Size of Alternating-Current Grounding Electrode Conductor, NEC 250-66

The size of the grounding electrode conductor of a grounded or ungrounded ac system shall not be less than given in Table 250-66. 

Exception: 
a. Where connected to made electrodes as in Section 250-83 (c) or (d), that portion of the grounding electrode conductor that is the sole connection to the grounding electrode shall not be required to be larger than No. 6 copper wire or No. 4 aluminum wire.

b. Where connected to a concrete-encased electrode as in Section 250-81(c), that portion of the grounding electrode conductor that is the sole connection to the grounding electrode shall not be required to be larger than No. 4 copper wire.

c. Where connected to a ground ring as in Section 250-81(d), that portion of the grounding electrode conductor that is the sole connection to the grounding electrode shall not be required to be larger than the conductor used for the ground ring. 

 

TABLE 250-66
GROUNDING ELECTRODE CONDUCTOR (GEC)
for
Alternate-Current Systems
Size of Largest Service-Entrance Conductor or Equivalent Area for Parallel Conductors

Size of Grounding Electrode Conductor (GEC)

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 kcmil 4 2
Over 3/0 thru 350 kcmil Over 250 thru 500 kcmil 2 1/0
Over 350 thru 600 kcmil Over 500 thru 900 kcmil 1/0 3/0
Over 600 thru 1100 kcmil Over 900 thru 1750 kcmil 2/0 4/0
Over 1100 kcmil Over 1750 kcmil 3/0 250 kcmil