A grounding electrode conductor (GEC) from an electrical service to a ground rod, pipe or plate type electrode is never required to be larger than 6 AWG copper or 4 AWG aluminum unless the GEC continues on from the rod, pipe or plate to connect another electrode that may require a larger GEC such as a concrete encased electrode, building steel or metal underground water pipe that qualifies as a grounding electrode.

The minimum size GEC permitted to connect a concrete encased electrode is 4 AWG copper. Building steel and metal underground water piping that qualifies as a grounding electrode may require a larger GEC since the GEC to these electrodes is sized based on the size of the service entrance conductors and Table 250.66. It would not be code compliant to run a 6 AWG copper GEC from the service to a ground rod, and then run a GEC bond jumper from the rod to another grounding electrode that required a larger GEC such as a concrete encased electrode.

Consider a ground rod for example; If the GEC starts at the service and goes to a ground rod or group of ground rods and stops there, the GEC is never required to be larger than 6 AWG copper or 4 AWG aluminum. If the GEC continues from the rod to another electrode of a different type that requires a larger GEC, then the GEC from the service to the first rod will be under sized for the furthest grounding electrode.

Similar code language appears in 250.66(B & C) which applies to concrete encased electrodes and ground rings.

Below is a preview of Article 250. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2017 NEC^® edition of NFPA 70.

2020 Code Language:

250.66(A) Connections to a Rod, Pipe, or Plate Electrode(s). If the grounding electrode conductor or bonding jumper connected to a single or multiple rod, pipe, or plate electrode(s), or any combination thereof, as described in 250.52(A)(5) or (A)(7), does not extend on to other types of electrodes that require a larger size conductor, the grounding electrode conductor shall not be required to be larger than 6 AWG copper wire or 4 AWG aluminum wire.