It is important to identify the time when the wiring was first installed.
In the USA, for many years there were home grown codes (and some still are) and although they used the NEC as a basis for them, there were some inspectors who had different ideas about the way in which certain installations, and circuits (to include services and feeders) should be run.
Just the other day, I did a visual inspection of the basements and utility rooms (not to include the dwelling units) in 6 apartment buildings built around 60 years ago.
I was able to recognize some of the different methods, or so-called non conforming methods and I took many pictures.
When I was ready to prepare my suggestions I looked at each picture and decided that some were not to be called a hazard today because of the rule that is now in the Code.
For an example, the service conductors were the old rubber covered types (they were underground conduit laterals run to each service “end box”) and the various nipples, and conduits entries were without the required insulation at the bushings for 4 AWG or larger.
We can’t say that this is illegal, because the rule was not in the code when the wiring was installed.
Also, the NEC should not be considered as a retroactive document.
It is only during the last few code cycles that we have been required to install a GFCI when the “grounded” receptacle was replaced in areas that now call for one such as in 210.8.
I can tell you that the older installations were based on the codes written early on like in the 30’, 40’s or even 50’s.
When I was first starting out, I was introduced to installations around the NYC 5 borough areas, and came across some systems that were very interesting, such as the old “A” base meters (many still in existence) and knob and tube still in use today, and sometimes still allowed according to the NEC.
The questions asked here on this board should be with some history on the date of the installation (can we still find the date in the toilet tank?) to avoid discussing the rules of today when the wiring in a 60 year old building is being discussed.
That’s good for those who want to become Code Scholars, but the new kids 20 years from now, who will make the typical inspections that a Home Inspector makes, will look back into an older code.