5.4 Equipment Grounding Conductors & Enclosure Bonding

5.4 Equipment Grounding Conductors & Enclosure Bonding

While the Grounding Electrode System connects to the earth and Main Bonding Jumpers establish the supply-side neutral link, the Equipment Grounding Conductor (EGC) performs the everyday life-safety work across every branch circuit and feeder. The EGC is the low-impedance metallic highway that routes short-circuit ground-fault current straight from an energized tool, appliance, or motor back to the panelboard, tripping the circuit breaker before electrical fires or fatal shocks occur.

On the Wisconsin Journeyman examination, questions on EGC sizing, permitted wiring methods, proportional up-sizing for voltage drop, and device bonding appear regularly. Electricians must navigate NEC 250.118, Table 250.122, and NEC 250.146 with rapid precision.

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1. Permitted Types of EGCs (NEC 250.118)

An Equipment Grounding Conductor does not have to be a green wire. NEC 250.118 recognizes fourteen distinct wiring methods and metallic pathways as compliant EGCs:

1. Conductor Types: Copper, aluminum, or copper-clad aluminum; insulated, covered, or bare.
2. Rigid Metal Conduit (RMC) and Intermediate Metal Conduit (IMC).
3. Electrical Metallic Tubing (EMT): The steel wall of EMT is fully listed and recognized as an effective EGC when installed with listed fittings made up tight.
4. Type AC Cable (Armored Cable): The steel or aluminum armor, in combination with the internal 16 AWG aluminum bonding strip in intimate contact with the armor, forms an approved EGC.
5. Type MC Cable (Metal-Clad Cable): Permitted as an EGC only where listed and identified for grounding (e.g., continuous corrugated aluminum sheath or interlocked armor with an integral bare bonding tape/wire combination such as MC-Tuff or MC-AP). Standard interlocked MC cable with only a bare green wire relies on that wire as the EGC.

Flexible Metal Conduits as EGCs: Strict Limitations

Flexible conduits have higher electrical impedance at their joints. Therefore, NEC 250.118 establishes rigid restrictions on using them as the sole EGC:

[!CAUTION] Flexibility After Installation Exception: If FMC or LFMC is installed specifically to allow flexibility after installation (such as connecting to a vibrating motor, transformer, or reciprocating pump), an equipment bonding jumper must be installed, regardless of conduit length or circuit amperage! The 6-foot / 20A rule applies only to stationary drop installations (such as lighting whips).

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2. Identification of EGCs (NEC 250.119)

NEC 250.119 governs how equipment grounding conductors must be visually identified:

• Color Code: The exterior finish must be green, green with one or more yellow stripes, or bare.
• Conductors 6 AWG and Smaller: Must have continuous green insulation or green insulation with yellow stripes along their entire length (cannot be re-identified with tape in the field), unless bare.
• Conductors 4 AWG and Larger: Permitted to be permanently re-identified at each termination and at every point where the conductor is accessible. Permitted re-identification methods include:
  1. Stripping insulation from the entire exposed length.
  2. Painting the exposed insulation green.
  3. Marking the exposed insulation with green tape or green adhesive labels encircling the conductor.

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3. Sizing EGCs (NEC Table 250.122)

Equipment Grounding Conductors are sized according to NEC Table 250.122 based on the rating or setting of the upstream overcurrent protective device (fuse or circuit breaker) protecting the circuit.

Table 250.122: Minimum Size Equipment Grounding Conductors for Grounding Raceway and Equipment

[!IMPORTANT] Key Principle: Notice that a 30A, 40A, and 60A overcurrent device all require the exact same copper EGC size: 10 AWG copper. This is a favorite trick on trade exams.

4. The Proportional Up-Sizing Rule for Voltage Drop (NEC 250.122(B))

When electrical conductors run long distances, electricians deliberately increase the gauge of the ungrounded phase conductors to combat voltage drop. For instance, an electrician might upsize a 30-ampere branch circuit from 10 AWG copper to 4 AWG copper to keep voltage drop under 3% over a 300-foot run.

When this occurs, NEC 250.122(B) triggers a mandatory adjustment:

"Where ungrounded conductors are increased in size from the minimum size that has sufficient ampacity for the intended installation, wire-type equipment grounding conductors, where installed, shall be increased in size proportionately according to the circular mil area of the ungrounded conductors."

The Proportional Sizing Formula

To determine the new required EGC size, use the ratio of circular mil areas from NEC Chapter 9, Table 8:

$\text{New EGC cmil} = \text{Original Table 122 EGC cmil} \times \left( \frac{\text{Actual Upsized Phase cmil}}{\text{Minimum Required Phase cmil}} \right)$

Worked Example 1: 30-Ampere Circuit Upsized for Voltage Drop

Given: A 30-ampere, 240-volt single-phase branch circuit is protected by a 30-ampere circuit breaker. Due to a 250-foot run, the phase conductors are upsized from 10 AWG copper to 4 AWG copper. Determine the minimum size copper equipment grounding conductor required.

• Step 1: Determine Minimum Required Phase Conductor Area:
  • A 30A circuit requires minimum 10 AWG copper (rated 30A at 60°C/75°C).
  • Look up 10 AWG in NEC Chapter 9, Table 8: $\mathbf{10,380\text{ circular mils}}$.
• Step 2: Determine Actual Upsized Phase Conductor Area:
  • The actual phase conductors installed are 4 AWG copper.
  • Look up 4 AWG in Chapter 9, Table 8: $\mathbf{41,740\text{ circular mils}}$.
• Step 3: Determine Base Table 250.122 EGC Area:
  • For a 30A breaker, Table 250.122 specifies a 10 AWG copper EGC ($10,380\text{ cmil}$).
• Step 4: Calculate the Upsizing Ratio: $\text{Ratio} = \frac{41,740\text{ cmil}}{10,380\text{ cmil}} = 4.0212$
• Step 5: Calculate New Required EGC Circular Mil Area: $\text{New EGC Area} = 10,380\text{ cmil} \times 4.0212 = 41,740\text{ circular mils}$
• Step 6: Select the Conductor from Chapter 9, Table 8:
  • Under Chapter 9, Table 8, a conductor with at least $41,740\text{ cmil}$ is 4 AWG copper ($41,740\text{ cmil}$).
• Conclusion: The equipment grounding conductor must be upsized from 10 AWG to 4 AWG copper!

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5. Multiple Circuits in a Single Raceway (NEC 250.122(C))

When multiple branch circuits or feeders are routed through a single conduit or cable tray, the NEC does not require pulling a separate EGC for every individual circuit:

• The Rule: A single common equipment grounding conductor is permitted in the raceway.
• Sizing Requirement: The common EGC must be sized based on the largest overcurrent device protecting any circuit conductor contained within that raceway.

Worked Example 2: Common EGC Sizing

Given: A conduit contains three separate branch circuits:

• Circuit 1: 15-ampere, 120V circuit (14 AWG THHN)
• Circuit 2: 20-ampere, 120V circuit (12 AWG THHN)
• Circuit 3: 50-ampere, 240V circuit (8 AWG THHN)

Find the minimum size single copper EGC that can serve all three circuits.

• Identify the largest overcurrent device: 50 Amperes.
• Refer to NEC Table 250.122 under 60 Amperes (covers 31A to 60A).
• Table 250.122 specifies a 10 AWG copper conductor.
• Conclusion: A single 10 AWG copper EGC satisfies code for all three circuits.

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6. Bonding Receptacles to Metal Boxes (NEC 250.146)

Under NEC 250.146, an equipment bonding jumper must be used to connect the grounding terminal of a grounding-type receptacle to a grounded metallic box. However, four important exceptions eliminate the need for a separate bonding jumper:

1. Surface-Mounted Boxes (NEC 250.146(A)): Where the metal box is surface-mounted, direct metal-to-metal contact between the receptacle mounting yoke and the box is permitted to ground the receptacle, provided the cover plate is secured with machine screws and at least one fiber washer is removed.
2. Self-Grounding Receptacles (NEC 250.146(B)): Where flush-mounted boxes are installed, standard receptacles cannot rely on screw contact because drywall mud rings create loose contact. However, listed self-grounding receptacles incorporate a spring-brass contact clip on one mounting screw that maintains continuous spring tension against the box, eliminating the bonding jumper.
3. Floor Boxes (NEC 250.146(C)): Receptacles installed in listed floor boxes designed to provide satisfactory ground continuity.

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7. Isolated Ground (IG) Receptacles (NEC 250.146(D) & 406.3(D))

In commercial, medical, data center, and broadcast studio environments, high-frequency electrical "noise" (electromagnetic interference from motors, lighting ballasts, and variable frequency drives) circulates across conventional equipment grounding conductors and metal conduits.

To prevent this noise from corrupting digital data, audio tracks, or medical instrumentation, engineers specify Isolated Ground Receptacles:

+-------------------------------------------------------------------------+
|                   ISOLATED GROUND RECEPTACLE (IG)                       |
|                   (NEC 250.146(D) & NEC 406.3(D))                       |
|                                                                         |
|  - Identification: ORANGE TRIANGLE on face of receptacle               |
|  - Construction: Grounding pin insulated from metal mounting yoke       |
|  - Grounding Path: Dedicated insulated green wire with yellow stripe    |
|  - Terminus: May pass through selected enclosures without connection; terminates as 250.146(D) permits       |
|  - SAFETY MANDATE: The metallic box MUST still be grounded by conduit!  |
+-------------------------------------------------------------------------+

Key Technical Rules for Isolated Ground Installations:

1. Visual Identification: Must be identified by an orange triangle located on the face of the receptacle. The receptacle body itself may be orange or any standard color.
2. Physical Isolation: The grounding terminal on the receptacle is intentionally insulated from the metallic mounting strap (yoke). Fastening the receptacle to a metal box does not ground the receptacle contact.
3. Wiring Requirement: An insulated equipment grounding conductor (green with one or more yellow stripes, or plain green marked at terminations) must run from the receptacle grounding screw, passing through all subpanels without touching their neutral or ground bars, to an applicable equipment grounding terminal for the derived system or service as permitted by NEC 250.146(D).
4. Critical Safety Mandate: The metal outlet box housing an isolated ground receptacle MUST still be grounded! The metal conduit or a standard separate EGC must ground the box. The isolated ground conductor grounds only the sensitive equipment plugged into the receptacle.

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8. Common Exam Traps & Practical Review

[!WARNING] Common Exam Traps on Section 5.4:

1. Table 250.122 30A-60A Bracket: Candidates frequently assume that a 60A breaker requires an 8 AWG or 6 AWG copper EGC. Check Table 250.122 carefully: 30A, 40A, and 60A breakers all require 10 AWG copper.
2. Voltage Drop Upsizing is Mandatory: If an exam question mentions that phase conductors were increased in gauge for voltage drop or distance, you must calculate the circular mil ratio and upsize the EGC proportionately under NEC 250.122(B).
3. Isolated Ground Receptacle Box Grounding: An exam question might ask: "Does an isolated ground conductor satisfy the grounding requirement for the metal outlet box?" The answer is no. The metal box must be independently grounded by the raceway system or a conventional EGC.