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Equipment Grounding Conductor Sizing Guide
An equipment grounding conductor provides the low-impedance fault-current return path from equipment enclosures back to the source — and it's sized from a genuinely different input than most other conductors on a circuit: the overcurrent device rating, not the wire's own ampacity. Getting it right means a direct Table 250.122 lookup, then checking whether the ungrounded conductors were upsized in a way that requires the EGC to scale up too.
Last updated: September 1, 2026
The equipment grounding conductor is easy to mis-size because it breaks the pattern set by every other conductor on the circuit — it isn't sized from ampacity, insulation rating, or the load it serves. It's sized from the overcurrent device protecting the circuit, with one specific follow-up check whenever the ungrounded conductors were upsized for another reason.
This guide covers how NEC Table 250.122 works, the 250.122(B) proportional-increase rule, copper vs. aluminum, and a full worked example.
How Table 250.122 Works
Table 250.122 sizes the EGC directly from the rating or setting of the automatic overcurrent device (breaker or fuse) protecting the circuit ahead of the equipment — with separate copper and aluminum/copper-clad aluminum columns, covering OCPD ratings from 15A branch circuits up to 6000A services.
| OCPD Rating | EGC — Copper | EGC — 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 |
Above 6000A, Table 250.122 no longer applies directly — NEC 250.122(A) instead requires the EGC to be at least 12.5% of the circular-mil area of the largest ungrounded conductor.
The 250.122(B) Proportional Increase
When ungrounded conductors are increased in size for voltage drop (or any reason other than ambient temperature correction or more than 3 current-carrying conductors), a wire-type EGC run with them must be increased in circular-mil area by the same proportion.
| Step | What to Do | Notes |
|---|---|---|
| 1 | Find the minimum required ungrounded conductor size | From an NEC Table 310.16 ampacity calculation for the actual load, insulation rating, and installation conditions |
| 2 | Find the actual installed ungrounded conductor size | The larger size actually being installed, typically to control voltage drop over a long run |
| 3 | Calculate the circular-mil-area ratio | Actual installed conductor CM ÷ minimum required conductor CM |
| 4 | Scale the table-derived EGC size by that ratio | Table 250.122 EGC size CM × ratio from step 3 |
| 5 | Round up to the next standard size | Never round down, and never leave the result at a non-standard value |
| 6 | Cap at the ungrounded conductor's own size | The EGC is never required to be larger than the ungrounded conductor it runs with |
Worked Example — 100A Feeder Upsized for Voltage Drop
100A Copper Feeder, Ungrounded Conductors Upsized From 8 AWG to 6 AWG
Illustrative example
| Step | Calculation | Result |
|---|---|---|
| Table 250.122 lookup | 100A OCPD → copper column | 8 AWG copper |
| Minimum required ungrounded size | From ampacity calculation | 8 AWG (16,510 CM) |
| Actual installed ungrounded size | Upsized for voltage drop | 6 AWG (26,240 CM) |
| Proportional ratio | 26,240 ÷ 16,510 | ≈ 1.59× |
| Final required EGC size | 8 AWG (16,510 CM) × 1.59 ≈ 26,250 CM → next standard size up | 6 AWG copper |
Without the 250.122(B) check, a plain OCPD-rating lookup alone would have suggested the smaller 8 AWG EGC size — technically undersized relative to the actual upsized ungrounded conductors, even though the OCPD rating itself never changed.
Common Mistakes
Sizing the EGC From Conductor Ampacity Instead of the OCPD Rating
Table 250.122 keys strictly off the rating of the overcurrent device protecting the circuit — not the ampacity of the circuit's own conductors, which is frequently a different number.
Forgetting the 250.122(B) Check After a Voltage-Drop Upsize
Upsizing ungrounded conductors for voltage drop is common on long runs, but the resulting need to proportionally upsize the EGC is easy to overlook — leaving a technically undersized EGC even though the plain OCPD-rating lookup looks compliant.
Confusing the EGC With the Grounding Electrode Conductor
These are two different conductors, sized from two different tables (250.122 vs. 250.66), serving two different purposes. Using this calculator's result to size a grounding electrode conductor — or vice versa — produces the wrong answer for either.
Dividing the EGC Size Across Parallel Raceways
Where circuit conductors run in parallel across multiple raceways, each raceway needs its own full-size EGC based on the circuit's total OCPD rating — the EGC size is not divided among the raceways the way current is.
Applying an Outdated Table Value for Large Aluminum Services
The 2020 NEC revised the 5000A/6000A aluminum EGC row from 1200 kcmil to 1250 kcmil — several older or unverified 'wire size chart' sources still publish the outdated figure.
Relevant Standards and References
Equipment/protective grounding conductor sizing is code-specific — the underlying safety principle is universal, but the exact published tables and rules vary by code and edition.
| Region | Relevant Codes / Guidance |
|---|---|
| United States | NEC (National Electrical Code) Table 250.122 for EGC sizing, 250.122(B) for the proportional-increase rule, 250.118 for qualifying raceway-as-EGC methods |
| Europe / UK | BS 7671 (IET Wiring Regulations) sizes the circuit protective conductor from its own separate table, commonly as a ratio to the phase conductor's cross-sectional area — structured differently from the NEC's OCPD-rating-based approach |
| General guidance | Equipment/protective grounding conductor sizing rules are code-specific and vary by edition — always confirm against the exact table and edition your local authority has adopted before finalizing a real installation. |
Final Verdict
Correct EGC sizing is an OCPD-rating table lookup followed by a voltage-drop upsize check — skipping the second step is the single most common way an EGC ends up technically undersized on an otherwise fully code-compliant circuit.
- Start from NEC Table 250.122 using the circuit's actual OCPD (breaker/fuse) rating, not the conductor's ampacity.
- Check whether the ungrounded conductors were upsized for voltage drop or a similar reason — if so, apply the 250.122(B) proportional increase.
- Round the proportional result up to the next standard size, and never size the EGC larger than the ungrounded conductor itself.
- Confirm whether a wire-type EGC is actually needed, or whether the raceway itself qualifies as the EGC per NEC 250.118.
- For parallel conductor runs, size each raceway's own full EGC from the full circuit OCPD rating — it isn't divided.
- Never substitute this sizing for grounding electrode conductor (Table 250.66) sizing — they're different conductors for different purposes.
Related calculators
Use these calculators when you need to turn this reference information into project quantities:
- Equipment Grounding Conductor Size Calculator
Find the minimum EGC size per NEC Table 250.122, with the 250.122(B) upsize check applied automatically.
- Grounding Electrode Conductor Size Calculator
Find the minimum GEC size per NEC Table 250.66, with electrode-type caps applied automatically.
- Wire Size Calculator
Find the correct AWG wire size from load current, run length, and derating conditions.
- Circuit Breaker Size Calculator
Size the breaker that pairs with a given wire and load.
Related resources
- Grounding Electrode Conductor Sizing Guide
Complete guide to sizing a grounding electrode conductor (GEC) — how NEC Table 250.66 works, the three electrode-specific size caps, copper vs. aluminum, and a worked example.
- Wire Gauge Selection Guide
Complete guide to selecting the correct wire gauge (AWG) — base ampacity by insulation rating, the derating factors that reduce it (conductor count, ambient temperature), the small-conductor breaker cap, and a worked example.