I would like to know how to find the wire cross sectional area (mm^2) for high voltage cables for electric vehicles.
I also would like to know how to choose the number of strands.
System voltage is around 800V. So, I am thinking my cable should be chosen for 1000VDC.
The 'size' of a wire in mm2 refers to the area of the copper conductor only, which is determined by the current, not the voltage.
Until you get to tens of thousands of volts, where the conductor diameter does matter for surface electric field, the size of the copper conductor is unrelated to the voltage.
For voltages below a few thousand volts, it's the thickness and type of insulation that determines the voltage rating.
Although the system voltage is nominally 800 V, you can expect transients well above that in normal use, and then yet bigger ones in abnormal use. I would be very surprised if the regulations around electric vehicles allowed a rating as low as 1000 V for wiring an 800 V system.
The required insulation thickness and type will be driven as much by its physical strength as by its electrical strength. You don't want the insulation round a cable to be damaged or punctured during installation, destroying its insulation properties.
There is no simple single formula for the area of copper wire as a function of current, as it's the softening temperature of the insulation that determines how much heat the wire can produce, and therefore what current it can carry. A single wire in free air in a cool ambient with high temperature insulation like PTFE or fibre-glass can be rated to carry a far higher current than bunched PVC insulated wires in a conduit in a high ambient.
With all those caveats, 10 A/mm2 will get you into the right sort of ballpark for wires up to 10 A or so. Wikipedia gives a table of ampacity versus wire gauge for various ambient temperatures. However check what assumptions it makes, automobiles can have a very high ambient, and I would not be surprised if the high operating voltage needs a lower assumed temperature rise for the insulation than a for a building.
When the voltage gets very low, say 12 V for a distance of a few metres, then the voltage drop from end to end may become the limiting factor, and require an increase in the copper area above that needed for heat. This will not be a problem at 800 V until you get to hundreds of metres.
