11.4 Main, System & Supply-Side Bonding Jumpers

11.4 Main, System & Supply-Side Bonding Jumpers

Bonding jumpers are the conductive linchpins that maintain electrical continuity across enclosures, disconnects, transformers, and raceway discontinuities. Under NEC Article 100, a bonding jumper is defined as "a reliable conductor to ensure the required electrical conductivity between metal parts required to be electrically connected." An open or undersized bonding jumper breaks the effective ground-fault current path, preventing overcurrent protective devices from opening during ground faults and leaving metal cabinets energized at lethal line voltages.

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The Fundamental Boundary: Supply Side vs. Load Side

Commercial electricians must recognize the critical code boundary between the supply side and the load side of an overcurrent protective device:

1. Supply Side (Ahead of OCPD): Upstream of the service disconnect or between a transformer secondary and its first disconnect, there is no downstream overcurrent device to limit fault duration. Ground faults on the supply side are cleared only by upstream utility primary cutouts or network protectors, which may take seconds to blow under fault currents reaching tens of thousands of amperes. Therefore, all supply-side bonding conductors (MBJ, SBJ, SSBJ) must be sized using NEC Table 250.102(C)(1).
2. Load Side (After OCPD): Downstream of an overcurrent device (branch circuits and feeders), any ground-fault current is cleared rapidly by that specific breaker or fuse. Therefore, load-side equipment bonding jumpers (EBJ) are sized using NEC Table 250.122 based on the ampere rating of that upstream OCPD.

                                THE BONDING JUMPER BOUNDARY

  [ UTILITY SOURCE / TRANSFORMER ]
                 |
                 | <--- SUPPLY SIDE (No Premises OCPD!)
                 |      Conductors Sized per Table 250.102(C)(1)
                 v
     [ Supply-Side Bonding Jumper (SSBJ) ]
                 |
                 v
  [ MAIN SERVICE DISCONNECTING MEANS ]
     [ Main Bonding Jumper (MBJ) ] ===> Bonds Neutral to Enclosure & Ground Bus
                 |
    =========================== [ SERVICE OVERCURRENT PROTECTIVE DEVICE ]
                 |
                 | <--- LOAD SIDE (Protected by Premises OCPD)
                 |      Conductors Sized per Table 250.122
                 v
     [ Equipment Bonding Jumper (EBJ) ]

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Main Bonding Jumper (MBJ, NEC 250.28)

The Main Bonding Jumper (MBJ) is the single most critical electrical safety link in any building electrical service. Installed at the service disconnecting means, the MBJ completes the ground-fault current loop by bonding the grounded service conductor (neutral) to the equipment grounding conductor and the service disconnect enclosure.

Construction and Material Requirements (NEC 250.28(A))

The MBJ must be constructed of copper, aluminum, copper-clad aluminum, or other corrosion-resistant material. It may take the form of:

• A wire conductor.
• A flat metal busbar.
• A specialized bonding screw (standardly provided by panel manufacturers with a distinct green finish that must be clearly visible after installation).

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

The MBJ is sized based on the size of the largest ungrounded service-entrance conductor or the sum of the circular mil areas of parallel conductors per phase:

The 12.5% Rule for Large Services

When the total cross-sectional area of the ungrounded phase conductors exceeds 1100 kcmil copper (or 1750 kcmil aluminum), Table 250.102(C)(1) no longer provides a fixed conductor size. Instead, Note 1 to Table 250.102(C)(1) mandates that the bonding jumper must have an area of not less than 12.5% of the total cross-sectional area of the largest ungrounded phase conductor (or equivalent area for parallel sets):

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

Worked Example: 2000A Commercial Service

A commercial service entrance is supplied by five parallel conduits per phase, with each conduit containing a 500 kcmil copper ungrounded conductor:

1. Calculate Total Phase Cross-Sectional Area: $\text{Total Area} = 5 \times 500\text{ kcmil} = 2{,}500\text{ kcmil} = 2{,}500{,}000\text{ circular mils}$
2. Apply the 12.5% Rule: $\text{Required MBJ Area} = 2{,}500{,}000\text{ cmil} \times 0.125 = 312{,}500\text{ circular mils}$
3. Select Standard Conductor Size from NEC Chapter 9, Table 8:
   • 250 kcmil = $250{,}000\text{ cmil}$ (Too small!)
   • 300 kcmil = $300{,}000\text{ cmil}$ (Too small!)
   • 350 kcmil = $350{,}000\text{ cmil}$ (Exceeds $312{,}500\text{ cmil}$ requirement)

Therefore, the single Main Bonding Jumper must be at least 350 kcmil copper!

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Separately Derived Systems & System Bonding Jumpers (SBJ, NEC 250.30)

Under NEC Article 100, a Separately Derived System (SDS) is a premises wiring system whose power is derived from a generator, transformer, or converter windings and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.

The most common commercial SDS is a 480V Delta primary to 208Y/120V Wye secondary dry-type step-down transformer. The secondary neutral terminal (X0) is completely isolated electrically from the 480V utility supply. To establish a safe reference to ground and create a fault-clearing return path, a System Bonding Jumper (SBJ) must be installed.

               SEPARATELY DERIVED SYSTEM (TRANSFORMER GROUNDING)
480V Delta Primary                                    208Y/120V Wye Secondary
(Ungrounded Supply)                                   (Separately Derived System)
===================                                   ===========================
Phase A -------------------+                     +--- Phase A (120V to N)
                           )                     (
Phase B -------------------+                     +--- Phase B (120V to N)
                           )                     (
Phase C -------------------+                     +--- Phase C (120V to N)
                                                 (
                                                 +--- Grounded Neutral Terminal (X0)
                                                      |
                                        +-------------+
                                        | (SYSTEM BONDING JUMPER - SBJ)
                                        v (Sized per Table 250.102(C)(1))
                 [ Transformer Metal Enclosure ]
                                        |
                                        v (GEC Sized per Table 250.66)
                  [ Grounding Electrode: Building Steel ]

Where is the SBJ Installed? (NEC 250.30(A)(1))

The code permits the SBJ to be installed at any single point from the secondary source to the first disconnecting means:

• Option 1 (Inside the Transformer): Bond terminal X0 directly to the transformer enclosure using an SBJ wire or busbar.
• Option 2 (Inside the First Panelboard Disconnect): Route an insulated neutral conductor from X0 to the panelboard neutral bus, and install the SBJ between the neutral bus and the panelboard enclosure.

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

A Supply-Side Bonding Jumper (SSBJ) is installed on the supply side of service equipment (e.g., between a utility metering trough and the service disconnect switch, or between an outdoor transformer and an indoor service switchboard) or between a separately derived system transformer and its first downstream disconnect.

• Sizing Rule: Sized from NEC Table 250.102(C)(1).
• Parallel Raceways (NEC 250.102(C)(2)): If ungrounded supply conductors are routed in parallel raceways, the SSBJ may be routed in parallel inside each raceway. The SSBJ in each individual conduit is sized based on the size of the ungrounded supply conductors within that specific raceway (not the sum of the entire parallel set!).

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Bonding Around Knockouts for Circuits Over 250V to Ground (NEC 250.97)

Commercial facilities commonly utilize 480Y/277V systems, where the voltage from any phase to ground is 277 volts (which exceeds 250 volts to ground). When metal conduit enters a panelboard or pull box containing pre-punched concentric or eccentric knockouts, electrical safety is severely threatened:

• The Hazard: Concentric knockouts consist of punched concentric metal rings joined by tiny metal tabs. Under a high-voltage ground fault (277V or 480V), high fault current arcing across the loose tabs will melt the metal, breaking electrical continuity before the breaker can clear.
• NEC 250.97 Mandate: For circuits operating at over 250 volts to ground, the electrical continuity of metal raceways containing concentric or eccentric knockouts must be ensured by one of the following methods:
  1. Threaded hubs or listed bosses.
  2. Bonding-type locknuts or listed grounding bushings equipped with bonding jumpers sized per Table 250.102(C)(1) or Table 250.122.
  3. Standard locknuts are permitted only if the box or enclosure is specifically listed and tested for grounding over 250V without bonding jumpers.

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Bonding of Building Piping Systems & Structural Steel (NEC 250.104)

All conductive metallic systems within a building must be bonded into the grounding system to prevent them from becoming energized by accidental contact with electrical wiring:

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

• Must be bonded to the service equipment enclosure, grounded conductor at the service, GEC, or grounding electrode.
• Sized per NEC Table 250.102(C)(1) based on service-entrance conductor size.
• The bonding jumper must bridge around water meters, filtration canisters, and dielectric unions.

2. Structural Metal Building Framing (NEC 250.104(C))

• Exposed structural metal building framing that is not intentionally grounded must be bonded to the service equipment enclosure, grounded neutral conductor, GEC, or electrode.
• Sized per NEC Table 250.102(C)(1) based on service conductor area.

3. Other Metal Piping Systems (NEC 250.104(B))

• Metal gas piping, compressed air piping, and chemical piping likely to become energized must be bonded.
• May be bonded using the Equipment Grounding Conductor (EGC) of the specific branch circuit that supplies the appliance connected to the piping (e.g., the EGC of the 120V circuit powering a gas furnace satisfies the bonding requirement for the gas pipe).