Hi,
I am having some trouble confirming how to derate a cable correctly. The current install is a 10mm thermosetting multicore (3 copper core 400V, SWA as earth) from dist board to UPS. There is a 63A BS88-2 gG fuse protecting the cable. Our calculations for the existing plant utilise 512.1.5 - where the cables are run to 70°C, so all cables are classed as thermoplastics, in order to negate all existing equipment checks on circuits affected by the upgrades.
Using table 4D4A, the best-case scenario described above is the cable is capable of 62A (ref. method E), but is protected by a 63A fuse. So we know we need to do something as this is before applying grouping & installation method derating factors.
The cable is a 15m run, on perforated tray & under suspended floor (again on perforated tray) bunched with other cabling (grouped as 8).
With reference to table 4A2, #47 installation method seems suitable, which defines the installation method as B.
Next I go to Table 4C1, row 1, grouped as 8. This gives a derating factor of 0.52.
What I am unclear on, is what do I apply the derating factor to? I assume it is the 62A as noted above, meaning the cable should be derated to 32.24A in this scenario. Is this correct, or is there a different value I should be using to apply the derating factor to?
Once I understand this, I can move on to the solution (load assessments to determine if the best solution is to increase the cable size, or reduce the fuse size (or both)).
Appreciate any clarification on this.
Regards, James
