Flexible cables in stud walls

yep, the world is not that precise - the tables say nothing about vertical or horizontal routing but it makes a good few degrees difference if the cable is pre-heated by more cable below it.

Even if the figure was 15A for the headline case, the basis of the tables is  a 40degree rise, to 70C from a 30C starting temperature, after some hours at that load to reach equilibrium. In the UK unless the location is near some heat producing kit, much more than a 25C start is unlikely, and rising to 80 instead of 70 does not explode the cable, it just goes a bit floppy, and gets painful to touch. To reassure yourself drop an offcut in your tea and watch it not melt like butter - or ideally your mate's tea. My point is there is an illusion of precision with the ratings - you may calculate 18A and agonise that the load may  be 20 occasionally- but a lot of other things have to go wrong at the same time to actually cause a real problem, even if your sum ends up being 10-20% out on the current rating.

Mike.

yep, the world is not that precise - the tables say nothing about vertical or horizontal routing but it makes a good few degrees difference if the cable is pre-heated by more cable below it.

Even if the figure was 15A for the headline case, the basis of the tables is  a 40degree rise, to 70C from a 30C starting temperature, after some hours at that load to reach equilibrium. In the UK unless the location is near some heat producing kit, much more than a 25C start is unlikely, and rising to 80 instead of 70 does not explode the cable, it just goes a bit floppy, and gets painful to touch. To reassure yourself drop an offcut in your tea and watch it not melt like butter - or ideally your mate's tea. My point is there is an illusion of precision with the ratings - you may calculate 18A and agonise that the load may  be 20 occasionally- but a lot of other things have to go wrong at the same time to actually cause a real problem, even if your sum ends up being 10-20% out on the current rating.

Mike.

Your point is understood. However, myself being a simple electrician, if anything were ever to go wrong with that bit of the wiring (perhaps resulting in a fire), I don't have the qualifications/experience to be able to be able to argue that I evaluated the flex to be acceptable in that scenario, whereas an experienced electrical engineer can argue their case with more gravitas, and be accepted by authorities more readily than I. I recal reading a post on the forum some time ago that perhaps unfairly said that electricians seem to think that if a cable carries more than an amp above its Iz/It, it'll burst into flames, whereas engineers know better. I don't believe we do think that, it's more that we have to go by stated values because we're not qualified to make allowances in ccc that differ from those provided.

T.