NEC Table 250.66
| Size of Largest Ungrounded Conductor 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.
aThis table also applies to the derived conductors of separately derived ac systems.
bSee installation restrictions in 250.64(A).
Source: NFPA 70
Background
The grounding electrode conductor (GEC) connects an electrical system’s grounded conductor to the grounding electrode system (GES). For example, in a panelboard the GEC connects the neutral bus bar to the buried ground rods, ground ring, building steel, concrete encased Ufer etc. in contact with the earth. These components make up the GES.
Sizing the GEC is correlated to the size of the ungrounded (current carrying) conductors. The larger the ungrounded conductors, the larger the GEC. In the event of a phase-to-ground fault the GEC must be large enough to survive for the power system’s protection to operate and clear the fault. The values listed in table 250.66 are therefore minimums.
It’s important to understand the difference between the grounding electrode conductor and an equipment grounding conductor (EGC). Specified in table 250.122 the EGC connects individual loads back to the equipment ground bar in a panelboard, motor control center, etc.
