Grounding Conductor Calculator (EGC / GEC)

How NEC grounding conductor sizing works

The NEC treats the equipment grounding conductor (EGC) and the grounding electrode conductor (GEC) as two distinct conductors with different sizing rules. They are not interchangeable terms — each connects to a different part of the grounding system and is sized from a different input.

The EGC (NEC 250.122) provides the fault-current return path from metal equipment enclosures and raceways back to the source. Its minimum size comes from Table 250.122 — look up the rating or setting of the OCPD protecting the circuit ahead of the equipment. A 100 A breaker requires at minimum an 8 AWG copper or 6 AWG aluminum EGC. Note that 250.122(B) may require upsizing the EGC proportionally when the ungrounded conductors are increased for voltage-drop reasons.

The GEC (NEC 250.66) bonds the grounding electrode system (ground rod, metal water pipe, concrete-encased electrode, etc.) to the grounded conductor at the service. Its size is determined by the largest ungrounded service-entrance conductor — not the OCPD. Table 250.66 caps at 3/0 AWG copper / 250 kcmil aluminum regardless of service size. Electrodes such as rods and pipes have additional caps under 250.66(A–C): a single ground rod never requires more than 6 AWG copper to the rod clamp, even on a large service.