Let's clarify the context of NEC sections 250.66 and 250.
122. They address different aspects of equipment grounding conductors (EGCs) and grounding electrode conductors (GECs). 250.66 (Grounding Electrode Conductor Sizing)

This section deals with the grounding electrode conductor (GEC), which connects the grounded conductor (neutral) or the equipment grounding bus at the service to the grounding electrode (like a ground rod, water pipe, or concrete-encased electrode). The size of the GEC is based on the size of the service entrance conductors (or derived system conductors).

250.122 (Equipment Grounding Conductor Sizing)

This section addresses the equipment grounding conductor (EGC), which runs with the circuit conductors (hot and neutral) to equipment to provide a low-impedance path for fault current back to the source. The size of the EGC is based on the size of the overcurrent protective device (circuit breaker or fuse) protecting the circuit.

In short: 250.66 is about grounding the system (service), and 250.122 is about grounding the equipment connected to a circuit.

Therefore, both sections apply, but in different situations. They aren't mutually exclusive.

How they interact

You use 250.66 to determine the size of the grounding electrode conductor (GEC) for the service. This is based on the size of the service entrance conductors. Whether the largest conductor is #3/0 is irrelevant to 250.122, but it will impact the size of the GEC you calculate using 250.66. You use 250.122 to determine the size of the equipment grounding conductor (EGC) within each branch circuit or feeder. This is based on the rating of the circuit breaker or fuse protecting that specific circuit.

Therefore, the presence of a #3/0 conductor doesn't nullify the need to follow 250.122. If your circuit breaker is rated at, say, 60 amps, you must size the EGC according to the 250.122 table based on that 60 amp overcurrent device. A larger conductor size for another circuit, doesn't change the requirement to have the correct EGC for that 60 amp circuit.

Example


Service Entrance

Let's say you have a service entrance with #3/0 AWG copper conductors. You'd use 250.66 to determine the minimum size of the grounding electrode conductor (GEC) based on #3/0 copper. Looking at Table 250.66, #3/0 copper service conductors require at least a #6 AWG copper GEC (assuming you're connecting to a ground rod, pipe, or plate electrode).
2.

Branch Circuit

Now, within that building, you have a 20 amp circuit breaker feeding a receptacle. According to 250.122 and Table 250.122, you need a #12 AWG copper EGC for that 20 amp circuit, regardless of the #3/0 service entrance conductors or the #6 AWG GEC.

In Summary: Always apply both 250.66 and 250.
122. 250.66 is for sizing the GEC based on the service entrance conductors. 250.122 is for sizing the EGC in each circuit based on the overcurrent protective device. The size of the largest conductor for the service only comes into play when sizing the grounding electrode conductor (GEC) according to 250.66. It has no bearing on the EGC sizing per 250.
122.* Never undersize either the GEC or the EGC. Oversizing is acceptable, but not required.Always refer to the most recent version of the NEC for the definitive answer and consult with a qualified electrician for specific applications. These explanations are for informational purposes only and should not be substituted for professional electrical advice. Flag for review