AMK said:

Option 1 is not a private supply, because it still relies on the public electricity supply system at the HV level.

See my later response on "source". A transformer is a source (if you take the diagrams in BS 7671 at face value)

BS 7671 covers circuits supplied at nominal voltages up to and including 1000V AC or 1500V DC. It also applies to Extra Low Voltage (ELV) ranges and Low Voltage (LV) ranges, across a variety of frequencies.

Thus oprion 1 is null and void!

Or more to the point, HV supply is not taken into account, thus transformer would be private supply 

Respectfully I disagree with your response. A private supply is defined as a supply that comes from a non-public source of energy, The HV supply is still a public source of energy, even if it is not considered. The transformer does not alter the source of energy, only the voltage level. Therefore, the transformer is not a private supply, but a public supply with a private transformer. 

The key thing about a private transformer - one where only you have the secondary side - usually the HV is shared with others (!) , is that you are free to set the taps (or call in the DNO to set the taps to your needs, depending on contract agreement and who has suitably qualified staff )

This means that the voltage drop allowance normally reserved for the street cables is yours to play with - which if you have a large campus you will probably need to.

Mike

PS A private supply is defined as a supply that comes from a non-public source of energy, The HV supply is still a public source of energy, even if it is not considered.

I think the authors of BS7671, and a number of designers of large sites do not agree with that interpretation.  :-)

fully agree with you. l

I think we need to distinguish between supply/ origin and source. 525.202 clearly states the volt drop measurement should origin of the installation (usually the supply terminals) and the socket outlet....

BS7671 only applies to installations 1000v AC / 1500 DC and below. Thus it would deem the supply terminals on the LV side of the transformer as the oringin of the installation. If those terminals are privately owned, it would be considered a private supply.

Annex 4 - 6.4, Table 4Ab - Voltage drop (ii) Low Voltage installation supplied from private LV supply.

The HV supply or the source of energy is not considered for the purposes of Volt drop, but Origin of the LV supply is.

If those terminals are privately owned, it would be considered a private supply.

But the DNOs themselves were privatised after they took over from the old Electricity Boards - they may be regulated, provide a supply to the public, but their equipment isn't publicly owned.

   - Andy.

Much like the railways!

Peter Hartley said:

Thus it would deem the supply terminals on the LV side of the transformer as the oringin of the installation. If those terminals are privately owned, it would be considered a private supply.

Agreed, although ownership is not the issue because the transformer could be leased, but still be private.

It seems reasonable to regard the LV terminals of a private transformer as the origin, but otherwise it is the service head.

The real point is whether the transformer supplies any other customer or not. So if it is just the one, it is possible to compensate for voltage drop within the installation. However if there is more than one customer, if the wick is turned up because of voltage drop in one installation, the voltage may be too high for another so there is less scope for adjustment.