My cooker circuit is currently fused at 30A and supplied by a (rather
ancient) 4mm cable. As part of the rewiring the circuit will now be
fed from a 32A MCB and my electrician has suggested using two new 2.5mm
cables to supply it instead.
While it seems to make sense to me, I'm a little worried that he's only
suggesting it so that he can use up the 2.5mm he's got left...
I'm probably being a bit too paranoid and I'd be very grateful if
someone could please confirm whether it is acceptable to use two 2.5mm
cables in this fashion.
That would be my take on it too. I don't know much about the latest
wiring regs but I would have thought that paralleling cables is not
considered best practice. What happens if one cable works loose or is
accidentally cut frinstance?
My instinct is that you should insist on a single cable, 4mm.
IIRC, the load the cable can safely take is proportional to the
cross-sectional area. So 2 x 2.5mm sqrd gives you 12.5 mm^2 but 4mm sqrd
gives you 16mm^2. The actual copper core diameter is much less than 2.5 or
4.0, but I guess the principle holds good.
Also, shouldn't the fuse (or trip) be rated to protect the cable? - so the
cable has got to be safe at 32 Amps.
It isn't quite like that. The quoted size of conductors refers to sq mm in
area, therefore 2.5sqmm x 2 = 5 sqmm, whereas 4 is just 4. Having said that,
I would use 6.0 to give bags of safety factor. You may have a bigger cooker
in the future.
Using 6mm or indeed 10mm on some cookers is the norm.
You forgot about the earth loop. Two 2.5 T&Es have a combined cpc of 3mm,
whereas 4mm I think (never bought any) has only 1.5mm cpc. It would be
possible to use longer runs using two 2.5mm cables.
Regardless, insist upon 6mm as the minimum size.
Cooker circuits are normally wired in 6mm t&e not 4mm and fused at 32a.
Ring main circuits are normally wired in 2.5mm t&e fused at 32a.
At the breaker of a ring both cables (2.5mm) are connected together as they
are at each socket outlet hence the term ring.(very few work loose)
Whilst very uncommon running a 32a cooker circuit as a dedecated ring is not
(as far as I am aware) against any regs'
However unless your cooker is miles from the mains the cost of a length of
6mm is nothing compaired to the hastle of running 2x2.5mm cables as a ring.
And most sparks carry 6mm as stock.
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