Voltage drop across a feeder circuit consisting of multiple parallel cables

Hi all, 
I have a question regarding voltage drop across a feeder circuit consisting of multiple parallel conductors per phase in a 3 phase system (arrayed out like in the attached file). I am trying to get an understanding of how to calculate voltage drop of the system and need some assistance.I usually assume that the current in each phase is equal and divide the current equally for each cable in a given phase. Then, I use the voltage drop steps in the following link:https://www.okonite.com/media/catalog/product/files/EHB_2018.pdf   (Section 3: Voltage Drop)But that I have a feeling that this doesn't really give the whole picture as there is mutual reactance of other cables not included in the voltage drop equation from okonite.I was wondering if anybody here can either provide a better model or point me to books/articles that can help me out?
Thanks!

Harry, I'm not a power guy but have done a fair bit of electrics.  I would say that at 50/60 Hz  you can disregard things like Mutual Inductance (except if you have a really long run, a good few hundred metres / yards or more) and skin effect (which reduces the effective diameter of a wire at high frequencies.   Even if they do come into play the effects should be very slight and not worth worrying about.    Power guys, any further comments?
Just add up the cross sectional area of all your A conductors to get an effective cross sectional area and use that in your calculations.Nice handbook from Okonite, lots of useful stuff...