I think it was introduced at the same time as the change of rating of ring circuit cable from 0.67x In to 20A - all so we could continue to use 2.5mm2 PVC T&E on 32A MCBs in thermally insulated buildings (as 0.67x32A (=21.44A) didn't jive with (what's now) 4D2A method A = 18.5A).

- Andy.

So what's wrong with reducing the load on XLPE cable so that the conductor runs at no more than 70 degrees (as in the OP's example), which would suggest that some advantage could still be gained from XLPE (note 2 not withstanding).
- Andy.

On the face of it BS7211 cable seems to have many advantages, higher capacity without increasing conductor size, making it cheaper than larger sizes of BS6004 to buy and easier to install. Also it can cope with the higher levels of insulation in buildings and is less of a risk in a fire with reduced fume hazards.

The problem seems to be there is the possibilty that existing electrical accessories are not compatible with the possible higher conductor temperature.

Considering that most electric showers use simple strip "choc" connectors, it would need careful consideration before subsituting a 6mm BS7211 for a 10mm BS6004 to supply an 10.8kW shower.

I suspect the cooker scenario of the original post would be unlikely to be a risk, as the cable would not get to full operating current limiting the conductor temperature. At Elex, Coventry I bought the IET book the formula is scanned from so I have that information to hand, however I go with the calcs already done rather than try and introduce my own.

Andy

I don't think that using 90°C cable gives you a greater current capacity if you want to limit the temperature to 70°C. When I first carried out the calculation to work out the temperature I used the 40A of the MCB but subsequently I thought the correct current should be the none fusing current of the MCB which is 1.45x40A.

This is backed up in Paul Cooks book which states:

Which makes sense as haven't the tables been produced by using the same formula?

The complete relevant paragraph can be found here: www.sju.co.uk/PaulCookCableTemp.pdf

Isn't that the formula we started with?



Not if my (and the OP's) maths are right. I assumed they were, hence my initial use of the 70-degree table in my 1st reply, but working backwards from the 90-degree tables seems to give significantly more generous results.

- Andy.