Sizing the grounding electrode conductor
The grounding electrode conductor is sized from Table 250.66 by the largest hot conductor supplying the system. Runs only to ground rods never need more than 6 AWG copper; runs only to a concrete-encased electrode never need more than 4 AWG copper.
What the code requires
The grounding electrode conductor connects the electrical system to its grounding electrodes, such as rods, a water pipe or a concrete-encased electrode. Table 250.66 sizes it from the largest ungrounded (hot) supply conductor. For parallel sets, the combined area of the conductors is used.
Supply conductor size, then electrode conductor size (Cu = copper, Al = aluminum): 2 AWG Cu or 1/0 Al and smaller, 8 Cu or 6 Al; 1 or 1/0 Cu, or 2/0 or 3/0 Al, 6 Cu or 4 Al; 2/0 or 3/0 Cu, or 4/0 or 250 kcmil Al, 4 Cu or 2 Al; over 3/0 through 350 kcmil Cu, or over 250 through 500 kcmil Al, 2 Cu or 1/0 Al.
Larger services: over 350 through 600 kcmil Cu, or over 500 through 900 kcmil Al, 1/0 Cu or 3/0 Al; over 600 through 1100 kcmil Cu, or over 900 through 1750 kcmil Al, 2/0 Cu or 4/0 Al; above that, 3/0 Cu or 250 kcmil Al.
On the job
- For a 200 A home service on 2/0 copper or 4/0 aluminum conductors (sized under 310.12), the conductor to the water pipe or Ufer is 4 AWG copper, while 6 AWG copper is enough for a run that goes only to ground rods.
- An undersized grounding electrode conductor to the water pipe is a common correction.
Exceptions that matter
- A section that runs only to rods, pipes or plates never needs to be larger than 6 AWG copper or 4 AWG aluminum or copper-clad aluminum.
- A section that runs only to a concrete-encased electrode never needs to be larger than 4 AWG copper.
- A section that runs only to a ground ring never needs to be larger than the ring conductor.
What changed in 2023
The 2023 edition applied the section to bonding jumpers that connect electrodes together and added copper-clad aluminum to the rod-only size limit; the table values did not change.
Related sections
- NEC 250.24: Grounding and bonding at the service
- NEC 250.32: Grounding at detached garages, ADUs and outbuildings
- NEC 250.50: Bonding all grounding electrodes together
- NEC 250.53: Ground rod installation and the 25-ohm rule
- NEC 250.64
- NEC 250.68: Grounding electrode connections and the 5 ft rule
Where this comes from
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Kyle Robert Anderson KBC Electric · CSLB #1073066 · C-10 Electrical · Licensed since 2021