95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fuses

[Chr!s]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I don't believe at any point we were trying to calculate watts/meter. We were taking those as a given (H).

We are trying to calculate the heat disipated via the cable under varying conditions, so wee need to us Watts/m as a measure, and if you want to "FORGETN THE SOODING TABLES" you need IEC 60287.

You need to consider many factor's, from the construction of the cable, to the A/C resistance taking into account the Skin effect and the Proximety effect, then you need to consider the external influences, then as i have said after 4 pages of equations you can calculate the Heat disipation of the cable, the tables etc in BS 7671 are not far off the calculted.

 

[Engineer54]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Yep, good idea, forget the soddin table altogether!!! lol!!

 

[Lucien Nunes]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I think there is confusion about the need to apply grouping factors to paralleled cables because different contributors have assumed the CSAs have been arrived at in different ways, and started with different tabulated CCCs...

Those saying no derating is needed, have assumed that a single cable of sufficient Iz with CSA Qmm² is split into n grouped parallel cables of Q/n mm². This gives equal CSA and greater heat dissipating area compared to the single cable, so no derating is needed. In fact the current density can be increased somewhat.
Example: Take one fully loaded 6.0mm², tabulated CCC 41A, and split it into four 1.5's. Current in each 1.5mmm² cable 41/4= 10.25A This is well within tabulated rating (17.5A) and even if Cg=0.65 is applied to the cables (11.375A) they are still good for more than 10.25A. So you don't need to consider the grouping factor separately.

Those saying that derating is needed, have assumed that n separate cables of Q/n mm² rated at their individual tabulated CCC have been paralleled up and grouped to form one circuit. Since the Iz for a cable of CSA Q/n is much more than 1/n x (Iz of cable CSA=Q) this would result in too high a current density, therefore derating is needed.
Example: Take four 1.5mm² cables tabulated at 17.5A CCC and parallel them up to get a cable totalling 6.0mm². You cannot work this at 4x 17.5 = 70A if the cables are grouped together. But if you apply the Cg of 0.65, you can work it at 70 x 0.65 = 45.5A.

So the disagreement is all in the choice of which CCC to start with. Use that of the individual cables and you'll need to derate. Use that of a cable equal to the combined CSA and you don't.

 

[Dan]

Bump