1. A conservative method stated in AS/NZS 3008.1.1
The Standard AS/NZS 3008.1.1 states how to calculate the voltage drop for unbalanced 3 phase load in Section 4.6 - "a conservative method to calculate voltage drop for unbalanced multi-phase circuits would be assume balanced three-phase load condition and perform calculations using the current flowing in the heaviest-loaded phase".
2. Implement the method in cloud-based Cable Pro Web software
Enter the heaviest load of 3 phase into 'Current' in 'Load' section. Cable Pro Web will automatically perform the voltage drop calculation with the specified current. The calculated voltage drop is based on the assumption that the three-phase loads are balanced which is the conservative value as the Standard states.
 AS/NZS 3008.1.1:2017 Electrical installations—Selection of cables Part 1.1: Cables for alternating voltages up to and including 0.6/1 kV—Typical Australian installation conditions