Hi, I want to install a cooker circuit feeding two cookers one of 2.8kw and one of 4.6kw. I have done the initial sums and come up with a cable size of 6mm. However the cable is 15mts long from the consumer unit to the cooker switch, is bunched with two other cables for about 8mts and runs on its own inside an insulated stud wall of about six feet long. It is a modern consumer unit with mcb's. I have used the correction factors and come up with an answer of the circuit requiring a 25mm cable. I think I am going wrong somewhere in my maths or at least I hope I am. Could somebody please put me right with the details of coming to the correct answer.

As well as diversity you could also apply the principal of reg 433.3.1 (ii), if the cooker is an entirely resistive load, meaning that you would not have to base the cable rating on the protective device overload rating, as overload protection would not be relevant.

(In this case, the full load of 20A for the largest cooker would safely be carried by a 6 sq mm cable installed to method #103)

The fuse/cb would still afford fault protection if the Zs was low enough.

(I'm not suggesting this as a normal solution, even though it is within the regs, because too few installers actually understand this approach and it doesn't provide allowance for future changes to load characteristics. In this case, however, I doubt you'd have a problem.)

Carl,

a purely resistive load is, effectively, nothing more than a piece of conductor (unlike, say, a motor). The impedance of such a load is made up almost entirely of the simple resistance of that conductor.

Such a load could fail in short circuit or earth fault, or go open circuit, but is incapable of creating an overload.

So, in this case, the 20A (full) resistive load would easily be carried by a 6 sq mm cable at #103, with no risk of overload and therefore, no need to provide overload protection, but the circuit could be protected against fault currents by a 32A cb, simply by meeting the Zs limit for that breaker.

(But, as I've already stated, this would be something of a last resort in most circumstances.)

Thanks Guys. I have installed the circuit, livened it up and all testing ok including the Zs. I was going to use an rcbo but have saved a few quid by using a mcb. As an aside the board is a Hager and the few wholesalers  that I went in to didn't stock Hager fuses. Luckily the board had a spare 32a mcb anyway. Could I have used other makes of fuses or is this not good practice?

Or indeed if the cable is buried in the buildings fabric in a tradional fashion.

As Lec says, above, if the circuit is buried within the fabric of the building less than 50mm and not protected by an earthed metallic covering then an RCBO will be needed.

I am off to the patent office....

dingbat wrote on May 22^nd, 2009, 10:35am:

I have just got back from the patent office, now I have to go again...

TBH I never fit the cooker isolators with a socket.  I too see them as 'granny-ish'.