Sizing the Grounding Electrode Conductor

Size it off Table 250.66 from your service-entrance conductors, but remember the ceilings: a ground rod never needs bigger than #6 copper to it, and a concrete-encased electrode caps at #4.

People oversize the whole run because they do not read the exceptions. You do not need 3/0 going to a ground rod, and it will not make the earth any better at its job.

When it applies

Use this whenever you size the grounding electrode conductor, the wire that ties your service or system to the ground rods, water pipe, or concrete-encased electrode. You pull the size from the table based on the largest ungrounded service-entrance conductor, not the load.

Common mistakes

• Sizing the electrode conductor off the breaker or the load instead of the service conductor size.
• Missing the caps: to a made rod you never need bigger than a 6 copper, and to a concrete-encased electrode never bigger than a 4 copper, no matter what the table says.
• Running parallel service conductors and forgetting you total their area before entering the table.

People burn money running a 2/0 to a ground rod because the table said so, when the rod cap lets a 6 do the whole job. Read the exceptions, they save copper.