How to choose proper size of high power DC cable connections
An amp of DC current through a cable will generate the same# heating as an amp of AC current through a cable.

(# Actually slightly less. The resistance of a cable at DC is less than at AC. This is because there is no AC skin effect so the resistance is a few percent less.)

If you select a cable for DC current, using AC cable sizing guides / cable ampacities, your DC cable size will be conservative with respect to heating.

Note that various de-rating factors apply based on the installation environment of the cable. Proximity to other cables (which produce more heat), installation in a enclosed air space (which reduces air circulation i.e. cooling air), etc.

In Australia, the standard AS/NZS 3008 "Selection of Cables" covers all these de-rating factors in great detail.



Voltage drop is calculated using Ohm's Law and the DC conductor resistance. 

The DC cable resistance is published in all respectable cable catalogues. This includes AC cable catalogues - the DC conductor resistance is still published.

The resistance is usually given at 20 degrees C, adjust it to your maximum operating temperature using the temperature coefficient of resistance.

If your cable catalogue does not mention a maximum operating temperature, it is approximately 75C for PVC insulation, 90C for XLPE, and 110C for special high-temperature cable i.e. 110C-rated EPR cables.



Finally, for high voltage DC cabling there might be special considerations - normal 0.6/1kV AC power cable might not be suitable for 600V DC. I seem to recall that DC puts a different kind of stress on insulation than AC does. In your particular case, you are only dealing with 70 VDC so I wouldn't worry about it.