TryBuildCalcGround Ring Conductor Size Calculator (NEC 250.66(C))
Size the GEC to a ground ring.
🕒 Last updated: August 31, 2026
Inputs
ℹ️Governs both the largest-ungrounded-conductor column and the resulting GEC size — Table 250.66 uses genuinely different size boundaries for copper and aluminum.
ℹ️The largest ungrounded (phase) conductor at the service, feeder, or separately derived system — check the panel schedule or as-built drawings. For parallel conductor sets, use the total equivalent area.
ℹ️A rod/pipe/plate electrode caps the GEC at 6 AWG copper or 4 AWG aluminum (NEC 250.66(A)); a concrete-encased electrode caps it at 4 AWG copper (NEC 250.66(B)) — both only when it's the sole connection. A ground ring instead matches its own conductor size, per 250.66(C).
Cost
Required Grounding Electrode Conductor: 2 AWG (Copper)
Sized directly from NEC Table 250.66, no electrode-type cap applies
NEC Table 250.66 Lookup
Largest ungrounded conductor: 4/0 AWG
Table row: Over 3/0 AWG through 350 kcmil (Cu) / Over 250 through 500 kcmil (Al)
Table-derived GEC size: 2 AWG
Electrode Type Check
Electrode type: Ground ring (sole connection)
Ring conductor minimum: 2 AWG copper — the GEC to the ring must match the ring's own conductor size (NEC 250.66(C))
Assumptions Used
NEC Table 250.66 (grounding electrode conductor size for AC systems), NEC 250.66(A)/(B)/(C) electrode-specific caps, NEC 250.64(A) aluminum earth-contact restriction. Assumes a single, homogeneous conductor material for both the ungrounded conductor and the GEC. This is a reference estimate — confirm against the code edition adopted in your jurisdiction and a licensed electrician before final installation.
Want the full walkthrough on Table 250.66, the electrode-specific caps, and connector types? Grounding Electrode Conductor Sizing Guide →
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Grounding Electrode Conductor Size Calculator.
Ground ring electrode GEC sizing
A ground ring is a bare copper conductor buried around a building's perimeter, at least 30 inches deep and typically at least 20 feet long, in direct contact with earth — most common in larger commercial or industrial installations, or where a high-resistance soil condition makes other electrode types less effective.
Unlike the rod/pipe/plate and concrete-encased caps, NEC 250.66(C) works differently: the GEC connecting to a ground ring simply has to match the ring's own conductor size, and that ring conductor itself must be at least 2 AWG copper. This page is pre-set to a 4/0 AWG copper largest ungrounded conductor with the ground ring electrode type selected.
- The ground ring conductor itself must be at least 2 AWG copper, per NEC 250.52(A)(4) and 250.66(C).
- The GEC to the ring matches the ring conductor's actual installed size — confirm that size directly, rather than relying only on the table-derived size shown here.
- A ground ring must be buried at least 30 inches deep, encircling the building or structure.
Grounding Electrode Conductor Formula: How Is the Size Determined?
Two steps: a direct NEC Table 250.66 lookup, then an electrode-specific cap check.
Step 1 — Table 250.66 Lookup
Convert Largest Ungrounded Conductor Size to circular mils (AWG table lookup, or kcmil x 1000)
Find the Table 250.66 row whose range contains that value, for the selected material (Cu or Al)
Table GEC Size = that row's GEC size, in the same material
Table 250.66 has 7 rows, each covering a range of largest-ungrounded-conductor sizes, with separate copper and aluminum/copper-clad aluminum columns — the two materials have genuinely different range boundaries, not a scaled version of each other.
Step 2 — Electrode-Type Cap
Rod/pipe/plate (sole connection): Final GEC = the SMALLER of Table GEC Size and 6 AWG copper / 4 AWG aluminum (NEC 250.66(A))
Concrete-encased electrode (sole connection): Final GEC = the SMALLER of Table GEC Size and 4 AWG copper (NEC 250.66(B)) — copper only, no aluminum cap is published
Ground ring (sole connection): GEC matches the ring's own conductor, minimum 2 AWG copper (NEC 250.66(C))
Other/multiple electrodes: Final GEC = Table GEC Size, no cap applied
These caps only apply to the portion of the conductor that is the SOLE connection to that specific electrode type — a conductor that also serves another purpose is not automatically capped the same way. Aluminum GEC directly connected to a rod/pipe/plate, concrete-encased, or ground-ring electrode is flagged separately, since NEC 250.64(A) restricts aluminum from direct earth/masonry contact regardless of the size cap.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Largest ungrounded conductor | 4/0 AWG, copper | Sets the Table 250.66 row and material column |
| Electrode type | Ground ring (sole connection) | Determines which electrode-specific cap, if any, applies |
Step 1 — Table 250.66 Lookup
| Calculation | Result |
|---|---|
| Table row | Over 3/0 AWG through 350 kcmil (Cu) / Over 250 through 500 kcmil (Al) |
| Table-derived GEC size | 2 AWG |
Step 2 — Electrode-Type Cap
| Calculation | Result |
|---|---|
| Cap check | GEC matches ring conductor, minimum 2 AWG (NEC 250.66(C)) |
| Final recommended size | 2 AWG (copper) |
Therefore, for a 4/0 AWG copper largest ungrounded conductor connecting to ground ring (sole connection), you need approximately 2 AWG copper for the grounding electrode conductor.
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Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.