The voltage drop information box shows the calculated value, and the border is traffic light coloured to indicate if the cable is adequate.
The CSA and maximum allowable voltage drop can be set.
When determining the correct cable size for a required voltage drop, it is advisable not to decrease the CSA of the cable. Reducing the CSA may reduce the current rating to unacceptable limits. The design current and cable rating are shown in the information box, to help the user avoid this situation.
Our impedance calculation is by BS EN 60609 (IEC 60609), and you can see the formulae we use in the follow post: Impedance. Rather than using tables (BS 7671 for example), by calculating impedance the results are more accurate and tailored to the specific cable. We can use a wider variety of cables, installation arrangements, frequencies, voltages, and other parameters than covered by BS 7671. For the resistance element of impedance, we use IEC 60228 (supplemented by other methods if required) and consider the conductor operating temperature of the cable; you can see details of that in the following post: Conductor Resistance.
The actual clculation of voltage drop is in compliance with CENELC technical report CLC/TR 50480, and you can see details on this at: Voltage Drop.