Equipment Grounding Conductor Sizing (NEC 250.122)

Why this matters

The equipment grounding conductor (EGC) is the low-impedance fault-current path that lets an overcurrent device open in a ground fault. Size it correctly and a short to ground trips the breaker fast enough that the metal enclosure of a faulted appliance never reaches a dangerous touch voltage. Size it wrong (too small, or omitted on a long run that needed upsizing for voltage-drop conductor changes) and the breaker takes too long to clear, the enclosure rises in voltage, and someone touching it across a parallel path through their body gets the current. NEC 250.122 is the rule. It is short, often misread, and has one mandatory upsizing path that catches inexperienced installers.

The base rule: NEC 250.122(A)

NEC 250.122(A) sizes the EGC based on the rating of the overcurrent protective device (OCPD) ahead of the equipment, not the load amperage. Table 250.122 gives the relationship:

• 15A OCPD: 14 AWG copper EGC minimum
• 20A OCPD: 12 AWG copper EGC minimum
• 30A and 40A OCPD: 10 AWG copper EGC minimum
• 60A OCPD: 10 AWG copper EGC minimum
• 100A OCPD: 8 AWG copper EGC minimum
• 200A OCPD: 6 AWG copper EGC minimum
• 300A OCPD: 4 AWG copper EGC minimum
• 400A OCPD: 3 AWG copper EGC minimum
• (Table continues to higher ampacities)

Aluminum sizes are one or two AWG sizes larger per the same table. Verify against the current code-cycle Table 250.122 because exact aluminum entries change.

The upsizing rule: NEC 250.122(B)

This is the rule that gets missed. If the phase conductors are increased in size for any reason (voltage drop, ampacity adjustment for raceway fill, conductor sharing), the EGC must be increased in size proportionally.

Specifically: the EGC must be increased by the same circular-mil ratio that the phase conductors were increased.

Worked example: a 200A feeder with phase conductors that code-minimum would be 250 kcmil copper. The installer upsized the phase conductors to 350 kcmil for voltage drop on a long run. The EGC for a 200A OCPD per Table 250.122 is 6 AWG copper (26,240 cmil).

The phase-conductor upsizing ratio: 350,000 / 250,000 = 1.40 (40 percent increase).

The EGC must be upsized by the same ratio: 26,240 x 1.40 = 36,736 cmil. The next standard size up is 4 AWG copper at 41,740 cmil. So the EGC becomes 4 AWG.

This calculation is required by code. An inspector seeing 350 kcmil phase conductors and a 6 AWG EGC on a 200A feeder will fail the install.

The parallel-conductor rule: NEC 250.122(F)

When phase conductors are run in parallel (multiple sets of conductors in separate raceways for high-ampacity feeders), the EGC must be run in each raceway and sized for the OCPD ahead of the parallel set, not divided down by the parallel count.

Example: 800A feeder run as 2 sets of 500 kcmil copper in 2 raceways. The EGC for an 800A OCPD per Table 250.122 is 1/0 AWG copper. Each of the two raceways must contain a 1/0 AWG copper EGC. You cannot run a smaller EGC in each raceway and add them up.

EGC vs grounded conductor (neutral) vs GEC

Three things get confused:

• Equipment grounding conductor (EGC). Bonds metal enclosures together and back to the service neutral-ground bond. Carries fault current only during a ground fault. Sized per 250.122.
• Grounded conductor (neutral). The current-carrying return on a single-phase or three-phase system. Sized per 220.61 (see the related neutral-sizing article).
• Grounding electrode conductor (GEC). Connects the service neutral-ground bond to the grounding electrode system (ground rods, water pipe, Ufer, etc.). Sized per 250.66.

All three have different sizing rules and serve different functions. Mixing them up creates code-noncompliant installs and dangerous bonding.

Common installation mistakes

• Forgetting 250.122(B) upsizing on long runs. This is the highest-frequency miss. Inspector catches it; rework is expensive when raceway is already pulled.
• Sizing EGC to load current instead of OCPD rating. Table 250.122 reads off the breaker, not the load. A 20A breaker feeding a 12A load still requires the 12 AWG EGC per the 20A row.
• Combining EGC and GEC sizes. The GEC at the service is sized per Table 250.66 off the service entrance conductor size. The EGC for a branch or feeder is sized per Table 250.122 off the OCPD. These tables produce different results and one is not interchangeable with the other.
• Omitting EGC in conduit because the conduit is metal. NEC 250.118 lists the metal raceway types that qualify as an EGC. Some metal raceways do (rigid metal conduit, intermediate metal conduit, electrical metallic tubing under specific conditions). Some do not (flexible metal conduit beyond a defined length, liquidtight in certain configurations). When the raceway does not qualify, a separate wire-type EGC must be run inside.

Documentation

For any feeder or branch circuit where 250.122(B) upsizing was applied, the design notes should record:

• OCPD rating
• Code-minimum phase conductor size
• Actual phase conductor size installed (and the reason: VD or ampacity adjustment)
• Code-minimum EGC size per Table 250.122
• Ratio applied per 250.122(B)
• Actual EGC size installed

For parallel feeders, document the EGC in each raceway separately, sized to the full OCPD per 250.122(F).

Inspection check

When the inspector walks the install, expect Table 250.122 to be opened against the panel schedule. Be ready with the calculation if any conductor exceeds the code minimum.

References

• NEC 2023, Article 250.122, Size of Equipment Grounding Conductors
• NEC 2023, Article 250.66, Size of Grounding Electrode Conductor
• NEC 2023, Article 250.118, Types of Equipment Grounding Conductors
• NEC 2023, Article 220.61, Feeder or Service Neutral Load (companion article for neutral sizing)
• NEC 2023, Article 250.4, General Requirements for Grounding and Bonding
• IEEE Std 142 (Green Book), Recommended Practice for Grounding of Industrial and Commercial Power Systems
• UL 467, Standard for Grounding and Bonding Equipment