Grounding Electrode Conductor Sizing
The grounding electrode conductor (GEC) from the largest ungrounded service conductor per NEC Table 250.66.
Example
You enter
- Largest ungrounded service conductor (kcmil) 250
- Service / GEC material copper
- Electrode type rod-pipe-plate
You get
- Required GEC 6
- Table 250.66 base 2
Checked against published worked examples from: IAEI Magazine (Keith Lofland).
Details, formula, and sources
ScopeWith 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 or tableGEC 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 sole connection not required to be larger than the ring conductor (at least 2 AWG copper, 250.52(A)(4)), so capped at 2 AWG copper for the minimum ring; water-pipe / structural-steel electrode takes the full table size.
Source and editionNEC 2023 (NFPA 70). Section 250.66 and Table 250.66.
Free accessFree read-only access at nfpa.org/freeaccess.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesservice_kcmil, material, electrode_type, required_gec, base_gec
- Table mapping only the threshold-to-size mapping is encoded (the egc-sizing / wire-ampacity precedent), not a reproduced NFPA tableNEC Table 250.66
- Electrode caps 250.66(A) rod/pipe/plate, 250.66(B) concrete-encased, 250.66(C) ground ring; the cap is the smaller of the table size and the electrode limitNEC 250.66(A)-(C)
- Material the chosen material sizes both the service-conductor column and the GEC; copper defaultuser input