💡 What the numbers mean
Adjust the inputs to see how Table 250.66 and the electrode-specific rules interact.
🔍 Reading the trace
The source conductor material picks the input row of Table 250.66; the GEC material picks the output column. Only after that lookup does an electrode-specific rule — 250.66(A), (B) or (C) — get a chance to reduce the answer, and only on the conductor segment it governs.
Press “New Challenge” to start.
Text is paraphrased to respect NFPA copyright — only section numbers and short labels are quoted. Consult the adopted code book for exact language.
Table 250.66 relates the largest ungrounded conductor (or equivalent area for parallel conductors) to the minimum GEC size. The source-conductor material selects which input range applies; the GEC material selects which output column is read. Sizing is based on conductor area, not ampacity — a critical distinction.
| Source Cu | Source Al / Cu-clad Al | GEC Cu | GEC Al / Cu-clad Al |
|---|
Where the conductor is the sole connection to rod/pipe/plate electrode(s) and does not extend on to other electrode types requiring a larger conductor, the GEC portion need not exceed 6 AWG Cu / 4 AWG Al. If the conductor continues on to a more demanding electrode, size it for that electrode instead.
The portion that is the sole connection to a concrete-encased electrode need not exceed 4 AWG copper. Aluminum/Cu-clad Al in contact with concrete, masonry, or earth is prohibited by 250.64(A), so this calculator sizes the concrete-encased connection in the copper column.
The portion that is the sole connection to a ground ring need not exceed the actual ring conductor size. The ring itself must be bare copper, minimum 2 AWG (250.52(A)(4)). This calculator uses the ring size you enter, not a hard-coded value.
No 250.66(A)–(C) maximum-size cap applies; use the full Table 250.66 value unless another code rule or electrode-system arrangement changes the conductor segment being sized. A metal underground water pipe electrode also requires a supplemental electrode per 250.53(D)(2).
Aluminum and copper-clad aluminum grounding electrode conductors are not permitted where in direct contact with masonry or the earth, or where subject to corrosive conditions. Where used outdoors, they may not be terminated within 450 mm (18 in.) of the earth. Because rod/pipe/plate and ground-ring connections are at or below grade in nearly every real installation, a code-compliant size in aluminum can still be a non-compliant installation. This calculator warns whenever aluminum is selected.
The GEC for a separately derived system is sized from Table 250.66 based on the derived-system ungrounded conductor size. For a single separately derived grounded system, see 250.30(A)(5). The same concept applies: match the GEC to the system conductor, then apply any electrode-specific rule.
Use the equivalent area (per-conductor size × number of sets) for the table lookup: two sets of 500 kcmil = 1000 kcmil equivalent. Paralleled conductors are generally limited to 1/0 AWG and larger (310.10(G)) unless a specific exception applies.
- Table 250.66 Note 1 (general case) — multiple sets of service-entrance conductors with unequal corresponding conductors: the equivalent size is the largest sum of the areas of the corresponding conductors of each set. This tool covers only the equal-size, equal-count special case.
- Table 250.66 Note 2 — where there are no service-entrance conductors, size from the equivalent area of the largest conductor required for the load.
- DC systems — GEC sizing follows 250.166, a separate rule.
- 250.64(D) GEC taps to multiple service disconnects, and 250.64(E) ferrous raceway bonding for the GEC.
- The supplemental electrode bonding jumper required with a water-pipe electrode (sized on its own, with its own 250.66(A) cap where it is the sole connection to a rod).
GEC vs. EGC — They Are Not the Same Thing
Grounding conductors are not one-size-fits-all just because the words sound related.
GEC — Grounding Electrode Conductor
- Connects the system to the grounding electrode
- Sized from Table 250.66 — by largest ungrounded conductor area
- One common conductor per service or derived system
- Stabilizes voltage; path for lightning / surge / stray higher-voltage events
- NOT the normal equipment ground-fault clearing path
EGC — Equipment Grounding Conductor
- Connects equipment back to the source
- Sized from Table 250.122 — by OCPD rating
- Typically one per circuit — 250.122(C) permits a single EGC for multiple circuits in the same raceway or cable tray, sized for the largest OCPD
- Low-impedance fault-return path
- Must clear the overcurrent device
Electrode Type Comparison
| Electrode | NEC Section | Max Cu GEC | Max Al GEC | Notes |
|---|---|---|---|---|
| Rod/Pipe/Plate | 250.66(A) | 6 AWG | 4 AWG | Cap only where sole connection; 250.64(A) usually rules out Al at the electrode |
| Concrete-Encased | 250.66(B) | 4 AWG | — | Copper — 250.64(A) prohibits Al in contact with concrete/earth |
| Ground Ring | 250.66(C) | Ring size | — | Cap = actual ring conductor (min 2 AWG bare Cu) |
| Water Pipe | Table 250.66 | Per table | Per table | No cap; requires a supplemental electrode, 250.53(D)(2) |
| In-Ground Support / Steel | Table 250.66 | Per table | Per table | No cap |
Common Mistakes
- Confusing GEC and EGC tables. Table 250.66 ≠ Table 250.122.
- Confusing source vs GEC material. The source-conductor material picks the input range; the GEC material picks the output column. They can differ (aluminum feeders with a copper GEC is routine).
- Applying the rod/pipe/plate cap to a shared common conductor. The 6 AWG Cu cap applies only where the conductor is the sole connection to the rod/pipe/plate electrode.
- Not combining parallel conductor areas. Two 500 kcmil = 1000 kcmil equivalent.
- Forgetting supplemental electrode rules. A water pipe alone requires a supplement per 250.53(D)(2).
- Specifying aluminum where it cannot legally land. 250.64(A) keeps Al and Cu-clad Al out of contact with earth, masonry, and corrosive conditions — and off the last 18 in. above grade outdoors.
What to Notice
- GEC size jumps at specific thresholds — it is a step function, not a linear scale.
- Aluminum/Cu-clad-Al GECs are larger than copper GECs for the same source conductor.
- The maximum Cu GEC from Table 250.66 is 3/0 AWG; the maximum Al is 250 kcmil. The table stops there no matter how large the service gets.
- Electrode-specific caps can dramatically reduce the required GEC — but only on the segment they govern.
Why This Matters
The GEC is the connection between your electrical system and earth. It is sized by the largest ungrounded conductor, not by the load. Getting it wrong leaves the grounding electrode system improperly sized for stabilizing voltage and for handling lightning, surge, and accidental higher-voltage contact. The GEC is not the normal equipment ground-fault clearing path — that is the job of the equipment grounding conductor sized from Table 250.122.