Sizing Neutrals from TX to LV Panel

What is the rating for each of these transformers?


You are saying Al singles? Why not use copper AWAs? That should do the trick unless your transformers are very big.

Hi John


In response to your questions, the transformers are both 1250kVA  with outgoing 2500A  NW25H1 ACB's (micrologic 5.0) with overload (lr) 0.7.


My mistake, the latest design drawing from UKPN states that the cables can be (up to) 3 x 630mm AWA per phase and 2 x 630mm per neutral.


The reason I stated Al singles is because I did the original design for this months ago to an older UKPN design, but UKPN how now updated their preferences and released a revised design.


Thanks


Adrian

It rather depends how much neutral current you expect - if all 3 phases are driving the same load and it is classical (resistive heating or inductive motors) then there will be little neutral current to worry about.  Indeed at one time (well into the early 1970s anyway) cables with reduced neutrals (relative to live) were the order of the day. However,   one phase may be unloaded, so the imbalance (which is the neutral current) is comparable to one phase current, or worse you may have loads with high harmonic content, so the cancellation does not occur - then the neutral current can in some cases be higher than any one  phase current.

If the load  balancing is good accross the phases and the load harmonic distortion and power factor is low, then a 2/3 sized neutral could be used with impunity.  however JPs suggestion of copper is probably better.

1250KVA  suggests  circa 1.6kA per phase  at full load ?  - what is your most onerous environment ? Hopefully not buried....


according to   Nexans for copper core One core  of 630mm2 is

Permissible short circuit current conductor 1s

90.1kA

Perm. current rating buried 20°C - trefoil formation

595A

Perm. current rating in air 30°C - trefoil formation

1088A

Perm. current rating in air 30°C - flat formation

1454A

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

I think you've got yourself into that awkward position of being handed a requirement by a DNO (which for them on the network is perfectly normal to use reduced neutrals, as their documentation states the available KVA is substantially balanced) and then by design or default you are becoming an iDNO or a BNO.


You then usually have to comply with BS7671 and the rules for iDNO or BNO adoption, until you deliver power to each apartment and BS 7671 becomes the requirement.


In my world, we wouldn't tolerate a reduced neutral (in many cases we have 150 or 200% neutral conductors due to the harmonic rich nature of loads) - for domestic housing it won't usually be an issue (even with an ACB at 50% neutral)


Equally, you need to take a sensible assessment of the actual load - whilst the TX may be rated at 1250KVA, what is actually going through it - in air ratings should easily be less than 3 x 500mm2, even buried rating should be feasible for the same conductor size, if the diversity assessment is sensible. At 630mm2, in copper, it's pretty well bomb proof


Regards


OMS

Thanks OMS.  

Correctly assumed yes, UKPN has basically labelled the works (as I have described) within their  schematic as 'BNO'.

I have been very BS7671 with the application of diversity, knowing that in reality, figures and loads will be lower than those currently stated.

With that said, and following the input from you and others, I feel somewhat more confident that a half sized neutral in this instance will not be an issue, especially if I stick to providing pretty much the maximum sized neutrals.


Adrian

OK - whatever diversity you've applied, be prepared to be shocked as to just how much diversity the DNO have already assumed.


Even for electrically heated flats, they would use a frighteningly small number of kW per flat compared to what might be thought to be required. In practice, with decent ENATS spec transformers, you can push them easily to 150% for limited time with no problems (particularly as the peak heating load coincides with the coldest outdoor ambient.


Cheap and nasty split DX aircon, is however, another matter (both for load and harmonics)


Regards


OMS

I have just had come in 2 very similar cable calls for 2 x 1400A cable runs from transformers to feed pillars. RFI to get more information.

Does UKPN have to supply own the transformers?


Regards

So you need some software that can do this attached



https://www.dropbox.com/s/48hx9j2cnt5o7fu/Report Export IET forum.pdf?dl=0