NEC Table 250.66 is used for sizing grounding electrode conductors for alternating current systems. As defined in Article 100, the Grounding Electrode Conductor is used to connect the system grounded conductor or the equipment to a grounding electrode or to a point on the grounding electrode system.

(GEC) installed from service equipment or a separately derived system to a water pipe grounding electrode. The GEC is required by 250.66 to be sized based on the size of the ungrounded supply conductors. The supply conductors could be service conductors or, in the case of separately derived systems, feeder conductors from a generator or other power

TABLE 250.66 Grounding Electrode Conductor for Alternating-Current Systems Size of Largest Ungrounded Service-Entrance Conductor or Equivalent Area for Parallel

Table 250.122 — Grounding Conductor Size. Minimum Size Equipment Grounding Conductors for Grounding Raceway and Equipment

This chart displays the size of a ground conductor for a circuit based on the ampere rating of the circuit protection devices.

The grounding electrode conductor and bonding jumpers must adhere to minimum size requirements in Table 250.66, with exceptions noted in sections 250.66 (A) to (C). The table specifies sizes based on the largest ungrounded conductor, for both copper and aluminum.

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Determine the size of the grounding electrode conductor used for a service equipment with service-entrance conductor of 3/0 AWG. Use Table 250.66 to size the grounding electrode conductor. According to the third row, “2/0 or 3/0” the size …

The grounding electrode conductors shall be sized based on the size of the service entrance conductors as required in Table E3603.4. (250.66)

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 ...