TryBuildCalcGrounding Electrode Conductor Size Calculator (NEC Table 250.66)
Find your required GEC size instantly.
🕒 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: 2/0 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: Other / multiple electrodes (metal water pipe, structural steel, etc.)
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.
General grounding electrode conductor size estimate
This page gives the full combined result — the Table 250.66 lookup, any electrode-specific cap, and the final recommended GEC size — from one largest-ungrounded-conductor-size, material, and electrode-type selection.
It's the same calculator behind every other grounding electrode conductor variant on this site, just without a specific scenario pre-selected — useful for a general estimate before finalizing the exact installation with a licensed electrician.
- Looks up NEC Table 250.66 directly from your largest ungrounded conductor size and material.
- Automatically applies the correct electrode-specific cap for rod/pipe/plate, concrete-encased, or ground ring electrodes.
- Supports copper and aluminum, with an aluminum earth-contact warning where relevant.
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 | 2/0 AWG, copper | Sets the Table 250.66 row and material column |
| Electrode type | Other / multiple electrodes (metal water pipe, structural steel, etc.) | 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 2/0 AWG copper largest ungrounded conductor connecting to other / multiple electrodes (metal water pipe, structural steel, etc.), 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.