The requirement for separately derived systems (transformers) to be connected to the building Grounding Electrode System (GES) came into the National Electrical Code (NEC) in stages. It wasn't a single year that completely mandated it, but rather a progression towards that requirement. Here's a breakdown: Pre-NEC 1971

Generally, grounding was less comprehensive. There was a focus on grounding the transformer enclosure and neutral to something, but not necessarily a comprehensive connection to the building's GES. Local codes might have had varying requirements.

NEC 1971

This is where things began to formalize. NEC 250-26 introduced more specific requirements for separately derived systems. The emphasis was on grounding the neutral conductor of the derived system and connecting it to a nearby grounded structural steel member or grounding electrode. While "nearby" wasn't precisely defined, it started pushing the connection towards a more robust grounding system.

NEC 1975

Refined the requirements for grounding the neutral conductor of separately derived systems and connecting to a grounding electrode. While not explicitly requiring connection to the building's main GES, the intent was to achieve a low-impedance path to ground.

NEC 1978

Further clarified the requirements and refined the grounding methods for separately derived systems.

NEC 1981

This is a significant point. The 1981 NEC, Section 250-26(c), started to mandate the connection to the nearest available of these grounding electrodes: (1) Effectively grounded metal structure of the building, (2) Effectively grounded metal water pipe, or (3) other electrodes as specified in Section 250-81 (now 250.52). This essentially required connection to the building's grounding electrode system in most cases.

Later Editions (1984 onwards)

The subsequent editions of the NEC continued to refine and clarify these requirements, making the connection to the building's GES more definitive.

In Summary

While the concept of grounding separately derived systems existed before, the requirement for connection to the building's grounding electrode system (GES) became a standard practice around

NEC 1981. Prior to that, the grounding was often to the nearest grounded metal structure or water pipe, which could be part of the GES, but it wasn't explicitly mandated.Therefore, while elements were in place earlier, 1981 NEC is generally considered the turning point where connection to the building GES became a more clearly defined and enforceable requirement. Flag for review