7.3 Equipment Grounding Conductors (EGC) Sizing (NEC Table 250.122)

7.3 Equipment Grounding Conductors (EGC) Sizing (NEC Table 250.122)

Exam Focus: EGC sizing per NEC Table 250.122, voltage drop proportional adjustment rule (250.122(B)), raceways permitted as EGCs (250.118), FMC/LFMC 6-ft limitations, and parallel circuit EGC sizing in multiple conduits (250.122(F)).

The primary function of an Equipment Grounding Conductor (EGC) is to connect non-current-carrying metal parts of equipment, raceways, and enclosures back to the system grounded conductor at the service disconnect or source of a separately derived system. This forms an effective ground-fault current path—a low-impedance circuit that facilitates the rapid operation of the overcurrent protective device (breaker or fuse) during a ground fault.

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Equipment Grounding Conductor Sizing Table (NEC Table 250.122)

Unlike Grounding Electrode Conductors (which are sized per Table 250.66 based on conductor size), Equipment Grounding Conductors are sized per Table 250.122 based strictly on the rating of the automatic overcurrent protective device (OCPD) protecting the circuit.

[!IMPORTANT] NEC 250.122(A) Limitation: An Equipment Grounding Conductor is never required to be larger than the ungrounded (phase) circuit conductors supplying the equipment.

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Proportional Adjustment for Voltage Drop (NEC 250.122(B))

When ungrounded circuit conductors are increased in size to compensate for voltage drop or long distance runs, the equipment grounding conductor MUST be increased proportionally in circular mil area.

                  VOLTAGE DROP EGC ADJUSTMENT STEPS
+-------------------------------------------------------------------------+
| Step 1: Identify Original Min Conductor Size for OCPD Ampacity (Table 310.16)|
| Step 2: Find Circular Mil (cmil) Area of Original & Upsized Conductors |
| Step 3: Compute Multiplier = (Upsized cmil Area) / (Original cmil Area) |
| Step 4: Multiply Table 250.122 Min EGC cmil Area by Multiplier          |
| Step 5: Select Final EGC Size from NEC Chapter 9, Table 8               |
+-------------------------------------------------------------------------+

Worked Calculation Example: Voltage Drop Adjustment

Scenario: A 100A 208V 3-phase feeder circuit is protected by a 100A circuit breaker. Due to a long 350-foot run, the phase conductors are increased from 3 AWG THHN Copper (standard for 100A) to 1/0 AWG THHN Copper. What minimum size copper EGC must be installed?

• Step 1: Determine Circular Mil (cmil) Areas (NEC Chapter 9, Table 8):
  • Original conductor for 100A (3 AWG Cu) = 52,620 cmil.
  • Upsized conductor installed (1/0 AWG Cu) = 105,600 cmil.
  • Minimum Table 250.122 EGC for 100A OCPD (8 AWG Cu) = 16,510 cmil.
• Step 2: Calculate Upsize Proportional Multiplier: $\text{Multiplier} = \frac{105,600\text{ cmil}}{52,620\text{ cmil}} = 2.0068$
• Step 3: Calculate Required EGC Circular Mil Area: $\text{New EGC cmil} = 16,510\text{ cmil} \times 2.0068 = 33,133\text{ cmil}$
• Step 4: Select EGC Conductor Size from Chapter 9, Table 8:
  • 6 AWG Cu = 26,240 cmil (Too small!).
  • 4 AWG Cu = 41,740 cmil (Sufficient!).
• Conclusion: The equipment grounding conductor must be increased from 8 AWG Cu to 4 AWG Cu.

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Permitted Equipment Grounding Conductors (NEC 250.118)

NEC 250.118 lists fourteen acceptable types of equipment grounding conductors, including wire-type conductors and metallic raceways.

                     METALLIC RACEWAYS AS EGCs (NEC 250.118)
+-------------------------------------------------------------------------+
| PERMITTED WITHOUT LENGTH LIMITATION:                                     |
| - Rigid Metal Conduit (RMC)                                             |
| - Intermediate Metal Conduit (IMC)                                      |
| - Electrical Metallic Tubing (EMT)                                      |
+-------------------------------------------------------------------------+
| STRICTLY LIMITED (Max 6 ft length & Max 20A/30A OCPD):                 |
| - Flexible Metal Conduit (FMC): Max 6 ft ground path, max 20A OCPD.     |
| - Liquidtight FMC (LFMC): Max 6 ft ground path; max 20A (3/8"-1/2") or |
|   max 30A (3/4"-1-1/4") OCPD.                                           |
+-------------------------------------------------------------------------+

Detailed FMC and LFMC Grounding Limitations

1. Flexible Metal Conduit (FMC) (250.118(5)): Permitted as sole EGC only if:
   • Total length in any ground return path does NOT exceed 6 ft (1.8 m).
   • Circuit conductors contained are protected by OCPD rated 20 amperes or less.
   • Conduit is terminated in listed fittings.
2. Liquidtight Flexible Metal Conduit (LFMC) (250.118(6)): Permitted as sole EGC only if:
   • Total length in ground return path does NOT exceed 6 ft (1.8 m).
   • For 3/8 in. and 1/2 in. trade size: OCPD rated 20 amperes or less.
   • For 3/4 in. through 1-1/4 in. trade size: OCPD rated 30 amperes or less.
   • If LFMC length exceeds 6 ft or OCPD exceeds 20A/30A, an equipment bonding jumper or wire EGC MUST be installed!

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EGC Sizing in Parallel Circuits (NEC 250.122(F))

[!CAUTION] Critical Parallel EGC Rule: When conductors are run in parallel in multiple raceways or cables per NEC 310.10(G), a full-sized Equipment Grounding Conductor must be installed in EACH raceway or cable assembly.

• The EGC in each parallel conduit is sized based on the total rating of the overcurrent protective device protecting the entire parallel circuit.
• Example: An 800A feeder is installed in two parallel 4-inch PVC conduits. Table 250.122 requires a 1/0 AWG Copper EGC for an 800A OCPD. You must install a full-sized 1/0 AWG Copper EGC inside EACH of the two conduits (you cannot split it into 2 AWG in each conduit!).