tony sayer wrote:

What's wrong with running two lumps of 2.5 twin and earth, as the cooker
hob is rated at 5.5 kW which is 23 amps and within the ratings of 2.5
cable in free space, the oven no more than around 3Kw IIRC. Two lumps of
2.5 will go into the cooker outlet, but it seems wrong.

Comments anyone?.....

It /might/ be compliant, since there is no risk of overload, but you
need to consider a few things:

1. Ambient temperature (it can get warm near a cooker): the 2.5mm^2
cable "in free space" would be rated at 27A at 30deg. C. At 40 degrees
Table 4C1 tells us that the ambient temperature factor (Ca) is 0.87
which brings the rating down to 23.5A which is on the limit (5.5kW at
230V is closer to 24A by the way, not 23A).

2. Grouping factors: there'd be a further derating if your two 2.5
cables were side by side. This factor (Cg) is 0.8 for two bunched
cables (Table 4B1). Applying both the Ca and Cg factors the effective
rating of the 2.5mm^2 cable is now down to under 19A...

3. Short circuit protection: the circuit is fused or MCB-protected at
more than the cable rating. This may be OK since overload can't occur,
but you do need to check that, in the event of a s/c or earth fault at
the end of the longer of your two 2.5mm^2 legs, the fault would be
cleared before the cable overheated [Reg. 434-03-03]. The following
information is needed to do the necessary calculations: (a) type and
rating of protective device, (b) circuit length, (c) circuit cable
size(s) and type(s), including CPC sizes, (d) type of installation
earthing - whether TN-S (cable sheath earth) or TN-C-S (PME).

You're definitely into marginal territory here. Most cooker outlets
have a terminal capacity of 10mm^2 or more. You might well be able to
get two 4mm^2 cables in, which would put things on much safer ground,
certainly so far as points 1 and 2 are concerned. You could also
consider reducing the circuit protective device rating from 45A to 32A
(YIK that 24A + 13A 32A, but diversity is allowable here (first 10A
plus 30% of the remainder = 18A).

Another alternative is to supply the oven from a ring circuit - it may
well come with a 13A plug on in any case - leaving only one cable to go
from the cooker outlet to the hob.

--
Andy