Volt Drop Simplified Method from AS/NZS3000:2009 Table C8, please refer to the standard for greater details
VD up to 7% where a substation is located on the premises.
Maximum Demand of the cct, CB Rating, total connected load
Current input above is the current passing through a single cable.
e.g. 400A total over 4 parallel single core cables (assuming good even contact and conductor size per cable) 400 / 4 = 100A Per Cable use this number for calculation