It's easy enough to find size of cable for a particular rating, but what I want to know is - what is the rating of this cable ? Somewhere there OUGHT to be a table of sizes vs. rating, but I've not been able to find one. The only tables talk about e.g. 2.5 mm for ring main cables - OK in theory, but totally useless in practice ! This particular cable is the old standard twin & earth, red, black & yellow/green (all sleeved), in an outer white PVC skin. The overall size, measured with calipers, is 14mm wide x 7mm thick. It's in a duct behind the render, then in the space between floorboards & ceiling. So what's the rating of it ? Anyone ? Please ?
Cable sizes have changed over the years, it was diameter and number of strands, now it's the cross sectional area. In real terms getting a micrometer on the cable does not work, what you want is some lugs, so red is 1 or 1.5 mm and blue 2.5 mm and yellow 4 or 6 mm and if slack or tight will show 4 or 6 mm. After that it's stamped on the lug.
Once you know the size the next step is type, some cable is rated 60 degs C normally flex, then 70 degs C standard twin and earth, (PVC) and 90 degs C for the like of ali-tube cable (LSZH). Mineral insulated can go even higher but the problem is even if the cable can run at 250 degs C at that temperature it would likely set other things on fire.
Then we have the route, there are letters to cover most routes, but twin and earth has 100, 101, 102, and 103 this http://www.batt.co.uk/upload/files/curr ... 876143.pdf link to Batt cables shows how 2.5mm cable can carry between 13.5 and 27 amp. For a ring final we need 20 amp cable so with 2.5mm either method 100 or 102 which limits insulation to no more than 100 mm thick. It also in the regulations shows how it should be clipped as if clipped to even a wooden beam that beam will help dissipate the heat.
Looking at the Batt cables spec it would seem you have 6 mm cable. But since you say white and earth is green/yellow not bare this could be wrong.
There are other manufactures but I tend to use Batt cables site myself. I really cringe when I go to places like B&Q and it says 2.5 mm is 21 amp cable, it's not that easy. We also need to compensate for heat and running cables together, but not really with domestic unless run with hot pipes.
So it would seem 6mm was correct. I have questioned cooker cable size many times, although there has been a move to using 10mm cable, when using separate oven and hob the problem is you are only sure the oven is OK with a 32A feed, over that standard feed under fault conditions the internal wiring could fail before the fuse or MCB opens, so even with a 10mm cable using a MCB over 32A could cause problems.
What we need is a 16A fused connection unit for oven feeds, but they are simply not made.
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