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Grounding Electrode Conductor Size Selector
Table 250-66 Calculator
by electrician2.com

Size of Largest COPPER Service-Entrance Conductor or Equilvalent Area for parallel Conductors
Size of copper
Grounding
Electrode
Conductor
Size of Aluminum
or Copper Clad
Grounding Electrode
Conductor
Size of Largest Aluminum Service-Entrance Conductor or Equilvalent Area for parallel Conductors
Size of copper
Grounding
Electrode
Conductor
Size of Aluminum
or Copper Clad
Grounding Electrode
Conductor
Notes:
1. Where multiple sets of service-entrance conductors are used 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.
2. 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.
3. Where the grounding electrode conductor is connected to made electrodes that portion of the 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.
4. Where the grounding electrode conductor is connected to a concrete-encased electrode that portion of the conductor that is the sole connection to the grounding electrode shall not be required to be larger than No. 4 copper wire.
5. Where the grounding electrode conductor is connected to a ground ring not smaller than No. 2. that portion of the 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.