, still have the cables sized for the peak loads, *and*
comply with the manufacturer's guarantee-preserving desire for a 20A
protective device on the hob." But frankly, you'd be setting yourself up
for unnecessary inconvenience on the one time a year you do run the
cooking appliances full blast, and it'd be a bizarrely exact
lowish-resistance fault in the hob which would make it safely-fused at
20A but not at 30A: the circuit fuse is protecting the *cable*, not the
appliance, and the hob's not capable of producing an 'overload' of the
cable's capacity, as opposed to a 'short-circuit'.
So - in summary - connect the hob to the load terminals of the existing
cooker switch using 4mmsq T&E; also connect an FCU for your oven to
those load terminals; if you can, swap the 30A rewireable for a 30A MCB
(easy if you've an older-style Wylex consumer unit - they sell
plug-compatible MCBs to replace the rewireable carriers).
Unless, of course, you hear wiser from other contributors!