With reference to 433.1.204 and cable as installed has min capacity of 20A if protected by a 30A or 32A. If the protective device is reduced to 20A how is the new minimum as installed capacity calculated or arrived at ? I've been looking in the Electrical Installation Design Guide, but the answer is avoiding my eyes.
Here we are again discussing rings, cable current rating, and plugging in several electric fires, washing machines, fryers, etc. There is a fundamental error in all of the comments which say "cable current rating" of which I cannot understand why this error is continuously made by electricians. I have discussed it at length several times before, but obviously, the point was missed or not understood. The tables of cable current rating and installation method are for permanently loaded cables, with a period of 24 hours. The properties to consider are power dissipation and thermal mass. The power is low and the mass surprisingly high. The only reason that the distribution network survives at all is this simple fact. It takes a long time to get the cable hot. Actually, you all know this because damaged cables in domestics from internal heating are completely unknown (implied by Zoomup asking for empirical evidence). The temperature at which PVC melts is 185 degrees C. It will go very soft at about 150-160 degrees but not fall away. So a safe temperature for a cable is MUCH higher than the tables in BS7671 suggest. There is loss of plasticiser to consider, but this doesn't matter much unless the cable is flexed (when it will be very stiff but can be fractured). This is the real effect of operation at excess temperature, loss of flexible life. A 2.5mm cable carrying 32A gets fairly warm but easily handled still. If buried in thermal insulation it will get hotter, but you will have to wait some time. There is no sudden failure, catching fire, or anything much else. I suggest that some experiments along these lines should be mandatory for all electricians, then a much better understanding of cable ratings should result, particularly for showers and cookers!
Here we are again discussing rings, cable current rating, and plugging in several electric fires, washing machines, fryers, etc. There is a fundamental error in all of the comments which say "cable current rating" of which I cannot understand why this error is continuously made by electricians. I have discussed it at length several times before, but obviously, the point was missed or not understood. The tables of cable current rating and installation method are for permanently loaded cables, with a period of 24 hours. The properties to consider are power dissipation and thermal mass. The power is low and the mass surprisingly high. The only reason that the distribution network survives at all is this simple fact. It takes a long time to get the cable hot. Actually, you all know this because damaged cables in domestics from internal heating are completely unknown (implied by Zoomup asking for empirical evidence). The temperature at which PVC melts is 185 degrees C. It will go very soft at about 150-160 degrees but not fall away. So a safe temperature for a cable is MUCH higher than the tables in BS7671 suggest. There is loss of plasticiser to consider, but this doesn't matter much unless the cable is flexed (when it will be very stiff but can be fractured). This is the real effect of operation at excess temperature, loss of flexible life. A 2.5mm cable carrying 32A gets fairly warm but easily handled still. If buried in thermal insulation it will get hotter, but you will have to wait some time. There is no sudden failure, catching fire, or anything much else. I suggest that some experiments along these lines should be mandatory for all electricians, then a much better understanding of cable ratings should result, particularly for showers and cookers!
