7.2 Sizing the Equipment Grounding Conductor

7.2 Sizing the Equipment Grounding Conductor

Exam Fast Fact: One of the most critical table-navigation rules on the Colorado Journeyman exam is distinguishing between Table 250.66, Table 250.102(C)(1), and Table 250.122. Sizing a Grounding Electrode Conductor (GEC) or Main Bonding Jumper is based on the size of the ungrounded service conductors. Sizing an Equipment Grounding Conductor (EGC) under Table 250.122 is based strictly on the rating or setting of the overcurrent protective device (OCPD) ahead of the circuit! Never use conductor size to look up an EGC in Table 250.122 unless applying the proportional adjustment rule of 250.122(B).

An Equipment Grounding Conductor must possess sufficiently low impedance to facilitate the rapid operation of overcurrent devices under fault conditions while safely conducting the fault current without melting or catching fire. Sizing the EGC correctly requires an electrician to master NEC Table 250.122, understand proportional scaling for voltage drop under 250.122(B), manage shared raceways under 250.122(C), and enforce full-sized parallel conductor rules under 250.122(F).

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Sizing Fundamentals: NEC Table 250.122

Under NEC 250.122(A), wire-type copper, aluminum, or copper-clad aluminum equipment grounding conductors cannot be smaller than the sizes listed in NEC Table 250.122. The table is arranged by the ampere rating or setting of the automatic overcurrent protective device (circuit breaker or fuse) installed ahead of the equipment.

Master Reference: NEC Table 250.122 Minimum Size EGCs

The Conductor Ceiling Rule (NEC 250.122(A))

Code Mandate: "In no case shall the equipment grounding conductor be required to be larger than the ungrounded circuit conductors supplying the equipment."

If a small motor or control circuit has a 20A circuit breaker but utilizes 14 AWG copper branch conductors (permitted under specific tap or motor rules in Articles 430 or 725), Table 250.122 technically calls for a 12 AWG copper EGC. However, by virtue of the ceiling rule in 250.122(A), the equipment grounding conductor is never required to exceed the 14 AWG phase wire size.

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Proportional Adjustment for Conductor Upsizing (NEC 250.122(B))

One of the most mathematically demanding problems on the Colorado exam involves NEC 250.122(B): proportional increase of the equipment grounding conductor.

                      PROPORTIONAL EGC UPSIZING (NEC 250.122(B))
                                          │
      ┌───────────────────────────────────┴───────────────────────────────────┐
      ▼                                                                       ▼
STEP 1: CALCULATE MULTIPLIER                                            STEP 2: SCALE EGC AREA
                       Actual Phase Area (cmil)                                               
 Proportional Ratio = ───────────────────────────                        Required Area = Table 250.122 Area × Ratio
                      Minimum Phase Area (cmil)                                               
                                                                              ▼
                                                                        STEP 3: SELECT CONDUCTOR
                                                                        Select standard AWG from Ch. 9, Table 8
                                                                        with equal or greater circular mil area.

The Safety Physics Behind 250.122(B)

Why does the code force contractors to upsize the green ground wire when they upsize phase conductors for voltage drop over long distances?

1. Loop Impedance: When circuit conductors are pulled hundreds of feet, electrical resistance increases linearly ($R = \rho L / A$). To prevent excessive voltage drop at the load, the installer increases the phase wire cross-sectional area.
2. The Hazard of Leaving the EGC Small: If a ground fault occurs at the far end of the circuit and the phase wire is 2/0 AWG but the EGC was left at a standard 8 AWG, the ground return path has significantly higher resistance than the supply path.
3. Lethal Touch Potential: During the fraction of a second before the breaker clears, Ohm's law ($V = I \times R$) causes the high resistance of the small EGC to develop a massive voltage drop. The metallic equipment enclosure rises to 80V, 100V, or higher above true earth ground, creating a lethal shock hazard for anyone touching the machine.
4. Thermal Destruction: High fault current flowing through a long, undersized EGC can anneal the copper, melt the insulation, or burn open before the upstream breaker can trip magnetically.

Step-by-Step Calculation Procedure

To calculate the required upsized EGC under NEC 250.122(B), follow this rigorous four-step procedure:

• Step 1: Determine Minimum Required Phase Conductor Size: Identify the minimum conductor size that has sufficient ampacity for the circuit load or OCPD under NEC Article 310. Look up its circular mil (cmil) area in NEC Chapter 9, Table 8 (Conductor Properties).
• Step 2: Determine Actual Upsized Phase Conductor Size: Look up the circular mil area of the actual enlarged conductor installed in NEC Chapter 9, Table 8.
• Step 3: Calculate the Proportional Multiplier: $\text{Multiplier} = \frac{\text{Circular Mil Area of Actual Upsized Conductor}}{\text{Circular Mil Area of Minimum Required Conductor}}$
• Step 4: Calculate and Select the New EGC Size: Look up the standard EGC size from Table 250.122 for the circuit overcurrent device, find its circular mil area in Chapter 9 Table 8, and multiply it: $\text{New Required EGC cmil} = \text{Standard EGC cmil} \times \text{Multiplier}$ Find the conductor size in Chapter 9, Table 8 whose area is equal to or greater than this calculated value.

Chapter 9, Table 8 Circular Mil Reference

Fully Worked Examination Problem

Problem: A 60-ampere, 240-volt single-phase branch circuit supplies commercial parking lot lighting. The terminals are rated 75°C.

• The minimum conductor size required for a 60A circuit is 6 AWG copper THHN.
• Due to a 400-foot run, the installer increases the phase conductors to 1 AWG copper to limit voltage drop to under 3%.
• What is the minimum size copper equipment grounding conductor required for this installation?

Solution Steps:

1. Minimum phase conductor: 6 AWG copper = 26,240 cmil (from Ch. 9, Table 8).
2. Actual upsized phase conductor: 1 AWG copper = 83,690 cmil (from Ch. 9, Table 8).
3. Calculate the proportional multiplier: $\text{Multiplier} = \frac{83,690\text{ cmil}}{26,240\text{ cmil}} = 3.1894$
4. Standard EGC from Table 250.122 for a 60A device = 10 AWG copper.
5. Standard 10 AWG area = 10,380 cmil.
6. Calculate the new required EGC circular mil area: $\text{Required Area} = 10,380\text{ cmil} \times 3.1894 = 33,106\text{ cmil}$
7. Look up 33,106 cmil in Chapter 9, Table 8:
   • 6 AWG is 26,240 cmil (too small!)
   • 4 AWG is 41,740 cmil (greater than 33,106 cmil)
8. Conclusion: The equipment grounding conductor must be upsized from 10 AWG to 4 AWG copper.

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Multi-Circuit Raceways (NEC 250.122(C))

In commercial wiring, electricians routinely combine conductors from multiple branch circuits or feeders into a single raceway, wireway, or cable tray. Under NEC 250.122(C), you are not required to pull a separate equipment grounding conductor for each individual circuit.

The Single Common EGC Rule

A single equipment grounding conductor is permitted to serve all circuits contained within the same raceway, trench, or cable tray.

• Sizing Rule: The common EGC must be sized based on the largest overcurrent protective device protecting any circuit conductors installed in that raceway.

Worked Example: A 1-1/4" EMT raceway contains three circuits:

• Circuit 1: 20A 120V lighting circuit (12 AWG Cu)
• Circuit 2: 30A 208V water heater circuit (10 AWG Cu)
• Circuit 3: 70A 208V subpanel feeder (4 AWG Cu)

What size common copper EGC is required? The largest OCPD in the conduit is 70 amperes. Referring to Table 250.122, an overcurrent device rated 70A (in the 'Over 60 through 100 Amperes' bracket) requires an 8 AWG copper EGC. A single 8 AWG copper wire satisfies grounding requirements for all three circuits.

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Sizing EGCs in Parallel Raceways or Cables (NEC 250.122(F))

When large feeders are installed in parallel using multiple raceways or multi-conductor cables, sizing the equipment grounding conductor is subject to a strict life-safety rule that trip up many exam candidates: NEC 250.122(F)(1).

                             800A OVERCURRENT DEVICE
                                       │
            ┌──────────────────────────┴──────────────────────────┐
            ▼                                                     ▼
     RACEWAY #1 (Conduit)                                  RACEWAY #2 (Conduit)
 ┌───────────────────────────┐                         ┌───────────────────────────┐
 │ 3 - 500 kcmil Cu (Phases) │                         │ 3 - 500 kcmil Cu (Phases) │
 │ 1 - 500 kcmil Cu (Neutral)│                         │ 1 - 500 kcmil Cu (Neutral)│
 │                           │                         │                           │
 │ FULL-SIZED EGC:           │                         │ FULL-SIZED EGC:           │
 │ 1/0 AWG Copper            │                         │ 1/0 AWG Copper            │
 │ (NEC Table 250.122)       │                         │ (NEC Table 250.122)       │
 └───────────────────────────┘                         └───────────────────────────┘
     *DO NOT SPLIT EGC AREA!*                              *DO NOT SPLIT EGC AREA!* 

The "Never Split the EGC" Mandate (NEC 250.122(F)(1))

Where conductors are run in parallel in multiple raceways or cables:

NEC 250.122(F)(1)(a): "Each raceway or cable shall contain a full-sized equipment grounding conductor chosen from Table 250.122 based on the rating of the feeder overcurrent protective device."

Electricians cannot take the cross-sectional circular mil area of an 800A EGC and split it 50/50 between two conduits!

The Electrical Physics of Parallel Conduits

Why does the NEC forbid dividing the EGC in parallel raceways?

• Ground-fault current does not divide equally across parallel raceways. Because of high mutual inductive coupling between phase conductors and the raceway interior, a phase-to-ground fault occurring in Raceway #1 will return almost entirely (90%+) through the EGC located inside Raceway #1.
• If an installer splits the required 1/0 AWG EGC into two 4 AWG conductors (one in each pipe), the 4 AWG wire in Raceway #1 would be forced to carry the full short-circuit current of an 800A circuit. The 4 AWG conductor would instantly vaporize, arcing violently and destroying the feeder before the 800A breaker could trip!

Motor Circuit EGC Sizing (NEC 250.122(D))

For motor circuits, branch-circuit short-circuit and ground-fault protective devices (such as inverse-time breakers or dual-element fuses) are sized at 150% to 250% (or up to 800% for instantaneous trip breakers) of motor full-load amperes under Article 430.

• Under NEC 250.122(D)(1), the EGC must be sized based on the rating of the motor branch-circuit overcurrent device from Table 250.122.
• However, under 250.122(A), the EGC is not required to be larger than the motor circuit conductors.

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