6.3 Main Bonding Jumpers & Equipment Grounding Conductor Sizing

Main Bonding Jumpers & Equipment Grounding Conductor Sizing

Calculation Alert: Equipment Grounding Conductor (EGC) sizing is one of the highest-yield topics on the Washington 01 examination. Examiners regularly target two specific calculation scenarios: (1) calculating proportional EGC increases when ungrounded conductors are upsized for voltage drop under NEC 250.122(B), and (2) selecting the proper EGC size in parallel raceway runs under NEC 250.122(F). Both require rigorous step-by-step mathematical precision.

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1. Main Bonding Jumpers (MBJ) at Service Equipment (NEC 250.28)

The Main Bonding Jumper (MBJ) is the unspliced electrical connection installed at each service disconnecting means that connects the grounded circuit conductor (the neutral) to the equipment grounding conductor and the service disconnect enclosure (NEC 250.24(B) and 250.28).

• Function: Without the MBJ, a ground fault occurring anywhere in the facility would have no low-impedance electrical path to return to the utility transformer winding. Fault current would stall, leaving metal panels energized at line voltage.
• Permitted Construction: Wire, bus, screw, or strap. If a screw is used as the MBJ, it must have a green finish that is clearly visible after installation (NEC 250.28(B)).
• Sizing Rule (NEC 250.28(D)): Sized from NEC Table 250.102(C)(1) based on the largest ungrounded service-entrance conductor (or sum of parallel conductors per phase).

The 12.5% Bonding Rule for Large Services

When the total cross-sectional area of the largest ungrounded phase conductor exceeds 1,100 kcmil copper or 1,750 kcmil aluminum, Table 250.102(C)(1) no longer provides a direct wire gauge. Under Note 1 to Table 250.102(C)(1) and NEC 250.102(C)(1):

$\text{Minimum MBJ Area} = \text{Total Phase Circular Mil Area} \times 12.5\%\ (0.125)$

+-----------------------------------------------------------------------------------------+
|                        WORKED EXAMPLE: THE 12.5% BONDING JUMPER RULE                    |
|                                                                                         |
|  Service Specification:                                                                 |
|  - 480Y/277V, 3-phase, 4-wire commercial service.                                       |
|  - Fed by four parallel 500 kcmil copper THHN conductors per phase.                     |
|                                                                                         |
|  Step 1: Calculate Total Ungrounded Phase Area                                          |
|  - Total area = 4 conductors × 500 kcmil = 2,000 kcmil per phase.                       |
|                                                                                         |
|  Step 2: Check Against the 1,100 kcmil Threshold                                        |
|  - 2,000 kcmil > 1,100 kcmil -> The 12.5% calculation rule is mandatory!                |
|                                                                                         |
|  Step 3: Calculate Required Bonding Jumper Area                                         |
|  - Required Area = 2,000 kcmil × 0.125 = 250 kcmil copper.                              |
|                                                                                         |
|  Step 4: Select Conductor Size                                                          |
|  - Result: The copper Main Bonding Jumper must be not smaller than 250 kcmil copper!    |
+-----------------------------------------------------------------------------------------+

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2. System Bonding Jumpers (SBJ) for Separately Derived Systems (NEC 250.30)

A separately derived system is an electrical wiring system whose power is derived from generator, transformer, or converter windings and that has no direct electrical connection (including a solidly connected neutral) to supply conductors originating in another system.

• Dry-Type Step-Down Transformers: A typical 480V primary to 208Y/120V secondary building transformer is the most common separately derived system in commercial and industrial facilities.
• The System Bonding Jumper (NEC 250.30(A)(1)): An unspliced SBJ must connect the secondary grounded conductor (X0 neutral) to the equipment grounding conductor and secondary metal enclosure.
• The Single-Point Location Mandate: The SBJ must be installed at either the source (inside the transformer enclosure) or at the first system disconnecting means or overcurrent device (such as the secondary main distribution panel), but not at both locations unless 250.30(A)(1) Exception No. 2 applies (both are permitted only when doing so creates no parallel path for grounded-conductor current). An SBJ at both points normally creates an illegal parallel loop for secondary neutral current.
• Sizing: The SBJ is sized according to NEC Table 250.102(C)(1) based on the size of the secondary ungrounded phase conductors.

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3. Bonding Other Metallic Building Systems (NEC 250.104)

To prevent dangerous potential differences across non-electrical metal systems during lightning surges or direct electrical contact, the NEC mandates structural bonding:

Important Distinction: Piping systems "likely to become energized" (such as metal gas piping supplying a gas furnace or water heater) are permitted to be bonded by the equipment grounding conductor of the branch circuit supplying the appliance (NEC 250.104(B)). A separate Table 250.102(C)(1) bonding jumper is not required for gas piping unless local utility or municipal amendments mandate it.

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4. Permitted Equipment Grounding Conductors (NEC 250.118)

An Equipment Grounding Conductor (EGC) is the conductive path that connects normally non-current-carrying metal parts of equipment back to the electrical source. Under NEC 250.118, the following wiring methods are recognized as equipment grounding conductors:

1. Wire-Type Conductors: Copper, aluminum, or copper-clad aluminum conductor, insulated, covered, or bare.
2. Rigid Metal Conduit (RMC) & Intermediate Metal Conduit (IMC): Listed threaded metal conduit provides an excellent, permanent EGC through its heavy steel walls.
3. Electrical Metallic Tubing (EMT): Listed EMT raceway with set-screw or compression steel/zinc fittings is fully recognized as an equipment grounding conductor without requiring an internal green ground wire (though engineer specifications frequently require an insulated copper wire-type EGC as standard practice).
4. Flexible Metal Conduit (FMC) & Liquidtight (LFMC) Limitations:
   • Permitted as the sole EGC only where total length in the ground return path does not exceed 6 feet (1.8 m);
   • Circuit overcurrent protection is rated at 20 amperes or less for FMC (or up to 60 amperes for 3/4-inch to 1-1/4-inch LFMC);
   • And fittings are listed for grounding. If flexibility is required after installation or if the circuit exceeds these limits, a wire-type EGC must be pulled inside or bonded outside.
5. Type MC Cable (250.118(10)): MC cable that contains an insulated or bare EGC; interlocked-armor MC whose armor plus a bare aluminum grounding/bonding conductor is listed and identified as an EGC; or smooth or corrugated tube MC listed and identified as an EGC.

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5. Sizing Equipment Grounding Conductors (NEC Table 250.122)

Wire-type Equipment Grounding Conductors are sized strictly according to the rating of the upstream overcurrent protective device (fuse or circuit breaker), NOT by the physical size or ampacity of the circuit conductors:

Notice: For 40A and 60A overcurrent devices, the required copper EGC is 10 AWG in both cases. For a 200A breaker, the required copper EGC is 6 AWG.

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

When ungrounded phase conductors are increased in size from the minimum size required for the circuit ampacity (such as upsizing conductors to prevent excessive voltage drop on long runs), the wire-type equipment grounding conductor must be increased in size proportionally according to the circular mil area of the ungrounded conductors:

$\text{Area Multiplier} = \frac{\text{Actual Circular Mil Area of Upsized Ungrounded Conductor}}{\text{Minimum Circular Mil Area Required for Circuit Ampacity}}$

$\text{Minimum Required EGC Area} = \text{Standard Table 250.122 EGC Area} \times \text{Area Multiplier}$

Why This Rule Is Mandatory

If you increase the size of the phase conductors, their resistance drops significantly. This lower resistance allows prospective fault current to escalate. If the return equipment grounding conductor is left small, its higher resistance creates an unbalanced impedance divider. Under fault conditions, a large portion of the voltage drops across the small EGC, driving the potential of the metal equipment enclosure to dangerously elevated voltages above earth before the breaker can clear.

+-----------------------------------------------------------------------------------------+
|                   WORKED CALCULATION: PROPORTIONAL EGC ADJUSTMENT (250.122(B))          |
|                                                                                         |
|  Scenario:                                                                              |
|  - A 100-ampere feeder is protected by a 100 A breaker.                                 |
|  - Minimum conductor for the load: #3 AWG copper (75 C ampacity = 100 A).               |
|  - For voltage drop, the phase conductors are upsized to 1/0 AWG copper.                |
|                                                                                         |
|  Step 1: Circular mil areas (NEC Chapter 9, Table 8)                                    |
|  - #3 AWG = 52,620 cmil (minimum size)    1/0 AWG = 105,600 cmil (installed size)       |
|  - Table 250.122 EGC for 100 A = #8 AWG copper = 16,510 cmil                            |
|                                                                                         |
|  Step 2: Ratio = 105,600 / 52,620 = 2.007                                               |
|                                                                                         |
|  Step 3: New EGC area = 16,510 x 2.007 = 33,135 cmil                                    |
|                                                                                         |
|  Step 4: Select from Table 8                                                            |
|  - #6 AWG = 26,240 cmil (too small)    #4 AWG = 41,740 cmil (large enough)              |
|                                                                                         |
|  Result: the equipment grounding conductor must be upsized from #8 AWG to #4 AWG.       |
+-----------------------------------------------------------------------------------------+

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7. Parallel Raceway Installations & EGC Sizing (NEC 250.122(F))

Where circuit conductors are run in parallel in multiple raceways or cables (pursuant to NEC 310.10(G)), the equipment grounding conductor sizing rules are governed strictly by NEC 250.122(F):

The "Full Size in Every Pipe" Rule (NEC 250.122(F)(1))

Each parallel raceway or cable assembly must contain a full-size Equipment Grounding Conductor sized for the full ampere rating of the upstream overcurrent protective device from Table 250.122.

• Classic Exam Trap: Candidates often attempt to split or divide the EGC size by the number of parallel raceways (for example, assuming an 800A circuit in two conduits can use two 400A-sized EGCs). This is strictly prohibited.
• Physical Reason: If a phase-to-ground fault occurs inside Conduit #1, the fault current does not divide evenly between the two conduits. Most of the fault current travels on the EGC inside Conduit #1, because the lowest-impedance return path is the one closest to the faulted phase conductor. If that EGC was downsized, it would instantly fuse and melt before the 800A breaker could clear, creating an catastrophic arc flash and fire inside the conduit.

                    PARALLEL RACEWAY EGC INSTALLATION (NEC 250.122(F))

 [ 800A Breaker ]
      |
      +==== Raceway 1: Phase Conductors + FULL-SIZE 1/0 AWG Cu EGC =====> [ 800A Panelboard ]
      |
      +==== Raceway 2: Phase Conductors + FULL-SIZE 1/0 AWG Cu EGC =====>

  * Both conduits must contain a complete 1/0 AWG copper EGC (Table 250.122 for 800A).
  * Splitting or downsizing the EGC across conduits is a dangerous code violation!