What is the largest size grounding electrode conductor required for copper connected to a concrete-encased electrode?

For a concrete-encased electrode, which typically involves rebar or steel that is embedded in concrete, the National Electrical Code (NEC) specifies the size of the grounding electrode conductor (GEC) based on the size of the largest ungrounded service entrance conductor.

In the scenario of connecting copper to a concrete-encased electrode, the relevant table in the NEC indicates that if you have a conductor size of 4 AWG copper or larger, the minimum size grounding electrode conductor required is indeed 4 AWG copper. This is grounded in the requirement to ensure a low-resistance path for electrical faults, providing safety to people and minimizing damage to equipment.

The other sizes would be insufficient in providing the necessary conductivity and safety for effective grounding, as they do not meet the NEC requirements for a grounding electrode connected to this type of electrode.