TryBuildCalcEquipment Grounding Conductor Size Calculator (NEC Table 250.122)
Find your required EGC size instantly.
🕒 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: 8 AWG (Copper)
Sized directly from NEC Table 250.122, no proportional increase applies
NEC Table 250.122 Lookup
OCPD (breaker/fuse) rating: 100 A
Table row: Over 60 through 100 A
Table-derived EGC size: 8 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 →
What Is an Equipment Grounding Conductor Size Calculator?
An equipment grounding conductor (EGC) size calculator finds the minimum conductor size required to provide a low-impedance fault-current return path from equipment enclosures, raceways, and metal boxes back to the electrical source, per NEC Table 250.122. Unlike a grounding electrode conductor, it is sized from the circuit's overcurrent device (breaker or fuse) rating, not from the size of the ungrounded conductors themselves.
This is a different conductor, sized from a different table, than the grounding electrode conductor that bonds the system to earth — the EGC's job is to safely carry fault current back to the source so the overcurrent device trips, not to connect the system to earth.
Why the 250.122(B) upsize check matters:
- A plain OCPD-rating lookup alone can undersize the EGC when ungrounded conductors were increased for voltage drop
- NEC 250.122(B) requires the EGC to scale up in the same proportion as the ungrounded conductors were increased, in circular-mil area
- Missing this check leaves an EGC that is technically undersized relative to the actual installed circuit, even though the OCPD-rating lookup alone looks compliant
- Confusing this calculator with a grounding electrode conductor (Table 250.66) calculator is a common, code-significant mix-up
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 | 100 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 60 through 100 A |
| Table-derived EGC size | 8 AWG |
Step 2 — 250.122(B) Upsize Check
| Calculation | Result |
|---|---|
| Upsize check | Not checked — ungrounded conductors not marked as upsized |
| Final recommended size | 8 AWG (copper) |
Therefore, for a 100 A circuit, you need approximately 8 AWG copper for the equipment grounding conductor.
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓Circuit & System Inputs+-
- OCPD (breaker/fuse) rating confirmed from the actual installed device, not assumed from wire size
- EGC material confirmed as copper or aluminum, matching the actual installed conductor
- Low-impedance fault-current path confirmed end-to-end, back to the source
- Local code edition and any local amendments confirmed before finalizing
✓250.122(B) Upsize Check+-
- Upsize reason confirmed as voltage drop or similar — not ambient temperature or conductor-count derating
- Minimum required ungrounded conductor size correctly determined from ampacity tables, not guessed
- Actual installed ungrounded conductor size confirmed from the panel schedule or as-built drawings
✓Table 250.122 Application+-
- Correct Table 250.122 row read for the actual OCPD rating and material
- EGC sized from the OCPD rating, confirmed not to have been sized from conductor ampacity instead
✓Installation & Code Compliance+-
- Where a raceway itself serves as the EGC, its physical continuity and listing confirmed end-to-end
- Permit and inspection requirements confirmed for the scope of circuit/equipment work
- Circuit/equipment grounding work reviewed or performed by a licensed electrician
Full QC Checklist+−
Verification checklist for equipment grounding conductor (EGC) sizing and installation — covering circuit inputs, the 250.122(B) upsize check, Table 250.122 application, and installation code compliance. Use the Essential Checklist for critical checks before finalizing, expand to Full QC Checklist for complete verification.
✓Circuit & System Inputs+-
- OCPD (breaker/fuse) rating confirmed from the actual installed device, not assumed from wire size
- EGC material confirmed as copper or aluminum, matching the actual installed conductor
- Wire-type EGC vs. raceway-as-EGC method confirmed for the actual installation
- Low-impedance fault-current path confirmed end-to-end, back to the source
- Local code edition and any local amendments confirmed before finalizing
✓250.122(B) Upsize Check+-
- Upsize reason confirmed as voltage drop or similar — not ambient temperature or conductor-count derating
- Minimum required ungrounded conductor size correctly determined from ampacity tables, not guessed
- Actual installed ungrounded conductor size confirmed from the panel schedule or as-built drawings
- Proportional increase rounded up to the next standard conductor size, not left at a non-standard value
- Final EGC size confirmed not larger than the ungrounded conductor it runs with
✓Table 250.122 Application+-
- Correct Table 250.122 row read for the actual OCPD rating and material
- EGC sized from the OCPD rating, confirmed not to have been sized from conductor ampacity instead
- Each parallel raceway confirmed to carry its own full-size EGC, not a divided fraction
- For OCPD ratings above 6000A, the 12.5%-of-largest-ungrounded-conductor rule applied instead of Table 250.122
- Terminal and lug ratings confirmed compatible with the final EGC size and material after any upsize
✓Installation & Code Compliance+-
- EGC connections and splices made with a listed connector or an irreversible method
- Equipment bonding jumper sized correctly where used, matching or exceeding the circuit EGC
- Where a raceway itself serves as the EGC, its physical continuity and listing confirmed end-to-end
- Permit and inspection requirements confirmed for the scope of circuit/equipment work
- Circuit/equipment grounding work reviewed or performed by a licensed electrician
NEC Table 250.122 Reference
Minimum equipment grounding conductor size, by the rating or setting of the overcurrent device protecting the circuit, copper and aluminum/copper-clad aluminum.
| OCPD Rating (Amps) | EGC Size — Copper | EGC Size — Aluminum |
|---|---|---|
| 15 A or less | 14 AWG | 12 AWG |
| Over 15 through 20 A | 12 AWG | 10 AWG |
| Over 20 through 60 A | 10 AWG | 8 AWG |
| Over 60 through 100 A | 8 AWG | 6 AWG |
| Over 100 through 200 A | 6 AWG | 4 AWG |
| Over 200 through 300 A | 4 AWG | 2 AWG |
| Over 300 through 400 A | 3 AWG | 1 AWG |
| Over 400 through 500 A | 2 AWG | 1/0 AWG |
| Over 500 through 600 A | 1 AWG | 2/0 AWG |
| Over 600 through 800 A | 1/0 AWG | 3/0 AWG |
| Over 800 through 1000 A | 2/0 AWG | 4/0 AWG |
| Over 1000 through 1200 A | 3/0 AWG | 250 kcmil |
| Over 1200 through 1600 A | 4/0 AWG | 350 kcmil |
| Over 1600 through 2000 A | 250 kcmil | 400 kcmil |
| Over 2000 through 2500 A | 350 kcmil | 600 kcmil |
| Over 2500 through 3000 A | 400 kcmil | 600 kcmil |
| Over 3000 through 4000 A | 500 kcmil | 750 kcmil |
| Over 4000 through 5000 A | 700 kcmil | 1250 kcmil |
| Over 5000 through 6000 A | 800 kcmil | 1250 kcmil |
250.122(B) — Increased in Size
Where ungrounded conductors are increased in size for any reason other than ambient temperature correction or more than three current-carrying conductors — most commonly, voltage drop — a wire-type EGC run with them must be increased in circular-mil area in the same proportion.
An EGC is never required to be larger than the ungrounded (phase) conductor(s) it runs with, regardless of the proportional-increase calculation.
When should you use this equipment grounding conductor size calculator?
- Sizing the equipment grounding conductor for a new branch circuit, feeder, or piece of equipment.
- Checking whether ungrounded conductors upsized for voltage drop require a proportionally larger EGC per NEC 250.122(B).
- Cross-checking an electrician's proposed EGC size against an independent estimate.
- Comparing copper vs. aluminum EGC options for the same OCPD rating.
- Planning material and cost for an EGC run alongside a circuit's ungrounded conductors.
Quick EGC Sizing Tips
- Size from the OCPD (breaker/fuse) rating, not the ampacity of the circuit conductors — these are frequently different values.
- Always check whether the ungrounded conductors were upsized for voltage drop before finalizing the EGC size — this is the single most commonly missed step.
- Remember Table 250.122 sizes the equipment grounding conductor only — the grounding electrode conductor (NEC Table 250.66) is a separate, differently-sized conductor for a different purpose.
- For parallel conductor sets in multiple raceways, each raceway needs its own full-size EGC based on the circuit's total OCPD rating — it is not divided among the raceways.
- A wire-type EGC is not the only permitted method — rigid metal conduit, IMC, and certain other raceways can themselves qualify as the EGC where installed per NEC 250.118.
- Aluminum EGCs generally run one to two AWG sizes larger than the equivalent copper size at the same OCPD rating.
Common Mistakes
- Sizing the EGC from the circuit conductor's ampacity instead of the OCPD rating — Table 250.122 keys strictly off the overcurrent device.
- Forgetting the 250.122(B) proportional increase after upsizing ungrounded conductors for voltage drop.
- Confusing this calculator's result with grounding electrode conductor (GEC) sizing — they use different NEC tables entirely.
- Dividing the EGC size across multiple parallel raceways instead of running a full-size EGC in each one.
- Assuming a larger OCPD always means a proportionally larger EGC — Table 250.122's steps are not linear with amperage.
- Using an EGC size based on an old NEC edition — the 2020 NEC changed the 5000A/6000A aluminum row from 1200 kcmil to 1250 kcmil, and several published charts still show the outdated figure.
Limitations
- NEC-based only (US) — other regions use different circuit-protective-conductor rules (IEC/BS 7671 and similar) and this calculator does not model them.
- Does not derive the "minimum required" ungrounded conductor size from ampacity tables — enter it directly from your wire-size/voltage-drop calculation.
- Assumes a single, non-parallel raceway or cable run — parallel EGC sizing (each raceway needs its own full-size EGC per NEC 250.122(C)/(F)) must be verified manually.
- Does not model the 2020+ exception permitting a qualified person to determine an alternative EGC size by engineering judgment instead of the strict circular-mil proportion.
- Does not size grounding electrode conductors (NEC Table 250.66) — that is a different conductor with a different purpose and a different sizing table.
- OCPD ratings above 6000A require the 12.5%-of-largest-ungrounded-conductor rule instead of a direct table lookup — out of scope for this calculator.
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.