NEC Grounding Electrode Conductor (GEC) Sizing
NEC Table 250.66 sizes the Grounding Electrode Conductor by service-entrance conductor size.
NEC Article 250 of NFPA 70 (2023) governs grounding and bonding, and Table 250.66 is the master table for sizing the Grounding Electrode Conductor (GEC). The GEC is the conductor that ties the grounded service conductor (neutral) at the service equipment to the grounding electrode system - the ground rod, concrete-encased electrode (Ufer), building steel, or water pipe. Sizing it correctly requires reading 250.66 in tandem with the size of the service-entrance conductors.
What Article 250 Says
NEC 250.66 reads: "The size of the grounding electrode conductor of a grounded or ungrounded ac system shall not be less than given in Table 250.66, except as permitted in 250.66(A) through (C)." Translation: look up your largest service-entrance ungrounded conductor in the left column of Table 250.66, then read the GEC size from the right column.
Table 250.66 Verbatim
| Largest Ungrounded Service Conductor (Cu AWG/kcmil) | Largest Ungrounded Service Conductor (Al AWG/kcmil) | GEC Copper | GEC 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 |
GEC values per NEC 2023.
The Critical Exceptions in 250.66(A), (B), (C)
These exceptions are what most installers actually use:
- 250.66(A) - Connections to Rod, Pipe, or Plate Electrodes: The portion of the GEC that is the sole connection to a made electrode (ground rod, pipe, plate) shall not be required to be larger than 6 AWG copper or 4 AWG aluminum.
- 250.66(B) - Connections to Concrete-Encased Electrodes (Ufer): The portion of the GEC that is the sole connection to a concrete-encased electrode shall not be required to be larger than 4 AWG copper.
- 250.66(C) - Connections to Ground Rings: The portion of the GEC that is the sole connection to a ground ring shall not be required to be larger than the conductor used for the ground ring (typically 2 AWG copper).
These three exceptions are the reason a 400A service can have a 6 AWG GEC running to its ground rod while a #2 AWG GEC runs to the building steel.
GEC vs EGC - The Critical Distinction
| Attribute | GEC (Grounding Electrode Conductor) | EGC (Equipment Grounding Conductor) |
|---|---|---|
| NEC Article | 250.62 - 250.70 | 250.118 - 250.122 |
| Sizing Table | 250.66 | 250.122 |
| Path | Service grounded conductor -> grounding electrode | Equipment metal -> grounded service |
| Sized by | Largest service-entrance conductor | Overcurrent device upstream |
| Carries | Lightning, transient surge currents | Fault current |
| Required at | Service only (and SDS) | Every circuit |
See our EGC reference for the parallel EGC discussion.
Worked Example
Goal: Size the GEC for a 200A residential service with 4/0 aluminum SE cable to a ground rod plus a Ufer.
- Service conductors: 4/0 AL (per Table 310.12, 4/0 AL is acceptable for 200A dwelling service)
- Table 250.66 row: "4/0 AL" -> GEC = 2 AWG copper / 1/0 AL
- But - per 250.66(A), the run to the ground rod need not be larger than 6 AWG copper
- And per 250.66(B), the run to the Ufer need not be larger than 4 AWG copper
So the installer runs a single 4 AWG GEC from the neutral bus to the Ufer, then taps a 6 AWG continuation to the ground rod (with no splice in the main GEC). The 250.66(A) and (B) exceptions save expensive copper.
Installation Requirements - NEC 250.64
- 250.64(A) Aluminum prohibited in contact with masonry, earth, or wet locations
- 250.64(B) Securing required to prevent damage; running paths must be protected from physical damage
- 250.64(C) Continuous: GEC shall be installed in one continuous length without splice, except certain irreversible compression connections or busbars
- 250.64(E) Metallic raceways: Must be electrically continuous and bonded at both ends to prevent inductive choke
The Inductive Choke Problem
When a GEC is enclosed in a ferrous metallic raceway (EMT, RMC, IMC) and the raceway is not bonded at both ends, the changing magnetic field from a lightning impulse induces an opposing voltage in the steel pipe that effectively chokes the GEC's ability to dissipate the surge. NEC 250.64(E) requires bonding both ends of the raceway to short-circuit this effect. The simpler solution: do not put the GEC in a metallic raceway at all - run it exposed and protected by a 1/16-inch plate where physical damage is a concern, or use a nonmetallic raceway.
Cross-References
- NEC 250.50 - 250.70 - grounding electrode system requirements
- NEC 250.122 - EGC sizing (different table, different purpose)
- NEC 310.12 - dwelling service conductor sizing
- NEC 250.30 - separately derived system grounding
- NEC 250.94 - intersystem bonding termination
How WireFillChart Connects
Our conduit fill calculator is not a grounding calculator, but proper GEC sizing affects how many conductors fit through a service-entrance LB or main breaker enclosure. See what is an EGC for the related but distinct sizing concept, and our blog on conduit for 200A service for the full service-entrance picture.