Table 250.66 Size of Alternating-Current Grounding Electrode Conductor
The size of the grounding electrode conductor at the service,
at each building or structure where supplied by a feeder(s) or branch circuit(s),
or at a separately derived system of a grounded or ungrounded AC system
shall not be less than given in Table 250.66, except as permitted in 250.66(A)
through (C).
Size of Largest Ungrounded Conductors or Equivalent Area for
Parallel Conductors (AWG/kcmil)
Size of Grounding Electrode Conductor (AWG/kcmil)
Copper
Aluminum or Copper-Clad Aluminum
Copper
Aluminum or Copper-Clad Aluminum
2 or smaller
1/0 or smaller
8
6
1 or 1/0
2/0 or 3/0
6
4
2/0 or 3/0
4/0 or 250
4
2
Over 3/0 through 350
Over 250 through 500
2
1/0
Over 350 through 600
Over 500 through 900
1/0
3/0
Over 600 through 1100
Over 900 through 1750
2/0
4/0
Over 1100
Over 1750
3/0
250
Notes:
If multiple sets of service-entrance conductors connect directly to a service drop,
set of overhead service conductors, set of underground service conductors, or
service lateral, the equivalent size of the largest service-entrance conductor
shall be determined by the largest sum of the areas of the corresponding conductors
of each set.
Where there are no service-entrance conductors, the grounding electrode conductor
size shall be determined by the equivalent size of the largest service-entrance
conductor required for the load to be served.