Grounding Electrode Conductor Sizing
The grounding electrode conductor (GEC) from the largest ungrounded service conductor per NEC Table 250.66, with the 250.66(A)-(C) electrode caps applied: a rod/pipe/plate sole connection need not exceed 6 AWG copper, a concrete-encased (Ufer) 4 AWG copper, while a water-pipe or structural-steel electrode takes the full table size. The AHJ-adopted edition governs.
Formula and source
GEC from NEC Table 250.66 by the largest ungrounded service conductor (or equivalent parallel area), then the 250.66(A)-(C) electrode caps: rod/pipe/plate sole connection not larger than 6 AWG copper / 4 AWG aluminum; concrete-encased not larger than 4 AWG copper; ground ring not smaller than the ring conductor and not smaller than 2 AWG; water-pipe / structural-steel electrode takes the full table size.
NEC 2023 (NFPA 70). Section 250.66 and Table 250.66.
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