derating

So following having a read the other day of both the on site guide and the full BS7671 regs that I have, it dawned on me how much of a impact grouping of cables as per table 4C1 has, and how I suspect that what my electrician did back in 2013, violates the regs. I'm trying to work out that if...

Say you had a cable running horizontally through floor joists without any thermal insulation (assuming horizontal cable support requirements were met), how is one supposed to know how much to de-rate the cable by and what the de-rated current carrying capacity is for a given conductor size...

I'm currently renovating a my new home which is a late Victorian house, and I've taken plaster off a couple of stud walls in my office. With in the room, there are currently (The house was rewired around 7 years ago). 1 Run of 1.5mm T + E for the lighting Circuit (running from floor to ceiling...

After a bit of initial advice, please. Decided to internally insulate a solid brick outbuilding as it's freezing in winter... in the process will be needing to move and add more electrical sockets etc, going to use said building for general craft/diy type projects. I had intended to build...