NEC Table 250.66 sizes the grounding electrode conductor — the wire from the service enclosure to the ground rods, water pipe, or concrete-encased electrode. It is sized from the largest ungrounded service-entrance conductor, not from the service rating. For a typical 200 A residential service with 2/0 copper conductors, the GEC is 4 AWG copper.
| Largest ungrounded service conductor (copper) | Or equivalent aluminum | GEC copper | GEC aluminum |
|---|---|---|---|
| 2 or smaller | 1/0 or smaller | 8 AWG | 6 AWG |
| 1 or 1/0 | 2/0 or 3/0 | 6 AWG | 4 AWG |
| 2/0 or 3/0 | 4/0 or 250 kcmil | 4 AWG | 2 AWG |
| Over 3/0 through 350 kcmil | Over 250 through 500 kcmil | 2 AWG | 1/0 AWG |
| Over 350 through 600 kcmil | Over 500 through 900 kcmil | 1/0 AWG | 3/0 AWG |
| Over 600 through 1100 kcmil | Over 900 through 1750 kcmil | 2/0 AWG | 4/0 AWG |
| Over 1100 kcmil | Over 1750 kcmil | 3/0 AWG | 250 kcmil |
Table 250.66 gives the general rule, but three notes in 250.66 cap the GEC far below the table value depending on which electrode it connects to. These caps are why a 200 A house almost never needs a conductor larger than 6 AWG to its ground rods:
250.66(A) — rod, pipe, and plate electrodes. The portion of the conductor that is the sole connection to a rod, pipe, or plate electrode never needs to be larger than 6 AWG copper or 4 AWG aluminum.
250.66(B) — concrete-encased electrodes. The sole connection to a concrete-encased electrode (Ufer ground) never needs to be larger than 4 AWG copper.
250.66(C) — ground rings. The sole connection to a ground ring never needs to be larger than the conductor used for the ring itself, and never larger than 2 AWG copper.
The word "sole" matters. If one continuous conductor runs from the service to a water pipe and then on to a ground rod, the portion to the water pipe is sized from the full table and only the rod tap gets the 6 AWG cap.
200 A dwelling, 2/0 copper service conductors, ground rods only. Table 250.66 row "2/0 or 3/0" gives 4 AWG copper. But the electrodes are rods, so 250.66(A) caps it at 6 AWG copper. Answer: 6 AWG.
200 A dwelling, 2/0 copper service, water pipe electrode. The water pipe is not covered by any of the three caps, so the full table applies: 4 AWG copper.
400 A service, parallel 2 sets of 250 kcmil copper. Combined area is 500 kcmil, which falls in the "Over 350 through 600 kcmil" row: 1/0 copper to a water pipe or Ufer, still 6 AWG to a rod.
These two tables are constantly confused. Table 250.66 sizes the grounding electrode conductor, which bonds the system to earth and is sized from the service conductors. Table 250.122 sizes the equipment grounding conductor, which carries fault current back to the source and is sized from the overcurrent device rating. Different job, different table, different input.