Chris Pearson said:

Just a thought here. Is there any provision which specifies voltage drop at HV. Surely, if HV is low, Lv must also be low?

No specific provision in BS 7671 ... and agreed, but transformers with tap-changers and AVC can be used to overcome that (or, in the LV system, autotransformers with tap-changers)

Think we are drifting away from the question. What is a private supply in terms of BS7671? Option  1 or Option 2?

Peter Hartley said:

Think we are drifting away from the question. What is a private supply in terms of BS7671? Option  1 or Option 2?

Anything other than a "standard" ESQCR public LV supply is the answer. Not drifting, that was answered earlier, and the discussion progressed beyond it.

I have searched the web and found the definition of a private supply according to BS 7671. A private supply is a supply that comes from a source of energy other than a public electricity supply system, This definition is also in agreement with the one given in [Wikipedia], which references BS 7671:2018 as its source. However, I could not find the exact location of this definition in the BS 7671

So just for clarity, your answer is both Option 1 & Option 2 will be considered private supply?

So just for clarity, your answer is both Option 1 & Option 2 will be considered private supply?

I think there is some misunderstanding about my answer. I did not say that both Option 1 and Option 2 are private supplies. I said that only Option 2 is a private supply. Option 1 is not a private supply, because it still relies on the public electricity supply system at the HV level. The private transformer does not alter the source of energy, only the voltage level. Therefore, Option 1 is a public supply with a private transformer, not a private supply. 

It looks confusing in the chat. I actually replied to gkenyon. I understand what you are saying and thats what I thought. I'm also aware of the Internet definitions you quoted. I always thought the same as you. However a colleague said the same as Chris Pearson above. If you own the transformer you are able to adjust the tappings on the transformer to achieve peak efficiency on your volt drops. So I tend to agree with them. Just wish BS7671 was clear on this.

When we don’t have a clear answer, we call it a grey area. It’s like the weather in Manchester, always cloudy and uncertain. 

Peter Hartley said:

So just for clarity, your answer is both Option 1 & Option 2 will be considered private supply?

Yes, they could both be considered a private supply, although as I have said the over-riding factors are not really the drop within the installation, but conformity with Regulations 525.1 and 525.201.

I’d go with Graham’s final conclusion. My opinion has been changed 

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.