Sizing the Grounding Electrode Conductor and Ground Rods
The grounding electrode conductor connects the service to earth. Its size comes from a short table, with one important cap for rod-only electrodes.
GEC sizing table
| Largest ungrounded service conductor (copper) | GEC (copper) |
|---|---|
| ≤ 30 mm² | 8 mm² |
| 38–50 mm² | 14 mm² |
| 60–100 mm² | 22 mm² |
| 125–175 mm² | 30 mm² |
| 200–300 mm² | 38 mm² |
The ground-rod cap
The GEC run to a made electrode (ground rod, pipe electrode) is never required to be larger than 14 mm² copper, no matter how large the service. The connection to a concrete-encased electrode is capped at 22 mm². Only the run to a metal water pipe or structural steel follows the full table.
Ground rod specification
- Copper-clad steel or solid copper, minimum 16 mm diameter, 3 m long, driven full length.
- Top of rod at least 300 mm below grade, or the connection protected.
- Use an irreversible connection (exothermic weld or listed clamp) below grade.
The 25-ohm rule
A single rod electrode must be tested. If it does not achieve 25 Ω or less, a second rod is required, spaced at least 1.8 m from the first (one rod length is the practical minimum; further apart is better). Once two rods are installed, no further resistance testing is mandated by the basic rule.
What goes in the BOM
Ground rods, driving couplings and points for deep drives, exothermic weld kits or listed clamps, the GEC conductor length, and any ground bar or inspection well.
EngEst Pro sizes the grounding electrode conductor from your service size (applying the 14 mm² rod cap), and lists ground rods, connectors, and the GEC run in the electrical BOM.
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