Transformer Tails Sizing

Hi Broadgage, that is exactly what i had in mind, and therefore allowing me to use 2DE spacing factor in 4C5 and the ladder/clipped direct figures. I think UKPN use a GRP fibre glass walkway on their side of the trench to omit bonding and allow vent  etc.

What about making the top or roof of the trench out load bearing steel mesh, as is often used for elevated walkways in plantrooms.

This would allow almost unlimited ventilation and it might then be reasonable to consider the cables as being wall mounted rather than in an enclosed trench.


This sort of thing Steel walkway


No specific recomendation is made or implied for that particular brand. Link provided to show the TYPE of product to which I refer.

The length is only 6m straight from the TX into the Switch panel within an internal trench. The proposed conductor is copper but with an AWA. I would say that 3 x 3 x 1C 630mm2 + Neutrals is a reasonable size for a 2500A ACB turned down to 2125A as long as the vented trench metholody 117 mentioned above is used to allow the 2DE spacing, although not strictly an installation method to BS 7671 if used in trefoil.


There must be a standard-ish arrangement for TX tails for transformers sizes 1MVA, 1.25MVA and 1.5MVA

What terminations does your 2kA ACB have? What is the length of these cables? Have you thought of using Copper cables? How much space do you have at the Transformer terminals? There is a faulty assumption here somewhere!

Hi UKPN,


if I’m reading this correct your example in an enclosed trench would be 4 x 3 1C 1000mm AWA cables (rounded from 960) + 2 1000mm Neutrals.


and for a vented trench, 3 x 3 1C 1000mm AWA + 2 1000mm neutrals.


I would have thought both these arrangements would be impossible to terminate into an ACB.

Another factor to consider if a significant distance is involved is the energy cost of cable losses.

A loss of 1% is of course 15 KW. At current energy prices 15Kw will cost about £2 an hour.

For a long hour load, that could be about £16,000 a year. Spending even £30,000  on larger cables would be an excellent investment with a payback of less than two years.


Upsizing cables to reduce losses from say 2% down to 1% is well worth considering.


Is the transformer already existing and in in place ? If not, could it be placed closer to the load to save on the huge LV cables.

Okay, after further looking. 


For cables installed as per Installation method 117, with a ventilated trough. Which i would expect from an internal trough with an opening each end for connections to the TX and Switch panel, I can also specify a GRP ventilated lid. A 630mm cable from 4D3 can carry 771A horizontal flat spaced. This is the same figure if the cable was installed ladder horizontal flat spaced, I think therefore we can use the Trefoil method of the cable ladder as a reasonable substitute as there is no trefoil option in Installation method 117, this can then be combined with the grouping omission of 2De with neutrals in between.

Thanks Mike, Ive had a look at those links and still cannot find anything concrete.


Ive had a look at a couple of old jobs where an IDNO have sized the tails and it appears they do not use a grouping factor. 


An example is a 1500KVA TX, ACB set at 2125A supplied by 3 groups of 4 x 1C 500mm2 AWA cables in a trench between the sub and LV panel.


If each group can carry 731A in Trefoil, which gives a total capacity of 2139A.


I suppose the only way to get sensible sizes is to ensure that the trench has a degree of ventilation, which is possible due to being internal and also to specify a cable ladder to act as a heat sync. We could then potentially use table 4C5 and use the 2DE spacing to omit the grouping. Any kind of derating factor even at 0.8 would give ridcilous sizes it appears.