Invisible-Man said:
I would have thought that if you had wires running through a cavity for instance, then have it filled with insulation, this would be classed as buried in insulation! (meaning a 50% derating),
That's one of the two reasons why you're not supposed to run cables in cavities....
surely what I have done is no different from passing wires through studwork and then insulate with rockwall,
Passing
through is not too bad - 50mm of insulation means a 0.89 derating, and 100mm means 0.81.
I would have thought that wires laid within a modern floor would have insulation on one side, as in many walls (with no air circulation).
Insulation on one side is not so bad - if the cables are in contact with a thermally conductive surface on one side that is Installation Method 15, and 2.5mm² T/E is derated to 21A, which means it's OK for a 32A ring final.
I did not think that my situation would be classed as (buried in insulation)!
But they
are actually buried in insulation, are they not?
If however, this would be classed as buried in insulation or at least derated in some way, what are the consequencies, is it dangerous, for instance not throwing the typical MCB for that size wire, ie 32amp for 2x2.5mm ring main, causing possible fire hazard etc, or would it mean that the MCB would just keep throwing when loading or trying to run apperatus.
If we assume a free-air rating for 2.5mm² T/E of 30A, or if you want to be stricter, the highest rating quoted in BS7671 which is 27A for Reference Method 1 (clipped to a wall), then 50% derating gets you 13.5 - 15A, so it no longer complies with the regulations for ring-finals, which are specifically written to allow the peculiarly British ring-final circuit of 2.5mm² with an assumed worst-case cable rating of 20A.
As a guide you might revert to the pre Amendment 1 regulation 433-02-04 which required that the “
minimum current carrying capacity Iz of the cable be not less than 0.67 x the rated current setting In of the protective device”, and figure that a 20A MCB would make your circuit safe, but you would still be in contravention of the wiring regulations, and you would have to show that you'd calculated things properly, and had taken into account the accelerated aging of the cables to determine their lifespan.
But MCBs just detect the current flowing - they have no idea of how hot the cables are getting, so if you stick with a 32A MCB the answer is.....
....fire hazard.