NEC Table 250.66: Grounding Electrode Conductor for Alternating-Current Systems

Size of Largest Ungrounded Conductor or Equivalent Area for Parallel Conductors (AWG/kcmil)Size of Grounding Electrode Conductor (AWG/kcmil)
CopperAluminum or Copper-
Clad Aluminum
CopperAluminum or Copper-
Clad Aluminum
2 or smaller1/0 or smaller86
1 or 1/02/0 or 3/064
2/0 or 3/04/0 or 25042
Over 3/0 through 350Over 250 through 50021/0
Over 350 through 600Over 500 through 9001/03/0
Over 600 through 1100Over 900 through 17502/04/0
Over 1100Over 17503/0250

Source: NFPA 70