Final answer:
The smallest size copper grounding electrode wire for a transformer with #1 AWG secondary wires depends on the NEC and specific grounding system design; consult the NEC code or a licensed electrician for the correct size.
Step-by-step explanation:
The question pertains to the sizing of grounding electrode conductors in an electrical system involving a separately derived transformer with secondary wires of size #1 AWG copper. According to the National Electrical Code (NEC), there are specific rules that determine the size of grounding electrodes and grounding electrode conductors based on the size of the service conductors or secondary conductors of a transformer. The smallest size copper grounding electrode wire depends on the applicable electrical codes and the grounding electrode system design. However, without the specific code reference, we cannot provide an exact answer to the size of the grounding electrode conductor needed. It is recommended to consult the latest NEC code or a licensed electrician to determine the appropriate conductor size that would ensure electrical safety and code compliance.