Is it under any circumstances acceptable to use 1.5mm twin and earth cable to run an additional socket from an existing socket or is this cable only suitable for lighting circuits.
You may only use it if you feed the cable via a fused spur with a 13A fuse inserted before going on to the new socket. This protects the downsizing of cable as you suggest with regard to current carrying capacity.
For what it costs to buy the fused spur and associated patress/box , you might as well just get some 2.5mm and do it properly!
Technically it is ok to use 1.5mm² as it will safely carry 22A if installed to reference method 11 (on a perforated cable tray) or reference method 13 (free air), but I personally don't know any professional that would use 1.5mm². I always install a minimum of 2.5mm² for socket circuits.
If the 1.5mm² cable was installed to reference method 4 (enclosed in an insulted wall, etc.) then its current carrying capacity is only 14A, which is definatly not acceptable as you could draw 26A from a twin 13A socket, thus overloading the cable by 12A (that's a bad thing™)
The method that has been used is free air I suppose, it runs down the back of kitchen units then approx 3metres along the floor and into a double socket. I could likely replace it without too much trouble. It is the supply for a washing machine and fridge.
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