8.1 Main & System Bonding Jumpers

Main & System Bonding Jumpers

In the National Electrical Code (NEC), grounding and bonding represent two fundamentally different concepts with distinct safety objectives. Grounding is the intentional connection of an electrical system or equipment to the earth (ground) to stabilize phase-to-ground voltages against lightning strikes, line surges, and unintentional contact with higher-voltage lines. In contrast, bonding is the mechanical and electrical joining of metallic parts to establish an electrically continuous, permanent, low-impedance path back to the electrical source. For journeyman electricians in Kentucky, understanding the Main Bonding Jumper (MBJ), Supply-Side Bonding Jumper (SSBJ), and System Bonding Jumper (SBJ) is essential not only for passing the licensing examination but for preventing catastrophic fires and fatal electrical shocks.

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1. Why the Main Bonding Jumper Matters

Section 7.1 showed why the earth cannot clear a ground fault: a 120-volt fault through a 25-ohm ground rod produces less than 5 amperes, far too little to trip a breaker. The main bonding jumper is the link that lets fault current on equipment grounding conductors, raceways, and enclosures cross over to the grounded service conductor and return to the transformer. With that low-impedance loop complete, fault current reaches hundreds or thousands of amperes and the overcurrent device opens almost instantly. Every jumper in this section (main, system, and supply-side) exists to build that effective ground-fault current path back to the source (NEC 250.4(A)(5)). The differences between them are where they are installed and which table sizes them.

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2. Main Bonding Jumper (MBJ) Requirements (NEC 250.24(C) & 250.28)

Under NEC 250.24(C) (2023 numbering), an unspliced main bonding jumper must be installed within the enclosure for each service disconnecting means to connect the equipment grounding conductor(s) and the service-disconnect enclosure to the grounded conductor (neutral).

Construction and Fastening (NEC 250.28)

NEC 250.28 establishes rigid physical standards for main and system bonding jumpers:

1. Material (250.28(A)): Must be copper, aluminum, copper-clad aluminum, or other corrosion-resistant material. If of wire, it may be insulated, covered, or bare.
2. Construction (250.28(B)): May be a wire, busbar, screw, or similar suitable conductor.
   • The Green Bonding Screw: In residential and light-commercial loadcenters, manufacturers supply an identifiable green bonding screw. Under NEC 250.28(B), a screw used as a bonding jumper must be identified with a green finish that is visible with the screw installed.
3. Attachment Means (NEC 250.8 & 250.28(C)): Bonding jumpers must be secured using listed pressure connectors, terminal bars, exothermic welding, machine screws engaging not less than two threads or secured with a nut, or thread-forming machine screws engaging not less than two threads in the metal enclosure.
   • PROHIBITION: Under NEC 250.8(A), sheet metal screws (drywall screws, tek screws, or self-tapping sheet metal screws) shall not be used to connect bonding jumpers or grounding conductors to enclosures.

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3. Sizing MBJ & Supply-Side Bonding Jumpers (NEC Table 250.102(C)(1))

The sizing of Main Bonding Jumpers (MBJ) and Supply-Side Bonding Jumpers (SSBJ) is governed exclusively by NEC 250.28(D), 250.102(C), and NEC Table 250.102(C)(1). Sizing is determined strictly by the size of the largest ungrounded service-entrance conductor or the sum of the circular mil areas of the conductors if installed in parallel.

NEC Table 250.102(C)(1): Grounded Conductor, Main Bonding Jumper, and Supply-Side Bonding Jumper Sizing

Critical Exam Distinction: Do NOT confuse Table 250.102(C)(1) with Table 250.66 (Grounding Electrode Conductor) or Table 250.122 (Equipment Grounding Conductor). Table 250.102(C)(1) applies to conductors on the supply side of the service overcurrent device where fault currents are determined by utility transformer impedance, not premises breakers.

The 12.5% Rule for Large Conductors (NEC 250.102(C)(1) Note 1)

Where the ungrounded service-entrance conductors exceed 1100 kcmil copper or 1750 kcmil aluminum, the bonding jumper cannot simply be selected from the table rows; it must be calculated using the 12.5 percent rule:

$\text{Area of Bonding Jumper (kcmil)} = \text{Total Area of Phase Conductors (kcmil)} \times 0.125$

If the ungrounded supply conductors are installed in parallel in two or more raceways or cables, the sum of the circular mil areas of the largest ungrounded conductors in each parallel phase set is used.

Worked Example: 12.5% Sizing Calculation

Scenario: A 480Y/277V, 3-phase commercial service entrance is supplied by four parallel sets of $500\text{ kcmil}$ THHN copper conductors per phase in four separate conduits.

1. Calculate the total circular mil area for one phase: $\text{Total Area} = 4 \times 500\text{ kcmil} = 2{,}000\text{ kcmil}$
2. Compare against threshold: $2{,}000\text{ kcmil} > 1100\text{ kcmil}$ copper, so the 12.5% rule applies.
3. Calculate minimum bonding jumper circular mil area: $\text{Required Area} = 2{,}000\text{ kcmil} \times 0.125 = 250\text{ kcmil}$
4. Lookup standard conductor size: Under NEC Chapter 9, Table 8, a $250\text{ kcmil}$ copper conductor matches this exact value.

• Conclusion: The Main Bonding Jumper must be not smaller than 250 kcmil copper.

Supply-Side Bonding Jumpers (SSBJ, NEC 250.102(C))

A Supply-Side Bonding Jumper is an conductor installed on the supply side of a service or separately derived system to ensure electrical conductivity between metal parts of the service equipment. Common locations include bonding an external meter socket enclosure to the service disconnect enclosure, or bonding metal service raceways.

• Single Raceway / Common SSBJ: Sized per Table 250.102(C)(1) based on the total area of the ungrounded service conductors.
• Parallel Raceways (NEC 250.102(C)(2)): Where the supply conductors are paralleled in two or more raceways and an individual SSBJ is run with each raceway, each SSBJ is sized from Table 250.102(C)(1) using the ungrounded conductors in that raceway. A single SSBJ that bonds two or more raceways is sized from the total area of the ungrounded conductors it serves.

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4. System Bonding Jumpers for Separately Derived Systems (NEC 250.30(A)(1))

Under NEC Article 100, a Separately Derived System (SDS) is defined as:

Separately Derived System: An electrical source, other than a service, having no direct connection(s) to circuit conductors of any other electrical source other than those established by grounding and bonding connections.

The most common SDS encountered on the Kentucky Journeyman exam is a step-down dry-type transformer (e.g., $480\text{ V}$ 3-phase delta primary to $208\text{Y}/120\text{ V}$ 3-phase wye secondary). Because the transformer secondary windings create an entirely new electrical system with no direct metallic connection to the primary neutral, a System Bonding Jumper (SBJ) is mandatory to establish the ground-fault return path for the secondary circuits.

+-----------------------------------------------------------------------------------+
|              SEPARATELY DERIVED SYSTEM (TRANSFORMER) BONDING OPTIONS              |
+-----------------------------------------------------------------------------------+
|  OPTION 1: Bond at Source (Transformer Enclosure) - MOST COMMON                   |
|  - SBJ connects secondary neutral (X0) to transformer ground bar/enclosure.       |
|  - GEC connects secondary neutral (X0) to building grounding electrode system.    |
|  - Secondary: 3 hots + neutral + supply-side bonding jumper to first disconnect.  |
|  - Disconnect neutral bar is ISOLATED (no bonding screw installed).               |
+-----------------------------------------------------------------------------------+
|  OPTION 2: Bond at First Disconnecting Means Enclosure                            |
|  - 4-wire feeder (3 hots + neutral) runs from transformer to disconnect.          |
|  - SBJ connects secondary neutral to disconnect enclosure ground bar.             |
|  - GEC terminates at disconnect neutral bar.                                      |
|  - Transformer enclosure bonded via supply-side bonding jumper.                   |
+-----------------------------------------------------------------------------------+
|  STRICT PROHIBITION (NEC 250.30(A)(1)):                                           |
|  NEVER install an SBJ at BOTH the transformer AND the first disconnect!          |
|  Dual bonding creates objectionable circulating neutral current on the EGC.       |
+-----------------------------------------------------------------------------------+

Single-Point Connection Rule (NEC 250.30(A)(1))

NEC 250.30(A)(1) mandates that an unspliced system bonding jumper shall be installed at only one point on a separately derived system:

1. At the source of the separately derived system (inside the transformer enclosure at the X0 terminal), OR
2. At the first system disconnecting means or overcurrent device.

Installing bonding jumpers at both the transformer and the first disconnect panelboard places the metallic equipment grounding conductor in parallel with the secondary neutral conductor, resulting in dangerous circulating neutral current under normal operating conditions.

Sizing the System Bonding Jumper

Under NEC 250.28(D) and 250.30(A)(1), the System Bonding Jumper is sized using NEC Table 250.102(C)(1) based on the circular mil area of the secondary ungrounded phase conductors derived from the transformer.

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5. Strict Prohibition Against Load-Side Neutral-to-Ground Bonds (NEC 250.24(A)(5) & 250.142(B))

One of the most heavily tested life-safety mandates in Article 250 is the absolute prohibition against bonding the grounded neutral conductor to the equipment grounding conductor or metal enclosures anywhere on the load side of the service disconnecting means.

NEC 250.24(A)(5): "A grounded conductor shall not be connected to normally non-current-carrying metal parts of equipment, to equipment grounding conductors, or be connected to earth on the load side of the service disconnecting means except as otherwise permitted in this article."

NEC 250.142(B): "A grounded circuit conductor shall not be used for grounding non-current-carrying metal parts of equipment on the load side of the service disconnecting means..."

The Dangers of Objectionable Neutral Current (NEC 250.6)

When an electrician improperly installs a bonding screw or strap in a downstream subpanel (connecting the neutral bar to the subpanel metal enclosure), two parallel paths are created for normal load return current back to the main service panel:

1. The insulated neutral conductor.
2. The equipment grounding conductor, metal raceway (EMT/RMC), metallic cable armor, and any bonded building metal (water pipes, structural steel).

+-----------------------------------------------------------------------------------+
|                    THE MECHANICS OF OBJECTIONABLE NEUTRAL CURRENT                 |
|                                                                                   |
|  MAIN SERVICE PANEL (Bonded)                      SUBPANEL (ILLEGALLY BONDED)     |
|  +-------------------------+                      +-------------------------+     |
|  | Neutral Bar             | <==================> | Neutral Bar (Bonded!)   |     |
|  |    ^     (Main Bonding  |      Neutral (60%)   |    ^                    |     |
|  |    |      Jumper)       |                      |    |   Normal 120V Load |     |
|  |    v                    |   Metal Conduit /EGC |    |   Returns Here     |     |
|  | Ground Bar / Enclosure  | <------------------> | Ground Bar / Enclosure  |     |
|  +-------------------------+      Current (40%)   +-------------------------+     |
|                                                                                   |
|  HAZARDS CREATED:                                                                 |
|  1. Continuous circulating current across conduit couplings causes arcing/fire.   |
|  2. Breaking a conduit or water pipe exposes worker to full return voltage (SHOCK)|
|  3. Nuisance tripping of downstream GFCI and AFCI breakers.                       |
|  4. Electromagnetic interference (EMI) disrupting electronic networks.           |
+-----------------------------------------------------------------------------------+

Severe Consequences of Downstream Neutral-to-Ground Bonds:

1. Continuous Current on Metal Enclosures: Under Kirchhoff's Current Law, current divides inversely proportional to impedance. A significant percentage (often 30% to 50%) of normal neutral load current flows over metallic conduit, panel cabinets, and appliance metal enclosures.
2. Shock Hazard During Maintenance: If a plumber or electrician separates a metal water pipe or disconnects a metallic conduit fitting carrying this return current, their body can bridge the gap, placing them directly in series with the return current and resulting in severe shock or ventricular fibrillation.
3. Fire Hazard at High-Resistance Couplings: Loose set-screw fittings on EMT raceways carrying neutral return current can generate localized resistive heating and arcing, igniting adjacent combustible building framing.
4. Nuisance Tripping of GFCIs and AFCIs: Modern GFCI and AFCI circuit breakers monitor the balance between ungrounded and neutral currents. Circulating ground-loop currents cause immediate unbalance, resulting in erratic nuisance tripping.
5. Electromagnetic Field Interference (EMI): In a healthy circuit, the equal and opposite currents in hot and neutral conductors cancel their magnetic fields. When neutral current takes an alternate path through conduit or building steel, net magnetic field cancellation is lost, generating severe 60 Hz hum and inductive data errors in computer network cabling.

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6. Summary Comparison: MBJ vs. SSBJ vs. SBJ