Is the cable size predicted by these sums right on the edge of failing due to current rating ? If so it may be significant, at least for the neutral conductor.

If however the cable is nowhere near rated max current due to voltage drop dominating, then that is less serious. (the RMS voltage drop may be increased or reduced by 3rd harmonic depending on the relative phase of the 1F and 3F components.)

Also note that big electronic loads, like VFDs for air con pumps and larger server racks have built in correction and actually have a surprisingly sinusoidal load current.

How has the max demand for each building been estimated, and what is the biggest credible load ? if it is heating then the fact the waveform is terrible when it is lightly loaded and the heating is off may not matter.

I suspect there is will be some thinking behind this, but it may be sensible to ask.

Mike.

Thanks Mike. However, other electronic loads, such as computers, printers, lighting, and TVs, may have high harmonic content and may need to be accounted for when selecting the cable size. The max demand for each building and the biggest credible load is unknown and needs to be clarified. It is known that heating is not the biggest credible load

This is the design for the first run between the switchboard in the sub and the first building.

If I read those forms right  (always an uncertain assumption ) you are running a 150mm2 cable for a 60A load for 170m distance. !!

If  that was true it would be  one heck of an oversized cable. There would  be no heating problem. The soil could be replaced by vacuum. Voltage drops even at full load  negligible.

Which begs the question, are we expecting more load to be added. or if not, why is it not run in something thinner ? (you could reduce the cross section to about 16mm2 for that sort of load before thermal effects would be a concern. For such a length voltage drop dominates but even so probably 35mm2 or 50mm2 would be enough depending how much volt drop needs to be saved up for the final circuits at the end.)

So I assume that the 60A figure is not the full load, just that of one building, and a total max current of more like 200A-300A  is expected  - which if there are 5 houses to feed 3 phases  might be quite reasonable. It seems a bit odd. Perhaps a typo - Ib of 160 amps ?

Mike.

The first run from A to B is a 170-metre run (buried direct)with a 315 A BS 88 fuse for protection. The next run from B to C is 90 metres long (buried) and has a 250 A fuse. The run  from C to D is also 90 metres long (ladder) and has a 200 A fuse. The last segment from D to E is 97 metres long (Ladder) and has a 160 A fuse. I think the design needs to be questioned. The dist circuit enters each building into a isolator then busbar then isolator out up-to the next building.

That makes far more sense. Not sure of the significance of the Ib of 60A of the form, So now the question becomes is 315 A fuse sensible top cover for a 150mm2 cable. In the UK where the soil is rather cooler than the assumptions on that form yes.

Mike.

That makes far more sense. Not sure of the significance of the Ib of 60A of the form, So now the question becomes is 315 A fuse sensible top cover for a 150mm2 cable. In the UK where the soil is rather cooler than the assumptions on that form yes.

Mike.

I agree, about 40% more than the tabulated value for reference method D.