If you have a specific transformer in mind, then by far the most accurate method is to contact the owners (usually the DNO) or the maker if that can be determined - there should be a rating plate somewhere. However, in recent decades such things have been standardized by EU  regulations on efficiency, and that in turn means that the data for any transformer of the same wattage and construction will be similar. Is the transformer oil filled or solid resin ?

FWIW, I have a 2013 copy of the guide and that refers to a table D6 of  an ENA guide P 28 - it may be that the original reference material has been superseded and later versions have not caught up ?.

Mike

If you have a specific transformer in mind, then by far the most accurate method is to contact the owners (usually the DNO) or the maker if that can be determined - there should be a rating plate somewhere. However, in recent decades such things have been standardized by EU  regulations on efficiency, and that in turn means that the data for any transformer of the same wattage and construction will be similar. Is the transformer oil filled or solid resin ?

FWIW, I have a 2013 copy of the guide and that refers to a table D6 of  an ENA guide P 28 - it may be that the original reference material has been superseded and later versions have not caught up ?.

Mike

There is something that may be useful in annex D of the 1989 version of P28 which would have been current when the IET guide was first compiled.

Now, the latest (2017) version has lost that annex in favour of a more generic approach to voltage drop estimation, but has rather more information on modern types of load.

I suggest perusing both if you have the time:-)

Mike

Also, as I found it in passing, there is a very similarly titled "Electrical Installation Guide" by Schneider that can be downloaded from here

https://go.schneider-electric.com/WW_201903_2018-Electrical-Installation-Guide-A07456_EA-LP-EN.html

That has among other things has some very good general EMC advice but nothing special about transformer impedances