Grounding Electrode Conductor (GEC) Size Chart

Start from the largest ungrounded service-entrance conductor — the phase conductor, not the neutral — and pick the row it falls in, using the copper or aluminum column to match your service conductor material. Where the service is paralleled, add the circular mils of the parallel set first: two 350 kcmil copper conductors per phase read as 700 kcmil, which lands in the 700 to 1100 row. The GEC itself may be copper or aluminum regardless of what the service conductors are — but per NEC 250.64(A), aluminum grounding electrode conductors can't be used in direct contact with masonry or earth, or within 18 inches of the earth outdoors, which in practice keeps most electrode runs copper.

This chart sizes the conductor to the earth electrodes. The ground wire run with a circuit is the equipment grounding conductor, sized from the breaker rating under NEC 250.122 — see the ground wire (EGC) size chart. Using 250.122 for an electrode run typically oversizes small services and undersizes large ones, so it fails inspection in both directions. There is also a near-twin of this table for the main, system, and supply-side bonding jumpers and the grounded conductor — identical rows until the largest services, where it keeps growing instead of capping at 3/0: see the bonding jumper size chart. As always, verify against the code edition your jurisdiction enforces.