Is there any guidance on how derating factors should be applied if for example a 40m run of cable was surface clipped in 'free air' for all but 150mm of it's length, where it passed through insulation? Arguably the bit of the cable that would melt if overloaded would be the bit at the insulation as that is the 'worst case', and he 'safe' option would be to use the derating factor for cable installed in insulation, but arguably the ability of the cable to dissipate heat when fully enclosed in in insulation throughout it's length is very much different to a cable passing through a short distance of insulation, where the conductors would also conduct away heat from that location with some effectiveness.
What about a similar scenario with a cable running 38m on a tray in an open ceiling void, and the last 2m in a trunking to a distribution board with perhaps only a 25% fill?
Should you always calculate for the worst case, regardless off the length involved? Is there any published guidance on this?
Jason.
