TryBuildCalcAluminum Grounding Electrode Conductor Size Calculator (NEC Table 250.66, Aluminum Column)
Size an aluminum GEC.
🕒 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 (Aluminum)
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: 2/0 or 3/0 AWG (Cu) / 4/0 AWG or 250 kcmil (Al)
Table-derived GEC size: 2 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.
Aluminum grounding electrode conductor sizing
Table 250.66 provides a full aluminum/copper-clad aluminum column with its own size boundaries, genuinely different from the copper column at every row — an aluminum system does not simply use a scaled-up version of the copper sizes.
This page defaults to aluminum with no electrode-type cap — if your GEC connects solely to a ground rod, plate, or concrete-encased electrode, NEC 250.64(A) restricts aluminum from that direct earth/masonry contact, and the calculator will flag this for you automatically.
- Aluminum/copper-clad aluminum GEC sizing uses a separate table column, not a simple multiple of the copper sizes.
- NEC 250.64(A) restricts aluminum grounding conductors from direct earth or masonry contact, and from within 18 in. of earth outdoors.
- For a direct rod/pipe/plate or concrete-encased electrode connection, copper is standard practice even on an otherwise-aluminum system.
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, aluminum | 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 | 2 AWG |
Step 2 — Electrode-Type Cap
| Calculation | Result |
|---|---|
| Cap check | No electrode-specific cap applies |
| Final recommended size | 2 AWG (aluminum) |
Therefore, for a 4/0 AWG aluminum largest ungrounded conductor connecting to other / multiple electrodes (metal water pipe, structural steel, etc.), you need approximately 2 AWG aluminum for the grounding electrode conductor.
Related Calculators
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.