7.4 Bonding Requirements & Main Bonding Jumpers

7.4 Bonding Requirements & Main Bonding Jumpers

Exam Focus: Grounding vs. bonding definitions, Main Bonding Jumper (MBJ) function (250.28), System Bonding Jumper (SBJ) for separately derived systems (250.30), sizing per Table 250.102(C)(1), the 12.5% rule for conductors > 1100 kcmil Cu, water/gas/steel bonding rules (250.104), and service equipment knockout bonding (250.92).

While grounding connects electrical circuits to the earth, bonding connects non-current-carrying metallic parts of electrical equipment, raceways, and building systems together. Bonding creates an electrically continuous low-impedance path back to the electrical source so that ground-fault currents can flow unimpeded to trip overcurrent devices instantly.

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

1. Main Bonding Jumper (NEC 250.28)

In a grounded service, the Main Bonding Jumper (MBJ) connects the grounded system conductor (neutral) to the service equipment enclosure and equipment grounding bus at each service disconnecting means.

• Function: Establishes the fault-return bridge between metallic equipment enclosures and the neutral conductor coming from the utility transformer.
• Permitted Materials (250.28(A)): Wire, busbar, screw (with green finish visible), or similar suitable conductor.
• Un-grounded Systems: On ungrounded systems, the MBJ bonds the service enclosure to the grounding electrode conductor.

                    MAIN BONDING JUMPER (MBJ) LOCATION
+-------------------------------------------------------------------------+
| UTILITY TRANSFORMER                                                     |
| Neutral Center Tap ---------------------------------+                   |
+-----------------------------------------------------|-------------------+
                                                      | Grounded Neutral
                                                      v
+-------------------------------------------------------------------------+
| SERVICE DISCONNECT ENCLOSURE                                            |
|   Neutral BusBar <=======[ MAIN BONDING JUMPER ]=======> Enclosure /   |
|        |                       (NEC 250.28)               Ground Bus    |
|        v                                                       |        |
|     GEC to Earth                                               v        |
|                                                            EGC to Panel |
+-------------------------------------------------------------------------+

2. System Bonding Jumper (NEC 250.30)

For a separately derived system (SDS)—such as a step-down transformer or generator—a System Bonding Jumper (SBJ) connects the grounded conductor of the derived system to the equipment grounding conductor and source enclosure. It must be installed at the source of the SDS or at the first disconnecting means, but NOT at both locations.

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Sizing MBJs, SBJs, and Supply-Side Jumpers (NEC Table 250.102(C)(1))

Main bonding jumpers, system bonding jumpers, and supply-side bonding jumpers are sized using NEC Table 250.102(C)(1) based on the area of the largest ungrounded supply conductor.

[!IMPORTANT] The 12.5% Rule for Large Services (Table 250.102(C)(1) Note 1): Where the ungrounded supply conductors exceed 1100 kcmil Copper or 1750 kcmil Aluminum, the bonding jumper shall have an area not less than 12.5% of the total area of the ungrounded supply conductors per phase.

Worked Calculation Example: 12.5% Rule Sizing

Scenario: A 2000A 480V service is supplied by four parallel runs of 500 kcmil copper conductors per phase (total $4 \times 500 = 2,000\text{ kcmil}$ copper per phase). What minimum size copper Main Bonding Jumper is required?

• Step 1: Check Total kcmil Area:
  • Total ungrounded conductor area per phase = 2,000 kcmil Cu.
  • 2,000 kcmil exceeds 1100 kcmil, so Table 250.102(C)(1) cap applies: 12.5% Rule.
• Step 2: Calculate Required Minimum cmil Area: $\text{Required MBJ cmil} = 2,000,000\text{ cmil} \times 0.125 = 250,000\text{ cmil} \text{ (250 kcmil)}$
• Step 3: Select Conductor Size:
  • 250 kcmil Copper minimum MBJ wire or equivalent busbar.

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Specific Building Bonding Requirements (NEC 250.104)

NEC 250.104 dictates bonding rules for interior metal piping and structural building frames:

                    BUILDING BONDING REQUIREMENTS SUMMARY
+-------------------------------------------------------------------------+
| INTERIOR METAL WATER PIPING (NEC 250.104(A))                             |
| - Bonded to service enclosure, GEC, or grounding electrode system.     |
| - Sized per Table 250.102(C)(1) based on service conductor size.        |
+-------------------------------------------------------------------------+
| METAL GAS PIPING (NEC 250.104(B))                                       |
| - Bonded to service equipment, GEC, or circuit EGC.                     |
| - Sized per Table 250.122 based on OCPD of circuit likely to energize it.|
+-------------------------------------------------------------------------+
| STRUCTURAL METAL FRAMING (NEC 250.104(C))                               |
| - Exposed structural steel likely to become energized must be bonded.   |
| - Sized per Table 250.102(C)(1) based on service conductor size.        |
+-------------------------------------------------------------------------+

1. Bonding Metal Water Piping (NEC 250.104(A))

• Interior metal water piping systems must be bonded to the service equipment enclosure, the grounded conductor at the service, the GEC, or one or more electrodes.
• Sizing: Sized per Table 250.102(C)(1) based on largest service conductor.

2. Bonding Other Metal Piping / Gas Piping (NEC 250.104(B))

• Metal gas piping and other metal piping systems likely to become energized must be bonded to the service enclosure, GEC, or grounded conductor.
• Sizing: Sized per Table 250.122 based on the rating of the circuit likely to energize the piping (e.g., gas furnace branch circuit, gas water heater circuit).
• Practical Note: The equipment grounding conductor (EGC) of the appliance branch circuit supplying the equipment (e.g. 12 AWG Cu for a 20A furnace circuit) is explicitly permitted to satisfy the gas pipe bonding requirement!

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Service Equipment & Knockout Bonding (NEC 250.92)

Service equipment raceways and enclosures require a much higher level of bonding integrity than standard branch circuit equipment. Why? Because service conductors have no line-side overcurrent protection ahead of them to trip during a fault!

                 SERVICE ENCLOSURE BONDING VS BRANCH CIRCUITS
+-------------------------------------------------------------------------+
| SERVICE RACEWAY TERMINATION (NEC 250.92)                                |
| - Standard locknuts or double locknuts ALONE ARE PROHIBITED!            |
| - MUST USE: Bonding bushings with bonding jumpers, threaded hubs, or    |
|   bonding locknuts with wedge teeth.                                    |
| - Concentric / Eccentric knockouts REQUIRE bonding jumpers around rings!|
+-------------------------------------------------------------------------+
| BRANCH CIRCUIT RACEWAY TERMINATION (< 250V to Ground)                   |
| - Standard locknuts on painted enclosures permitted.                    |
+-------------------------------------------------------------------------+

[!CAUTION] Knockout Exam Trap (NEC 250.92(B)): Where service raceways enter service enclosures through concentric or eccentric knockouts, standard locknuts cannot guarantee electrical continuity. A grounding bushing with a bonding jumper sized per Table 250.102(C)(1) must be installed around the knockout ring to ensure a low-impedance ground-fault return path.