4.3 Main Bonding Jumpers, System Bonding, and Equipment Bonding

The Main Bonding Jumper Is the Service Fault-Path Bridge

At a grounded service, the main bonding jumper connects the grounded service conductor to the service equipment enclosure and equipment grounding conductor system. That connection is central to fault clearing. If an ungrounded service or feeder conductor contacts a service cabinet, current returns through the cabinet, bonding path, main bonding jumper, grounded service conductor, and utility source. Without the bonding jumper, the cabinet might sit energized with too little current to open the overcurrent device.

The main bonding jumper is also why service equipment is treated differently from load-side panels. At the service disconnect, the grounded conductor and equipment grounding paths are connected. Downstream, they are generally separated so normal neutral current does not divide onto equipment grounding conductors, raceways, enclosures, piping, or structural steel. Many exam questions hinge on whether the panel is service equipment or a feeder-supplied panelboard.

System Bonding Jumpers

A separately derived system, such as a transformer secondary with no direct electrical connection to the primary circuit conductors other than grounding and bonding connections, needs its own grounding and bonding analysis. The system bonding jumper connects the derived system grounded conductor to the equipment grounding conductor system at a permitted point. Depending on the installation, the bonding point may be at the source, at the first disconnecting means, or within rules that coordinate the derived system grounding electrode conductor and supply-side bonding jumper.

Do not assume every transformer secondary must have a neutral bond in the transformer case and another in the first panel. Duplicate bonding points can place normal grounded-conductor current on metal raceways and equipment grounding conductors. The master electrician must inspect the transformer, secondary disconnect, panelboard neutral bars, bonding screws, bonding straps, and grounding electrode conductor termination as a coordinated system.

Equipment Bonding in the Field

Equipment bonding includes metal raceways, boxes, cabinets, cable trays, wireways, motor frames, disconnect enclosures, transformer cases, panelboards, and likely-to-be-energized metal parts. The Code recognizes many equipment grounding conductor types, including wire-type conductors and some metal raceways or cable assemblies when installed under their conditions. The phrase installed under their conditions matters. A raceway with loose locknuts, excessive corrosion, missing fittings, or nonlisted transitions may not provide the same path shown on the drawings.

Bonding bushings and bonding jumpers deserve special attention at services, around concentric and eccentric knockouts, and where raceways contain service conductors or conductors over certain voltage thresholds. Paint, powder coating, reducing washers, oversized holes, and flexible raceway sections can all create uncertainty. A plan may say EMT equipment grounding path, but the finished job may include flexible metal conduit to equipment. The master electrician must know when the flexible raceway qualifies and when an equipment grounding conductor must be installed.

Sizing Concepts

Main bonding jumpers and system bonding jumpers are sized by rules tied to the largest ungrounded service, feeder, or derived-system conductors, with special treatment for parallel conductors and large conductor sets. Equipment bonding jumpers on the load side are commonly sized from overcurrent device ratings. The reason is functional. Source-side bonding jumpers must carry the available fault current associated with large service or derived conductors until the supply-side protective device operates.

Load-side equipment grounding and bonding conductors are coordinated with the overcurrent device protecting that circuit.

The exam may ask for a conductor size, but the first step is naming the conductor. Is it a main bonding jumper, system bonding jumper, supply-side bonding jumper, equipment bonding jumper, equipment grounding conductor, or grounding electrode conductor? The same physical copper wire could look similar in a diagram, but the table or rule changes by function.

Code Navigation and Design Review

A reliable navigation sequence is: determine whether the equipment is service, feeder, branch circuit, or separately derived system; locate the grounded conductor; locate the first disconnect; locate the permitted bonding point; identify all metal enclosures and raceways in the fault path; then size any required bonding jumper. If a question includes a neutral bar bonded in a subpanel, mark it. If a transformer secondary has both XO bonded in the transformer and a neutral bond in the panel, mark it. If the raceway is PVC, remember that a wire-type equipment grounding conductor or other recognized path is needed.

In supervision, bonding is a quality-control topic. Check torque records for lugs, listings for grounding bushings, continuity of raceway systems, compatibility of aluminum and copper conductors, corrosion protection, and whether removable parts such as doors rely on hinges or separate bonding jumpers as required by the equipment design. A beautiful one-line diagram does not clear a fault. The installed metallic path, conductor terminations, and source bonding point clear the fault.

Exam Trap Examples

If the question says a feeder to a detached building includes an equipment grounding conductor and also has a neutral bonded to the detached building panel cabinet, the usual modern answer is that the neutral bond is wrong unless a specific exception applies. If the question says a service raceway uses concentric knockouts and ordinary locknuts, the issue may be bonding reliability at service equipment. If the question asks whether a ground rod at a machine allows omission of the equipment grounding conductor, the answer is no.

Keep returning to the fault loop: source to fault, fault to bonding path, bonding path to source.

Structured Decision Aid

• Locate the service disconnect or source of a separately derived system before placing the bonding jumper.
• Do not duplicate neutral-to-ground bonds downstream where the system requires separation.
• Size bonding jumpers and equipment bonding paths from the correct conductor or overcurrent basis.
• Trace the fault-current path back to the source to test whether the answer clears a fault effectively.