Tables › GEC Sizing
Grounding Electrode Conductor Sizing NEC 250.66
Minimum size grounding electrode conductor based on the largest service-entrance conductor or equivalent area for each phase.
| Largest Service-Entrance Conductor or Equivalent Area per Phase (Copper) |
Largest Service-Entrance Conductor or Equivalent Area per Phase (Aluminum) |
Copper GEC | Aluminum or Cu-Clad Al GEC |
|---|---|---|---|
| 2 AWG or smaller | 1/0 AWG or smaller | 8 AWG | 6 AWG |
| 1 AWG or 1/0 AWG | 2/0 AWG or 3/0 AWG | 6 AWG | 4 AWG |
| 2/0 AWG or 3/0 AWG | 4/0 AWG or 250 kcmil | 4 AWG | 2 AWG |
| Over 3/0 AWG through 350 kcmil | Over 250 kcmil through 500 kcmil | 2 AWG | 1/0 AWG |
| Over 350 kcmil through 600 kcmil | Over 500 kcmil through 900 kcmil | 1/0 AWG | 3/0 AWG |
| Over 600 kcmil through 1100 kcmil | Over 900 kcmil through 1750 kcmil | 2/0 AWG | 4/0 AWG |
| Over 1100 kcmil | Over 1750 kcmil | 3/0 AWG | 250 kcmil |
For parallel service-entrance conductors, use the equivalent area of a single conductor per phase. Example: two 3/0 AWG per phase = combined area equivalent — look up the total circular mil area and find the single-conductor row that matches.
Sole Connection Exceptions
When the GEC connects to only one type of electrode, these maximum sizes apply instead of the full table above.
| Electrode Type | Max Copper GEC | Max Aluminum GEC | NEC Reference |
|---|---|---|---|
| Ground rod or pipe electrode | 6 AWG | 4 AWG | 250.66(A) |
| Concrete-encased electrode (Ufer) | 4 AWG | 4 AWG | 250.66(B) |
| Ground ring | Same as ring conductor | Same as ring conductor | 250.66(C) |
These caps only apply when the GEC is the sole connection to that specific electrode type. If the GEC also connects to other electrodes, size per the full table above.
GEC vs. EGC — What's the Difference?
The GEC (this table, 250.66) runs from the service equipment to the grounding electrode system — it bonds the electrical system to earth. The EGC (250.122) runs with circuit conductors back to the panel — it provides a fault return path. Different purpose, different sizing rules.
The GEC (this table, 250.66) runs from the service equipment to the grounding electrode system — it bonds the electrical system to earth. The EGC (250.122) runs with circuit conductors back to the panel — it provides a fault return path. Different purpose, different sizing rules.
Common Mistake
Sizing the GEC based on the breaker size instead of the service-entrance conductor size. A 200A service with 4/0 copper service-entrance conductors requires a #4 copper GEC (from the table above), not #6 (which would come from the 250.122 EGC table for 200A).
Sizing the GEC based on the breaker size instead of the service-entrance conductor size. A 200A service with 4/0 copper service-entrance conductors requires a #4 copper GEC (from the table above), not #6 (which would come from the 250.122 EGC table for 200A).
Ground Rod Exception
Running a 200A service to a ground rod? Per 250.66(A), the GEC to that rod never needs to be larger than #6 copper, regardless of the service-entrance conductor size. This is the most commonly used exception on residential services.
Running a 200A service to a ground rod? Per 250.66(A), the GEC to that rod never needs to be larger than #6 copper, regardless of the service-entrance conductor size. This is the most commonly used exception on residential services.
For reference only. Verify all results with the current NEC and local amendments. See Terms of Use.