In what year did the NEC first require that the neutral and ground have to be separated in a sub panel?
Grounding and earthing, or specifically, that the earth will act as an electrical conductor has been known for over 270 years. We have white papers dating back to 1746 describing the phenomena, and of course, Benjamin Franklin used grounding for his lightning protection systems in 1749.
We also have papers in 1803 and 1808 requiring the earthing of battery systems, and of course starting 1882 Thomas Edison mandating the grounding of his systems worldwide (including the separation of the return current conductor “ie neutral” from the grounding system).
The first code regarding mandatory grounding (and separation from the neutral) goes back to the American Institute of Electrical Engineers (AIEE) in October of 1884 where they stated in their “Transactions” publication: “there is no element or adjunct of electrical science more important in many respects then the terrestrial ball itself, when we consider it in its relationship to the electrical circuit…”
The NFPA issued its first requirement for grounding conductors that are separate from the neutral in March of 1896, with the NEC following up in 1897. Note: the NEC was not part of the NFPA umbrella until 1911.
So in answer to your question, the NEC has required the separation of the neutral from the ground since its very first publication in 1897.
Please refer to this article on when and where to make neutral-to-ground bonds, and possibly even more importantly, when NOT to make such bonds.
The Engineering Experts at E&S Grounding Solutions