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Sizing Neutrals from TX to LV Panel

Adrian B
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
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  • 0
    Hi

    No, cables not to be buried. Only 20m on ladder from TX to LV Panel.

    The actual project itself is 4 blocks of residential flats standing on a shared basement car park.  With regards to balancing etc., yes the system has been balanced well.  With regards to loads on the system, the largest individual loads will be lifts, some being provided with 100A TPN supplies.  Power factor is currently set circa. 0.97.

    So, I have played with using Al/AWA singles and Cu/AWA singles this morning (albeit with 3 neutrals as like I say, Amtech will not allow me to select 2), and my calcs show that I can have either 600mm Al/AWA (x3 per phase) singles, or, 630mm Cu/AWA (x3 per phase) singles.  So, not much difference either way to be honest.  The rest of the calculation is not effected by the use of one or other.

    But, going full circle, I still have the problem that UKPN state that I can only have 2x 630mm (max) neutrals and that the incoming ACB has a half sized neutral.

    So, going full circle, my assumption is that a half sized neutral on an installation of this type should not be an issue, seeing that I know I have balanced loads well as a part of my design and that harmonics are really not a huge issue as a part of a residential project like this?  (if my assumptions are utter rubbish, please forgive my ignorance).

    Thanks All

    Adrian
Reply
  • 0
    Hi

    No, cables not to be buried. Only 20m on ladder from TX to LV Panel.

    The actual project itself is 4 blocks of residential flats standing on a shared basement car park.  With regards to balancing etc., yes the system has been balanced well.  With regards to loads on the system, the largest individual loads will be lifts, some being provided with 100A TPN supplies.  Power factor is currently set circa. 0.97.

    So, I have played with using Al/AWA singles and Cu/AWA singles this morning (albeit with 3 neutrals as like I say, Amtech will not allow me to select 2), and my calcs show that I can have either 600mm Al/AWA (x3 per phase) singles, or, 630mm Cu/AWA (x3 per phase) singles.  So, not much difference either way to be honest.  The rest of the calculation is not effected by the use of one or other.

    But, going full circle, I still have the problem that UKPN state that I can only have 2x 630mm (max) neutrals and that the incoming ACB has a half sized neutral.

    So, going full circle, my assumption is that a half sized neutral on an installation of this type should not be an issue, seeing that I know I have balanced loads well as a part of my design and that harmonics are really not a huge issue as a part of a residential project like this?  (if my assumptions are utter rubbish, please forgive my ignorance).

    Thanks All

    Adrian
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