Current sharing

I assume you have a horizontal tray spaced from the ceiling or whatever. How much free space do you have around it? Two layers would add a little cost and effort, but you might save some cable size that way as well, a good payback. You seem to have neutrals, is that because the loads are single phase? The derating factor would then be 0.98 for the trays so very small. It looks too as though 150mm aluminium cables would be suitable with 90-degree rating, and also that there is less z term for voltage drop in trefoil, which you also need to watch, but you know that. I looked at Table 4J1 and 4J2. I assume you want singles for ease of installation, but 150mm 4 core looks a candidate to consider too. I would choose XLPE aluminium as the cost will be lower than copper and they are lighter.

Thank you David. I think there is no serious need to be too fastidious on this one. How the 600A load was arrived at may also be questionable. I think it was done on the back of a fag packet. Cost doesnt seem to be an issue. However, I just wanted to make  sure that I am doing things right. I thought even though one circuit, the conductors are equal sharing in parallel x 2 and only one tray so the de-rate would be 0.91 from Table 4C5 rather than 0.98.

Aluminium might be cheaper but penalty on volt drop which I wanted to keep to around 2%

Have you a view about connecting at 90 degrees as the cables will enter a panel board with T/M MCCBs?

The nice thing about my jobs nowadays, the overalls are seldom needed and I defer respectfully to the younger, fitter members of our species, although I do so with not a considerable tingle of envy. 

If this is a long hour load, then you/your customer should also consider the ongoing energy cost represented by cable losses. Presuming standard supply voltage, then 600 amps per phase is about 430 kw. 

Reducing cable losses from say 2% down to 1% will therefore save about  4.3 kw or at present electricity prices at least £1-30 an hour.

For a long hour load used for 5,000 hours a year, that is about £6,500 a year. A larger cable size could well pay for itself in reduced losses.

Very good point Broadgage. Another good reason why one should be armed with the load characteristics rather than an instruction to just stick a 600A supply in please!

The point of 90 degree is not quite as is often described here. You are derating for close bunching, but entry to switchgear does not really do this, and they spead out quite well inside. The ambient inside may be a bit higher, ventilation of switchgear is often not too good, but as usual this is not an exact or even make or break temperature. If you decide to go for large copper, ir might be worth pricing another HV supply and transformer too, there may not be much difference in price, and there would probably be an energy saving overall.

I may not be as young as you seem to imagine BTW! Note the grey beard!

Hi Lyle, if you are worried about the terminal temperature, you can always join to larger ends with bolt terminals in a big box close by. This might be a good idea anyway as single cables to the switchgear are generally easier and short lengths of big cable are fairly easily handled. Are you fitting an MCCB at the supply end or fuses? An MCCB with Earth trip would save you a lot of Earth conductor cable too and no problems with Zs at the load.