Many thanks. My kitchen sockets are in the form of a 4mm radial on a 32a MCB. The circuit is RCD protected. The last leg of the circuit supplies the combi boiler. At present, the cable goes on the outside of the boxed-in pipes under the boiler, clipped direct to the wall. This was only ever meant to be a temporary measure. Two years later, I am planning on re-boxing the pipes in a better looking box, and obviously it's a good time to relocate the cable. I want the 4mm T&E to be inside the box. The temperature inside the box will get quite high....obviously the central heating flow and return and hot water pipe will be enclosed in a small area. The cable will be around 10cm from the pipes inside the box (including the gas supply). This section of the circuit will be carrying only the supply for the boiler (0.6a). Is derating even necessary? The switched FCU with 3a fuse is at present located above the boxed pipes, just below the boiler. Should the fuse be located before the cable enters the box? If so, it's not practical to have the switch at that location, so would it be ok to use a seperate FCU (un-switched) before the box and just an un-fused DP isolator switch at the top of the box (to avoid having two fuses)? Am I worrying about it too much?
Also, would you put the cable in trunking inside the boxed area?