Derating factor for bunched cables in Ladder

ERA 69-30 parts 6 & 7 refer to cables laid in multilayer groups on trays. Haven’t looked at them for years and I’m not in the office so can’t remember whether the XLPE part 7 depends on the PVC part 6 or if they stand alone; there may also be a dependency on power loss tables in parts 3 & 5 respectively.

Be warned that the method is nothing like BS7671 appx 4; it won’t give you a simple factor for grouping.

This may be a non problem. If domestic or similar with a cut out fuse not exceeding 100 amps single phase, then obviously the sustained current cant exceed 100 amps in total.

If industrial, then similar arguments may apply, depending upon what size fuse  protects the submain feeding the D/B in question.

If the installation exists already, then  maybe there is less need to calculate a theoretical current rating based on what may well be a poor guess of load currents and duty cycles, to get a highly precise, but incorrect answer.

Tables are great for selection before erection, but very deliberately we usually end up with cables that run cooler and last longer than predicted, except perhaps where something silly has happened like the free air rating has been applied to something that is not free.

One could inspect it. Do any of the inner cables show signs of distress ? does the whole bundle feel warm?  Could one fit 'high tide'  irreversible temperature stickers at this inspection and look at them at the next ?

To do a proper thermal calculation that is not just a wild approximation, you would  need to know the short term (few minutes ) average heating per unit length of each cable and at the least the external surface area of the bundle (*). Ideally the cables expected to run hottest should be near the outside, but this is hard to control. Given the heating dependence on the square of current, the reduction of heating effect from say a halving of current (to 1/4 the power) is worthwhile. The corollary is that a small and apparently innocent increase in load, can rapidly be destructive.

* this captures the shape factor, a circle has minimum area, while the same bundle flattened out into more of a ribbon of cables has more exposes surface and is therefore far better cooled.  

Calculating cable loom rises, which is very similar to this problem is a full time job in some weight critical industries, and even then nothing beats the 110% load soak test and thermal camera.

Mike.