4.5 Equipment Grounding Conductors & Bonding Jumpers

Equipment Grounding Conductors & Bonding Jumpers

While the Grounding Electrode Conductor connects an electrical installation to earth, Equipment Grounding Conductors (EGCs) and Bonding Jumpers perform the critical life-safety function of clearing electrical faults. Without correctly sized and installed EGCs and bonding jumpers, an insulation breakdown will turn every metal conduit, junction box, panel cabinet, and appliance frame into an energized shock hazard. NEC Article 250 Parts V and VII dictate the approved types, sizing rules, raceway limitations, and bonding connections required to maintain an effective ground-fault current path.

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Permitted Equipment Grounding Conductors (NEC 250.118)

An Equipment Grounding Conductor is not limited to a green insulated wire. 2023 NEC 250.118(A) lists fourteen wiring methods and materials that may serve as an EGC, including:

1. Conductors: A copper, aluminum, or copper-clad aluminum conductor (bare, covered, or insulated).
2. Rigid Metal Conduit (RMC) and Intermediate Metal Conduit (IMC).
3. Electrical Metallic Tubing (EMT): Standard steel EMT is listed as an equipment grounding conductor in its own right when installed with listed fittings.
4. Type AC Armored Cable (BX): Armor of Type AC cable containing an internal bonding strip of copper or aluminum in direct contact with the armor.
5. Type MC Metal-Clad Cable: Listed Type MC cable with an interlocked armor and grounding conductor assembly identified for grounding.

Limitations on Flexible Metal Conduits (NEC 250.118(A)(5) & (A)(6))

Flexible conduits may only serve as the sole equipment grounding conductor under strictly limited conditions:

Flexible Metal Conduit (FMC) — NEC 250.118(A)(5)

FMC is permitted as an EGC only if all of the following conditions are met:

• The conduit terminates in listed fittings.
• The circuit conductors contained in the conduit are protected by overcurrent devices rated at 20 amperes or less.
• The conduit is trade size 1¼ or smaller.
• The combined length of FMC, FMT, and LFMC in the same ground-fault return path does not exceed 6 feet (1.8 m).
• The conduit is not installed to provide flexibility after installation (e.g., motor vibration or equipment positioning).

Liquidtight Flexible Metal Conduit (LFMC) — NEC 250.118(A)(6)

LFMC is permitted as an EGC only if all of the following conditions are met:

• The conduit terminates in listed fittings.
• For trade sizes $3/8\text{ inch}$ through $1/2\text{ inch}$, the conductors are protected by overcurrent devices rated at 20 amperes or less.
• For trade sizes $3/4\text{ inch}$ through $1-1/4\text{ inches}$, the conductors are protected by overcurrent devices rated at 60 amperes or less.
• The combined length of FMC, FMT, and LFMC in the same ground-fault return path does not exceed 6 feet (1.8 m).
• The conduit is not installed to provide flexibility after installation.

Exam Trap — Flexibility Equals Bonding Jumper: If FMC or LFMC is installed to allow for equipment vibration, motor movement, or thermal expansion (such as connecting to a commercial HVAC motor or transformer), it cannot serve as an EGC, regardless of length! An equipment bonding jumper (wire-type EGC) must be installed inside or outside the conduit per NEC 250.118(A)(5)(e), (A)(6)(e), and 250.102(E).

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Sizing Equipment Grounding Conductors: NEC Table 250.122

Unlike the GEC (which is sized from Table 250.66 based on conductor size), wire-type Equipment Grounding Conductors are sized from Table 250.122 based on the rating or setting of the overcurrent protective device (fuse or circuit breaker) protecting the circuit.

Table 250.122: Minimum Size Equipment Grounding Conductors

Multiple Circuits in One Raceway (NEC 250.122(C))

Where a single equipment grounding conductor is run in a raceway containing multiple circuits, a single common EGC is permitted. It must be sized based on the largest overcurrent device protecting any circuit in the raceway.

Parallel Conductors in Multiple Raceways (NEC 250.122(F))

Where circuit conductors are run in parallel in multiple raceways or cables (e.g., an 800A feeder split across two parallel conduits), a full-sized EGC based on the total 800A rating (1/0 AWG copper) must be installed in each raceway. Electricians cannot divide the EGC area across raceways!

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Proportional Conductor Upsizing Rule (NEC 250.122(B))

One of the most frequent calculation problems on the Nebraska Journeyman exam involves proportional EGC adjustment.

The Code Rule

Under NEC 250.122(B), where ungrounded circuit conductors are increased in size from the minimum size that has sufficient ampacity for the intended installation (most commonly to prevent excessive voltage drop on long distance runs), wire-type equipment grounding conductors must be increased in size proportionately according to the circular mil area of the ungrounded conductors.

The Proportional Formula

$\text{New EGC Area (cmil)} = \text{Standard EGC Area (cmil)} \times \frac{\text{Upsized Phase Area (cmil)}}{\text{Minimum Phase Area (cmil)}}$

Step-by-Step Journeyman Exam Calculation Example

• Scenario: A 30-ampere, 240-volt single-phase branch circuit supplies a remote air conditioning unit located 200 feet from the panel.
• Step 1: Identify Minimum Conductor Sizes:
  • A 30-A circuit uses minimum 10 AWG copper ungrounded conductors: 30 A in the 60°C column, and 240.4(D) also limits 10 AWG copper to 30-A protection.
  • Per Table 250.122, a 30A overcurrent device requires a minimum 10 AWG copper EGC.
• Step 2: Look up Circular Mil Areas in Chapter 9, Table 8:
  • Minimum ungrounded conductor (10 AWG) = 10,380 circular mils (cmil).
  • Standard EGC (10 AWG) = 10,380 circular mils (cmil).
• Step 3: Evaluate Upsized Conductor Size:
  • Due to voltage drop over the 200-foot run, the electrician upsizes the ungrounded phase conductors to 4 AWG copper.
  • From Chapter 9 Table 8, 4 AWG = 41,740 circular mils (cmil).
• Step 4: Calculate the Upsizing Ratio: $\text{Ratio} = \frac{41{,}740\text{ cmil}}{10{,}380\text{ cmil}} = 4.021$
• Step 5: Apply Ratio to Equipment Grounding Conductor: $\text{New EGC cmil} = 10{,}380\text{ cmil} \times 4.021 = 41{,}740\text{ cmil}$
• Step 6: Select Required EGC from Chapter 9 Table 8:
  • Searching Table 8 for a conductor area of at least 41,740 cmil yields 4 AWG copper (41,740 cmil).
  • Final Answer: The equipment grounding conductor must be upsized from 10 AWG to 4 AWG copper!

Exam Trap: Many candidates assume the EGC stays at 10 AWG because "the breaker is still only 30A." That violates 250.122(B). Conductors are usually upsized because the run is long. A minimum-size EGC on that same long run raises the impedance of the fault-return path, which lowers fault current and slows breaker tripping. Upsizing the EGC in proportion keeps the return path as effective as the supply path.

Main Bonding Jumper (MBJ) & System Bonding Jumper (SBJ)

The Main Bonding Jumper (MBJ) is the vital physical connection at the service equipment that connects the equipment grounding conductors and the service-disconnect enclosure to the grounded system conductor (neutral) per NEC 250.24(B) and 250.28.

               [Service Disconnect Enclosure]
      ┌──────────────────────────────────────────────┐
      │  Neutral Busbar (Grounded Conductor)         │
      │       │                                      │
      │       └─── [MAIN BONDING JUMPER (MBJ)] ──┐   │
      │            (Green Screw, Wire, or Strap) │   │
      │                                          ▼   │
      │  Equipment Ground Bus ──────────────── Enclosure Metal
      │       │
      │       └───> To Grounding Electrode System (GEC)
      └──────────────────────────────────────────────┘

Construction & Identification

• Material and Construction (NEC 250.28(A)–(B)): Copper, aluminum, copper-clad aluminum, or another corrosion-resistant material, in the form of a wire, bus, screw, or similar suitable conductor.
• Green Screw Identification (NEC 250.28(B)): Where a main bonding jumper is a screw, it must be identified with a green finish that is clearly visible with the screw installed.

Sizing the Main Bonding Jumper (NEC 250.28(D) & Table 250.102(C)(1))

The MBJ is sized from NEC Table 250.102(C)(1) based on the largest ungrounded service-entrance conductor (similar to Table 250.66 up to 1100 kcmil copper).

The 12.5% Rule for Large Services

Where the ungrounded service conductors exceed 1100 kcmil copper (or $1750\text{ kcmil}$ aluminum), the main bonding jumper must have an area not less than 12.5% of the area of the largest ungrounded phase conductor (or total sum of parallel conductors):

$\text{MBJ Area (cmil)} = \text{Total Phase Area (cmil)} \times 0.125$

• Calculation Example: A 3,000A service is supplied by six parallel 500 kcmil copper conductors per phase ($6 \times 500\text{ kcmil} = 3{,}000\text{ kcmil}$ total copper per phase). $\text{Required MBJ} = 3{,}000\text{ kcmil} \times 0.125 = 375\text{ kcmil}$ Looking up standard conductor sizes in Chapter 9 Table 8, the next larger standard conductor size is 400 kcmil copper.

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The Subpanel Neutral-Ground Separation Mandate

One of the most dangerous and widespread code violations found on jobsites is the failure to separate neutral and ground conductors in downstream subpanels.

The Code Rule (NEC 250.24(A)(5), 250.6 & 250.142(B))

• At Service Equipment Only: The neutral conductor is connected to ground at the main service equipment via the Main Bonding Jumper.
• On the Load Side of the Service Disconnect (Subpanels): A grounded conductor (neutral) shall not be connected to normally non-current-carrying metal parts of equipment or to equipment grounding conductors.

MAIN SERVICE PANEL (Bonded):          DOWNSTREAM SUBPANEL (Separated / Floating):  
┌───────────────────────────────┐     ┌───────────────────────────────┐            
│ [Neutral Bar] ──[MBJ]── [Encl]│     │ [Neutral Bar] (FLOATING / NO BOND SCREW)   
│      │                  │     │     │      │                        │            
│      │                  │     │     │      │ Isolated White Wires   │            
│      ▼                  ▼     │     │      ▼                        ▼            
│   Neutral              EGC    │     │   [EGC Bar] ─── BONDED TO METAL ENCLOSURE  
└──────┬──────────────────┬─────┘     └───────────────────────────────┘            
       │                  │                                                        
       │ 4-Wire Feeder    │ (Black, Red, White, Green/Bare)                        
       └──────────────────┘                                                        

In Every Subpanel, the Electrician Must:

1. Float the Neutral Bus: Remove the green bonding screw or disconnect the factory bonding strap that connects the neutral bus to the metal enclosure.
2. Install an Equipment Grounding Bar: Mount a separate equipment grounding busbar screwed directly to the metal enclosure to bond the cabinet.
3. Terminate Conductors Separately: Terminate all white neutral conductors exclusively on the isolated neutral bus; terminate all bare or green EGCs on the bonded grounding bus.
4. Provide a 4-Wire Feeder: Feed single-phase 120/240V subpanels with four conductors (two hots, one isolated neutral, and one equipment grounding conductor).

The Hazard of Downstream Neutral-Ground Bonding

When a bonding screw is mistakenly left in a subpanel, the neutral bus connects to the metal cabinet. Because the metal feeder conduit and the equipment grounding wire are also connected to that cabinet, the neutral and grounding paths end up wired in parallel!

1. Continuous Objectionable Circulating Current (NEC 250.6): Under normal operation, return load current leaves appliances and reaches the subpanel neutral bus. Instead of returning solely on the insulated neutral conductor, the current splits between the neutral wire and the equipment grounding conductors, metal conduits, building steel, and water pipes.
2. Shock Hazard on Enclosures: Current flowing over metallic conduits and enclosures creates continuous voltage drops. Anyone touching a subpanel cover or metal conduit while in contact with earth will receive an electrical shock.
3. Fire & Arcing Hazard: Where conduit fittings, locknuts, or pipe hangars are slightly loose, circulating neutral currents arc across metal joints, creating localized hot spots and structural fires.
4. Electromagnetic Interference (EMI): Splitting neutral current between conduits and conductors creates strong magnetic loops that cause severe hum in audio systems and corrupt computer network data.
5. Nuisance Tripping of GFCIs and Ground-Fault Equipment: Ground-fault protective devices sense the current imbalance and trip continuously or fail to trip during real ground faults.

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Main Panel vs. Subpanel Grounding & Bonding Summary