The challenge this gives me is that the OCPD is 10A rated, the design current is 2.826A, the 30% rule based on the multicore derating alone is 2.821A, and the fully de-rated capacity of the cores (allowing for a further group of the multicores in the installation) in the multicore cable cores is 6.11A. If I can apply the 30% rule so no derating for the multicore cable then we are in the clear. The cable is already installed and in all other respects calculates out fine.
The graph of derating factor vs number of cores isn't quite linear:
It nearly is in my region of interest though! Does anyone have a formula for this calculation (if there is one?).
I know this is so marginal as to not normally be worth bothering about, but we may be dealing with a particularly picky client organisation here.
Practically, the chances of someone going up a tower, replacing some or all of the 650W lanterns with 2kW lanterns and leaving them all turned on to the point where the cable is damaged is so close to b***** all as to really but be worth considering, but right is right and wrong is wrong!
It's also not so easy to replace the OCPDs with 6A ones, as these are in a theatrical dimmer unit so modifying that would likely void the warranties, never mind the approvals!
Any other creative solutions welcomed.
As a complete aside I wonder how they chose the numbers of cores to put in the table - multiple single phase circuits would always be an even number, as would multiple TPN circuits - only 3-phase delta circuits would get you a multiple of 3 loaded cores so how you get to 7,19 or 37 cores baffles me unless you put part of the circuit in one multicore and part in another which is a bad idea, or it's some kind of control application.
