TryBuildCalc400A Equipment Grounding Conductor Size Calculator (NEC Table 250.122)
Size the EGC for a 400A service.
🕒 Last updated: September 1, 2026
Inputs
ℹ️The equipment grounding conductor's own material — not necessarily the same as the circuit's ungrounded conductors. Table 250.122 uses genuinely different size boundaries for copper and aluminum — aluminum EGCs run one to two sizes larger than the equivalent copper size.
ℹ️The rating or setting of the automatic overcurrent device (breaker or fuse) protecting the circuit ahead of the equipment — not the ampacity of the circuit conductors themselves. Common standard sizes: 15, 20, 30, 40, 60, 100, 150, 200, 400A, up to 6000A for large services — an adjustable-trip breaker's actual setting (e.g. 201A) works too, since Table 250.122 keys off the OCPD's "rating or setting," not just standard sizes.
Ungrounded Conductors Upsized?
Cost
Required Equipment Grounding Conductor: 3 AWG (Copper)
Sized directly from NEC Table 250.122, no proportional increase applies
NEC Table 250.122 Lookup
OCPD (breaker/fuse) rating: 400 A
Table row: Over 300 through 400 A
Table-derived EGC size: 3 AWG
250.122(B) Upsize Check
Not checked — enable the upsize toggle if ungrounded conductors were increased for voltage drop or another reason
Assumptions Used
NEC Table 250.122 (minimum size equipment grounding conductors), NEC 250.122(B) proportional increase where ungrounded conductors are upsized. Assumes a single, homogeneous conductor material for both the ungrounded conductors and the EGC, and a single (non-parallel) raceway or cable run. 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 250.122, the 250.122(B) upsize rule, and how this differs from grounding electrode conductor sizing? Equipment Grounding Conductor Sizing Guide →
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Equipment Grounding Conductor Size Calculator.
EGC sizing for a 400A service or large feeder
400A is a common rating for larger residential services (especially combined dual-meter installations), light-commercial services, and large feeders.
This page is pre-set to a 400A circuit with a copper EGC — edit the inputs above to match your actual OCPD rating and EGC material.
- A 400A circuit needs a 3 AWG copper EGC per Table 250.122's 'over 300 through 400A' row.
- At this size, confirm whether the circuit uses parallel conductor sets across multiple raceways — each raceway needs its own full-size EGC, not a divided fraction.
- Check whether the ungrounded conductors were upsized for voltage drop over a long run, which can require a proportionally larger EGC per 250.122(B).
Equipment Grounding Conductor Formula: How Is the Size Determined?
Two steps: a direct NEC Table 250.122 lookup from the OCPD rating, then a 250.122(B) proportional-increase check.
Step 1 — Table 250.122 Lookup
Find the Table 250.122 row whose OCPD-rating range contains the circuit's breaker/fuse rating
Table EGC Size = that row's EGC size, for the selected material (Cu or Al)
Table 250.122 has 19 rows, each covering a range of overcurrent device ratings, with separate copper and aluminum/copper-clad aluminum columns — sized from the OCPD rating, not the ampacity of the circuit conductors themselves.
Step 2 — 250.122(B) Proportional Increase
If ungrounded conductors were increased for voltage drop (or another reason besides temperature/conductor-count derating):
Ratio = Actual Installed Ungrounded Conductor CM ÷ Minimum Required Ungrounded Conductor CM
Final EGC = next standard size ≥ (Table EGC Size CM × Ratio), never larger than the ungrounded conductor itself
A 2020+ NEC exception also permits a qualified person to determine an alternative EGC size using engineering judgment instead of the strict circular-mil proportion — that professional-judgment path isn't modeled here, since it depends on site-specific analysis this calculator can't perform.
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 |
|---|---|---|
| OCPD rating | 400 A | Sets the Table 250.122 row |
| EGC material | Copper | Sets the Table 250.122 column |
| Ungrounded conductors upsized? | No | Determines whether the 250.122(B) proportional increase applies |
Step 1 — Table 250.122 Lookup
| Calculation | Result |
|---|---|
| Table row | Over 300 through 400 A |
| Table-derived EGC size | 3 AWG |
Step 2 — 250.122(B) Upsize Check
| Calculation | Result |
|---|---|
| Upsize check | Not checked — ungrounded conductors not marked as upsized |
| Final recommended size | 3 AWG (copper) |
Therefore, for a 400 A circuit, you need approximately 3 AWG copper for the equipment grounding 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.