4.5 Bonding, Main Bonding Jumpers, and Effective Fault Path

Bonding clears faults

If an ungrounded conductor contacts a metal cabinet, the installation should produce enough fault current to open the overcurrent protective device quickly. That requires a low-impedance effective ground-fault current path. Bonding is how metal raceways, enclosures, equipment grounding conductors, service equipment, and the grounded conductor at the correct point become that path. Earth is not relied on as the effective fault path.

This is the most important conceptual split in Chapter 4. Grounding connects to earth. Bonding connects conductive parts together. The grounding electrode system may help stabilize voltage and dissipate surges, but a line-to-case fault in a panel is cleared by the metallic return path to the source, not by current wandering through soil.

Code-navigation table

Main bonding jumper

The main bonding jumper connects the grounded service conductor to the service disconnect enclosure and equipment grounding system. It may be a screw, bus, strap, wire, or other listed means. Its job is to connect the grounded conductor, service equipment enclosure, and equipment grounding conductors together at the service so a fault to the enclosure has a return path to the source.

At the service, this bond is required. On the load side of the service disconnect, repeating the bond is usually wrong. A downstream neutral-to-case bond puts normal neutral current on metal raceways, cable armor, equipment grounding conductors, and building steel. That can create shock hazards, objectionable current, nuisance current on metal parts, and interference with sensitive systems.

Sizing distinctions

Not every green or bare conductor is sized from the same table. Equipment grounding conductors are generally sized from the rating of the overcurrent device protecting the circuit. Grounding electrode conductors are sized from service conductor size. Main bonding jumpers and supply-side bonding jumpers have their own sizing rules. System bonding jumpers for separately derived systems also have specific requirements.

A useful exam translation is:

If a multiple-choice answer sizes a main bonding jumper from the feeder breaker table without checking the service bonding rule, be suspicious. If it sizes an equipment grounding conductor from the GEC table, be suspicious. The words are similar, but the rules are not interchangeable.

Effective ground-fault current path

An effective ground-fault current path must be electrically continuous, have capacity to carry the fault current likely to be imposed, and have low enough impedance to facilitate operation of the overcurrent protective device or ground-fault detector. Paint, loose locknuts, concentric knockouts without proper bonding, flexible raceways beyond their recognized limits, and corroded fittings can all weaken the path.

For service raceways, bonding is especially important because metal raceways containing service conductors are on the supply side of the service overcurrent device. Standard locknut contact may not be enough in some service conditions. Bonding bushings, bonding jumpers, or listed fittings may be required depending on raceway type, knockouts, voltage, and installation details.

Field cases

Case 1: A warehouse subpanel is fed from a 400 amp service switchboard. The installer lands feeder neutrals and equipment grounding conductors on the same bar and leaves the green bonding screw installed. The panel appears to work, but normal neutral current now divides onto the feeder raceway and grounding path. The fix is to isolate the neutral bar, bond the equipment grounding bar to the cabinet, and remove the improper neutral-to-case bond.

Case 2: A service mast uses metal raceway into a meter-main. The grounded conductor is bonded in the service equipment, but a concentric knockout and loose locknut leave the metal raceway with a questionable fault path. The field correction may involve listed bonding fittings or jumpers. On the exam, the answer will likely refer to bonding around impaired connections or ensuring an effective fault-current path.

Case 3: A detached garage feeder includes two ungrounded conductors, one neutral, and an equipment grounding conductor. The garage also has ground rods. The neutral must remain isolated in the garage panel. The rods are connected to the grounding electrode system at the garage, but they do not permit a neutral-to-case bond in the feeder panel.

Calculation setup

When a bonding sizing question appears, write down the conductor role before opening a table:

1. Is it a GEC, EGC, main bonding jumper, supply-side bonding jumper, or system bonding jumper?
2. Is it on the supply side or load side of the service overcurrent device?
3. Is it tied to service conductors, feeder conductors, or transformer secondary conductors?
4. Which table or percentage rule matches that role?
5. Does parallel conductor installation require special treatment?

That setup is faster than flipping through Article 250 by keyword. It also prevents one of the common journeyman exam misses: choosing the table you remember instead of the table the conductor role requires.

Exam traps

Neutral and ground are not synonyms. Grounded conductor means intentionally grounded circuit conductor, often the neutral. Grounding conductor can mean several things and must be read in context. Equipment grounding conductor is not the grounding electrode conductor. Main bonding jumper is not the same as system bonding jumper.

If a question asks what clears a ground fault, look for the effective ground-fault current path and the overcurrent protective device. If an answer says the earth carries enough current to trip the breaker, it is usually wrong. If a question asks where to bond the neutral, identify the service equipment or separately derived system bonding location before answering.