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.

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!

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 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.

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.