For concrete metallic reinforcing bar (rebar) to be used as a grounding electrode, which condition is NOT necessary?

The requirement that the rebar must be at least 3/8" in diameter is not necessary for it to serve as a grounding electrode. The primary purpose of using rebar as a grounding electrode is to ensure that the grounding system is effective and provides a low-resistance path to ground.

In practice, various diameters of rebar can effectively perform this function if other conditions are met—such as the rebar being properly encased in concrete, ensuring that the concrete is in contact with the earth, and that the rebar is bonded together to form a continuous system. The emphasis is on the overall effectiveness of the grounding system rather than strictly the diameter of the rebar itself. Thus, the diameter is not specified as a condition for rebar to qualify as a grounding electrode within the code requirements.