NEC Grounding Table 250.122 Explained: EGC Sizing, Rules & Examples
The NEC Grounding Table 250.122 is one of the most useful references for selecting the minimum size of a wire-type equipment grounding conductor (EGC). The table connects the rating or setting of the overcurrent protective device (OCPD) with the required EGC size.
Correct EGC sizing is important because the grounding conductor must provide a low-impedance fault-current path back to the source. This allows the circuit breaker or fuse to operate quickly during a ground fault.
Table of Contents
Table of Contents
NEC Grounding Table 250.122 Quick Reference
The NEC Grounding Table 250.122 provides minimum equipment grounding conductor sizes based on the rating of the overcurrent device. The values below are commonly used for copper and aluminum EGCs.
| OCPD Rating (Amps) | Copper EGC | Aluminum or Copper-Clad Aluminum EGC |
|---|---|---|
| 15 | 14 AWG | 12 AWG |
| 20 | 12 AWG | 10 AWG |
| 30 | 10 AWG | 10 AWG |
| 40 | 10 AWG | 10 AWG |
| 60 | 10 AWG | 10 AWG |
| 100 | 8 AWG | 6 AWG |
| 200 | 6 AWG | 4 AWG |
| 300 | 4 AWG | 2 AWG |
| 400 | 3 AWG | 1 AWG |
| 500 | 2 AWG | 1/0 AWG |
| 600 | 1 AWG | 2/0 AWG |
| 800 | 1/0 AWG | 3/0 AWG |
| 1000 | 2/0 AWG | 4/0 AWG |
| 1200 | 3/0 AWG | 250 kcmil |
| 1600 | 4/0 AWG | 350 kcmil |
| 2000 | 250 kcmil | 400 kcmil |
| 2500 | 350 kcmil | 600 kcmil |
| 3000 | 400 kcmil | 600 kcmil |
| 4000 | 500 kcmil | 800 kcmil |
| 5000 | 700 kcmil | 1200 kcmil |
| 6000 | 800 kcmil | 1200 kcmil |
Always verify the applicable NEC edition and local amendments before using a table value for an actual installation.
What Is NEC Table 250.122?
The NEC Grounding Table 250.122 is found in Article 250 and is specifically used for sizing wire-type equipment grounding conductors. It applies to conductors that provide a fault-current path for electrical equipment and raceways.
The basic selection process is straightforward:
- Identify the circuit’s overcurrent protective device.
- Find its ampere rating or setting in Table 250.122.
- Select the corresponding EGC size.
- Check whether the conductor must be increased because the ungrounded circuit conductors were increased in size.
- Confirm any additional requirements for special installations.
For example, a circuit protected by a 100-ampere breaker requires a minimum 8 AWG copper EGC under the table.
EGC Size by Breaker Rating
| Breaker Rating | Minimum Copper EGC | Typical Application |
|---|---|---|
| 15 A | 14 AWG | Branch circuits |
| 20 A | 12 AWG | General-purpose circuits |
| 30 A | 10 AWG | Equipment circuits |
| 40 A | 10 AWG | Larger branch circuits |
| 60 A | 10 AWG | Equipment and feeder circuits |
| 100 A | 8 AWG | Feeders and equipment |
| 200 A | 6 AWG | Large feeders |
| 400 A | 3 AWG | Large commercial equipment |
| 600 A | 1 AWG | Large feeders |
| 800 A | 1/0 AWG | Large distribution systems |
| 1000 A | 2/0 AWG | High-current equipment |
The EGC size is not simply chosen to match the phase conductor size. It is primarily determined by the OCPD, subject to the other requirements of Article 250.
Planning a complete grounding conductor installation? Use our earth cable size selection chart to compare conductor sizes across common electrical grounding applications and standards.
Equipment Grounding Conductor vs Grounding Electrode Conductor
One common mistake is treating an equipment grounding conductor and grounding electrode conductor as the same thing. They perform different functions and are sized under different NEC rules.
| Feature | Equipment Grounding Conductor | Grounding Electrode Conductor |
|---|---|---|
| Primary purpose | Carries fault current | Connects system/equipment to grounding electrode system |
| Main NEC reference | Table 250.122 | 250.66 and related sections |
| Sizing basis | OCPD rating/setting | Grounded conductor or applicable electrode rules |
| Typical connection | Equipment, raceway, panelboard | Grounding electrode system |
| Fault clearing role | Directly supports OCPD operation | Not normally the primary fault-clearing path |
| Examples | Copper EGC, metallic raceway | GEC to ground rod, building steel, concrete-encased electrode |
This distinction is critical when performing an earthing conductor size calculation because the correct NEC section depends on the conductor’s function.
How to Use NEC Table 250.122
Suppose a panel is supplied from a 200-ampere circuit breaker and the installation uses copper conductors.
Look at the 200-ampere row in the table. The minimum copper equipment grounding conductor is 6 AWG.
Therefore:
200 A OCPD → 6 AWG copper EGC
If aluminum or copper-clad aluminum is used, the corresponding table value is 4 AWG.
| Example | OCPD | Conductor Material | Minimum EGC |
|---|---|---|---|
| Example 1 | 20 A | Copper | 12 AWG |
| Example 2 | 60 A | Copper | 10 AWG |
| Example 3 | 100 A | Copper | 8 AWG |
| Example 4 | 200 A | Copper | 6 AWG |
| Example 5 | 400 A | Copper | 3 AWG |
| Example 6 | 100 A | Aluminum | 6 AWG |
| Example 7 | 200 A | Aluminum | 4 AWG |
These are minimum table selections before considering other applicable requirements.
What Happens When Phase Conductors Are Upsized?
This is an important part of the NEC Grounding Table 250.122 requirements.
If the ungrounded conductors are increased in size beyond the minimum required for the circuit, the wire-type EGC generally must also be increased proportionately according to circular mil area, as required by NEC 250.122(B).
For example, assume a circuit requires a 6 AWG copper phase conductor based on ampacity, but the installer increases the phase conductors to 2 AWG to reduce voltage drop.
You cannot automatically continue using the table’s minimum EGC without checking the proportional-sizing requirement.
Basic Proportional EGC Calculation
The general relationship can be expressed as:
New EGC area = Table EGC area × (New ungrounded conductor area ÷ Minimum ungrounded conductor area)
The calculation uses circular mil area rather than simply comparing AWG numbers.
| Item | Example |
|---|---|
| Minimum phase conductor | 6 AWG Cu |
| Upsized phase conductor | 2 AWG Cu |
| Table EGC | 6 AWG Cu |
| Reason for upsizing | Voltage drop |
| Additional requirement | Proportional EGC increase |
| Calculation basis | Circular mil area |
The final conductor size must be selected from an available standard conductor size that meets or exceeds the calculated area.
Need to compare grounding conductor requirements quickly? Our earthing conductor size guide helps you review conductor selection methods and common sizing requirements in one place.
Why Does NEC Require Proportional EGC Upsizing?
An equipment grounding conductor must be capable of carrying sufficient fault current to operate the protective device. Increasing the phase conductor size can reduce the impedance of the circuit’s fault-current path.
Without the proportional increase, a long circuit with significantly upsized phase conductors could have an EGC that is relatively small compared with the circuit conductors.
The rule helps maintain an effective fault-current path.
This becomes especially relevant for:
- Long feeder circuits
- Motor circuits
- Large commercial buildings
- Solar photovoltaic systems
- EV charging circuits
- Industrial equipment
- Circuits designed for reduced voltage drop
- Large outdoor feeders
Raceway as the Equipment Grounding Conductor
A wire is not always required to serve as the equipment grounding conductor. Under permitted conditions, metallic raceways and other recognized conductive paths can serve as EGCs.
Examples include certain installations using:
- EMT
- RMC
- IMC
- Listed metal raceways
- Other NEC-recognized metallic grounding paths
The raceway must be properly installed and electrically continuous. Connections, fittings and equipment must maintain the required grounding path.
| Grounding Path | Common Example | Key Consideration |
|---|---|---|
| Copper wire EGC | THHN/THWN copper | Size according to applicable NEC rules |
| Aluminum EGC | Aluminum feeder EGC | Apply material-specific requirements |
| EMT | Commercial installations | Maintain electrical continuity |
| RMC | Industrial installations | Properly bonded fittings required |
| IMC | Commercial/industrial | Proper installation required |
Common Mistakes When Using Table 250.122
Several sizing errors appear repeatedly in electrical installations.
1. Matching the EGC to the phase conductor
The EGC does not normally have to be the same size as the phase conductors. Its minimum size is established by the applicable NEC requirements, including Table 250.122.
2. Ignoring phase-conductor upsizing
This is one of the most important mistakes. If ungrounded conductors are increased for voltage drop or another reason, check 250.122(B).
3. Confusing GEC and EGC
A grounding electrode conductor is not sized from Table 250.122. Different NEC requirements apply.
4. Using an outdated NEC edition
NEC requirements can change between code editions. Local jurisdictions may also adopt amendments.
5. Ignoring special equipment
Motors, generators, separately derived systems, sensitive electronic equipment, PV systems and other applications can introduce additional requirements.
Want to calculate the conductor size instead of checking individual values manually? Use our earthing cable size calculation guide for a practical approach to grounding and earthing conductor selection.
NEC Grounding Table 250.122 vs Common EGC Sizes
The following simplified chart can help during preliminary design.
| OCPD | Copper EGC | Aluminum EGC |
|---|---|---|
| 15 A | 14 AWG | 12 AWG |
| 20 A | 12 AWG | 10 AWG |
| 30 A | 10 AWG | 10 AWG |
| 40 A | 10 AWG | 10 AWG |
| 60 A | 10 AWG | 10 AWG |
| 100 A | 8 AWG | 6 AWG |
| 200 A | 6 AWG | 4 AWG |
| 300 A | 4 AWG | 2 AWG |
| 400 A | 3 AWG | 1 AWG |
| 500 A | 2 AWG | 1/0 AWG |
| 600 A | 1 AWG | 2/0 AWG |
For installations above 600 amperes, consult the complete NEC table rather than extrapolating the values.
Frequently Asked Questions
What is Table 250.122 used for?
It is used primarily to determine the minimum size of wire-type equipment grounding conductors based on the rating or setting of the overcurrent protective device.
What size ground wire is needed for a 100 amp breaker?
For a standard wire-type EGC, Table 250.122 specifies a minimum 8 AWG copper or 6 AWG aluminum EGC for a 100-ampere OCPD, subject to other applicable requirements.
Does the ground wire need to be the same size as the phase wire?
No. The equipment grounding conductor is normally sized according to the applicable NEC requirements rather than simply matching the phase conductor. However, conductor upsizing can trigger proportional EGC upsizing under 250.122(B).
What is the difference between Table 250.122 and Table 250.66?
Table 250.122 is used for sizing equipment grounding conductors. Table 250.66 is associated with sizing grounding electrode conductors. They serve different purposes and should not be interchanged.
Does voltage drop affect equipment grounding conductor size?
Yes. When ungrounded conductors are increased in size for voltage drop or another reason, NEC 250.122(B) can require a proportional increase in the wire-type equipment grounding conductor based on circular mil area.
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Final Takeaway
The NEC Grounding Table 250.122 provides a practical starting point for selecting equipment grounding conductor sizes. The key is to identify the OCPD rating, select the appropriate copper or aluminum value, and then check for additional requirements.
Most importantly, do not stop at the table when the circuit conductors have been upsized. NEC 250.122(B), conductor material, installation method and special circuit requirements can all affect the final EGC selection. For design and field work, always use the NEC edition adopted by the authority having jurisdiction and verify the complete installation requirements.
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