1 MVA Tx cables

1 MVA is about 1350 amps per phase, the exact figure depending on whether the 1 MVA is presumed to be at 415 volts or at 433 volts.

Single core copper cables with aluminium armouring are commonly used for this sort of application. The 300mm size in that cable type is rated at about 700 amps by the makers of the cable, the exact figure depending on how installed.

So it seems doable if the transformer terminals can only accept a pair of 300mm cables.

Normally the connections are to studs so that several crimp tags may be fitted at once to each. This limitation sounds a little odd, worth checking how the connections are designed. It may be that there is some space limitation.

The way ive managed to get around this in the past, is ask for the tails to be on a ladder in the trench and also spaced 2DE to remove the grouping factor. BS 7671 doesnt seem to offer a good solution for TX tail sizing and this is the best ive come up with. Various IDNOs ive worked with usually offer to size the tails, and this is usually always lower than what i can come up with?

Is the trench to be sanded as you can usually get a bit more due to the lower resistivity.

A standard DNO transformer only has provision for two lugs per phase. You cannot back-to-back them to get more cables, as you don’t have the phase-to-phase clearance. You can get transformers with extended bushings to take more lugs, but that is not a standard offering. I agree with broadgage, that we would usually use 300mm2 copper single core cables, with aluminium wire armour. 

Regards,

Alan. 

Can you get 2 x 630mm2 copper in?  I have done this in the past on ‘standard transformers’.

Also are you sure its the Transformer Terminals that are the limitation.  Some DNO's provide a Circuit Breaker (either on the transformer or in a separate cabinet) for larger LV supplies (typ over 350/400kVA) and the supply is taken from the outgoing terminals of that?  This will depend where you are and what the DNO policy is.