TryBuildCalcConcrete-Encased Electrode GEC Size Calculator (Ufer Ground — NEC 250.66(B))
Size the GEC to a Ufer ground.
🕒 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: 4 AWG (Copper)
Sized directly from NEC Table 250.66, no electrode-type cap applies
NEC Table 250.66 Lookup
Largest ungrounded conductor: 3/0 AWG
Table row: 2/0 or 3/0 AWG (Cu) / 4/0 AWG or 250 kcmil (Al)
Table-derived GEC size: 4 AWG
Electrode Type Check
Electrode type: Concrete-encased electrode / Ufer ground (sole connection)
No electrode-specific cap applies — the table size governs
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.
Concrete-encased electrode (Ufer ground) GEC sizing
A concrete-encased electrode — commonly called a Ufer ground — is typically rebar or a bare copper conductor encased in at least 2 inches of concrete at the base of a footing or foundation, in direct contact with the earth. It's a highly effective, low-resistance electrode in most soil conditions.
This page is pre-set to a 3/0 AWG copper largest ungrounded conductor with the concrete-encased electrode type selected — edit the inputs above to match your actual service.
- The 4 AWG copper cap applies to the portion of the conductor that is the sole connection to the concrete-encased electrode.
- New construction commonly requires a concrete-encased electrode where the qualifying conditions of NEC 250.52(A)(3) are met.
- Connections that will be encased in the concrete pour need an exothermic weld or a connector specifically listed for direct concrete encasement, per NEC 250.68(A).
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 | 3/0 AWG, copper | Sets the Table 250.66 row and material column |
| Electrode type | Concrete-encased electrode / Ufer ground (sole connection) | Determines which electrode-specific cap, if any, applies |
Step 1 — Table 250.66 Lookup
| Calculation | Result |
|---|---|
| Table row | 2/0 or 3/0 AWG (Cu) / 4/0 AWG or 250 kcmil (Al) |
| Table-derived GEC size | 4 AWG |
Step 2 — Electrode-Type Cap
| Calculation | Result |
|---|---|
| Cap check | No electrode-specific cap applies |
| Final recommended size | 4 AWG (copper) |
Therefore, for a 3/0 AWG copper largest ungrounded conductor connecting to concrete-encased electrode / ufer ground (sole connection), you need approximately 4 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.