When cables are downsized without a fuse, question

The point to consider is the PSCC at the size transition. As I mentioned the other day, this is likely to be considerably smaller than that at the CU, and it makes a big difference to the adiabatic equation result. However, as Broadgage has said this size change is not a known point of failure in real installations, and this is because the equation works to help you in this case. As I have said many times before, cable current ratings are not the point where correct operation changes to complete failure, and there are many "if, but and then" points, all of which are considered to be worst-case at once. The current rating as tabulated is for cables loaded 24/7 at the maximum rating and at a particular ambient temperature. Diversity works because the cables take a very significant time to reach maximum temperature, and appliances like ovens and hobs do not take power continuously. In reality, the cable ratings for any domestic installation except electric heating are VERY conservative for all the above reasons. Just think a moment, when was the last time you touched a cable that was more than slightly warm, at 70C conductor temperature you would have let go fairly quickly, the answer is probably never in a domestic. A useful addition to the design guide would be a section to fully explain cable rating, protective device rating, and diversity, all of which are very badly understood by most electricians. You will see this illustrated by the endless discussion of ring final circuits and the alleged danger these present. Has anyone ever seen a melted cable on an RFC correctly installed? If so I would love a picture and full details of the installation. This could change if those who install cables in cavities continue their work, and insulation levels continue to increase. I consider this very bad practice, although it is cheap and easy, and saves much wall chasing.

The point to consider is the PSCC at the size transition. As I mentioned the other day, this is likely to be considerably smaller than that at the CU, and it makes a big difference to the adiabatic equation result. However, as Broadgage has said this size change is not a known point of failure in real installations, and this is because the equation works to help you in this case. As I have said many times before, cable current ratings are not the point where correct operation changes to complete failure, and there are many "if, but and then" points, all of which are considered to be worst-case at once. The current rating as tabulated is for cables loaded 24/7 at the maximum rating and at a particular ambient temperature. Diversity works because the cables take a very significant time to reach maximum temperature, and appliances like ovens and hobs do not take power continuously. In reality, the cable ratings for any domestic installation except electric heating are VERY conservative for all the above reasons. Just think a moment, when was the last time you touched a cable that was more than slightly warm, at 70C conductor temperature you would have let go fairly quickly, the answer is probably never in a domestic. A useful addition to the design guide would be a section to fully explain cable rating, protective device rating, and diversity, all of which are very badly understood by most electricians. You will see this illustrated by the endless discussion of ring final circuits and the alleged danger these present. Has anyone ever seen a melted cable on an RFC correctly installed? If so I would love a picture and full details of the installation. This could change if those who install cables in cavities continue their work, and insulation levels continue to increase. I consider this very bad practice, although it is cheap and easy, and saves much wall chasing.

I have come across many, many electric shower cables buried under lots of loft insulation, but have never seen any damaged insulation due to over heating in the main run. I have though seen many damaged cable ends at 45 Amp cord switches where the terminals were not tightened fully or became lose when the installer struggled to get the switch in position next to its box.( I squared R heating effect.)

Z.