Sizing Neutrals from TX to LV Panel

A bit of help/ advice needed if anyone has a moment to spare please.


I have carried out numerous cable calculations using Amtech (Trimble) over the years, but, have for the first time come accross an example where I cannot use the software.


Basically, I am looking at a project requiring 2 new TX's from UKPN, serving 2 new LV Switch Panels (Clients).  UKPN have kindly given me all the necessary info on the ACB's etc. and also tell me that I can have 3 No. Al singles per phase up to 630mm CSA, but, I can only have 2 No. Al singles for the Neutral.


Now my helpful friend from Amtech support has just advised that you cannot select 3 cables per phase and only have 2 cables for neutral using the software, leaving me in a position where I have to (dare I say it) manually calc. the nuetral size.


What I do know is that my Amtech calc tells me that I need 3x 600mm CSA Al cables per phase, and it currently tells me I need 3x 600mm CSA neutrals (which I can not have).


I also know that the max permitted neutral size is 630mm CSA, which now leaves me in a postion of concern that being only permitted 2 neutrals, they may calculate out to be too large for the UKPN TX's.


Any help would be hugely appreciated by someone who should know better than to rely on Amtech all the time.


Thanks in advance


Adrian