4.7 Grounding and Bonding Case Lab

Case Method

Grounding and bonding case questions often include too many details: service size, feeder raceway, transformer voltage, ground rods, water piping, panel neutral bars, pool equipment, and fault-current labels. The way through is to separate the facts into layers. First, identify each source. Second, identify each disconnect and overcurrent device. Third, mark grounded conductors and equipment grounding conductors. Fourth, mark grounding electrode conductors and electrodes. Fifth, draw one imagined fault and trace its return to the source.

This method prevents common exam mistakes. If you see a ground rod, you will not automatically call the equipment grounded. If you see continuity through a neutral bar, you will ask whether that neutral is permitted to bond to the enclosure at that location. If you see a transformer, you will ask whether the secondary is a separately derived system and where the system bonding jumper belongs.

Case 1: Service and Feeder Panel

A commercial building has a 120/208 volt service disconnect in a service room. The grounded service conductor is bonded to the service disconnect enclosure with a main bonding jumper. A feeder leaves the service equipment in PVC conduit to a distribution panel. The feeder includes phase conductors, an insulated neutral, and a copper equipment grounding conductor. In the distribution panel, the installer leaves the bonding screw installed in the neutral bar because the panel label includes one.

The service bonding is probably expected. The feeder panel bonding screw is the problem. Because the distribution panel is on the load side of the service disconnect, the neutral should be isolated from the cabinet in the ordinary arrangement. The equipment grounding conductor bonds the panel cabinet. If the neutral remains bonded, normal neutral current can return on both the neutral and the equipment grounding path.

The code-navigation step is: service equipment rules for the main bonding jumper, feeder rules for equipment grounding conductor, panelboard instructions for neutral isolation, and Article 250 rules on objectionable current and load-side grounding. The field correction is to remove the bonding screw or strap from the neutral bar, verify the equipment grounding bar bonds to the cabinet, and document terminations after torqueing.

Case 2: Transformer Secondary

A 480 volt feeder supplies a 75 kVA transformer with a 208Y/120 volt secondary. The secondary conductors run in rigid metal conduit to a nearby panel with a main breaker. XO is bonded in the transformer, and the grounding electrode conductor lands at the transformer. The secondary panel neutral bar is isolated. The raceway and any required supply-side bonding path connect the transformer and panel enclosures.

This can be a coherent arrangement if the system bonding jumper is at the transformer and the other conductors are installed under the applicable rules. A secondary line-to-case fault in the panel must return through the panel cabinet, raceway or equipment bonding path, transformer enclosure, system bonding jumper, XO point, and secondary winding. If the panel neutral bar were also bonded, the answer would change because duplicate bonding could place neutral current on the metal raceway.

The review questions are: Is the transformer secondary separately derived? Is the system grounded? Where is the system bonding jumper? Where is the grounding electrode conductor connected? Are secondary conductors protected according to their rules? Is the panel equipment rated for available short-circuit current? Are neutral and equipment grounding bars correctly separated or bonded according to that chosen point?

Case 3: Detached Pump House

A feeder supplies a detached pump house. The installer drives two rods at the pump house and bonds them to the panel cabinet. The feeder contains two ungrounded conductors and a neutral but no equipment grounding conductor. Metal water piping connects the pump house to the main building. The installer says the rods clear faults.

This is a red-flag case. Rods do not replace an equipment grounding conductor or other recognized equipment grounding path with the feeder. The metal water piping may create parallel paths and shock hazards, but it should not be treated as the intended neutral or equipment grounding conductor. In a modern exam setting, expect the feeder to include an equipment grounding conductor, the detached building electrodes to be bonded, and the neutral to be isolated from the equipment grounding system at the detached building unless a specific older exception is truly in play.

The supervisory response is not just fail. Identify the required feeder equipment grounding path, correct neutral isolation, bond the grounding electrode system at the detached structure, and evaluate the metal water piping bonding so it is not carrying normal neutral current. If the job is under an older adopted NEC or local amendment, verify the jurisdiction rule before accepting any exception.

Case 4: Pool Equipment Replacement

A service technician replaces a pool pump motor and connects the branch-circuit equipment grounding conductor. The existing bonding conductor to the motor bonding lug is left disconnected because the branch circuit has GFCI protection. This is a classic confusion between equipment grounding, GFCI, and equipotential bonding. The replacement motor still must be integrated with the pool bonding system when required by the pool article and equipment instructions.

The field judgment is to stop and verify the equipotential bonding network before leaving the installation. The bonding conductor should not be dismissed as redundant. The GFCI reduces shock risk by detecting imbalance and opening the circuit. The equipotential bonding system reduces voltage differences between conductive parts around the pool. They work together.

Exam Elimination Rules

When answer choices are close, eliminate any answer that says earth alone clears the fault. Eliminate answers that bond the neutral in every panel. Eliminate answers that ignore whether a generator neutral is switched. Eliminate answers that use a grounding electrode conductor as a branch-circuit equipment grounding conductor. Eliminate answers that solve pool bonding by installing only a rod.

Then choose the answer that preserves the source-return loop and the article-specific purpose. In service cases, look for the main bonding jumper and service raceway bonding. In feeder cases, look for equipment grounding conductor continuity and neutral isolation. In derived-system cases, look for one correct system bonding point. In special bonding cases, look for equipotential or metal-system bonding in addition to fault clearing.

Documentation and Leadership

A master electrician should leave a trail that another qualified person can inspect. Mark panel schedules and drawings with service versus feeder status. Label separately derived system bonding points. Record available fault current where required. Keep transformer and generator grounding details consistent between drawings, submittals, and field installation. During turnover, explain which bars are isolated, which are bonded, and where electrode conductors terminate.

The exam rewards the same discipline. Do not answer from memory of how a previous shop always did it. Answer from the described source, conductor functions, and Code navigation.

Structured Decision Aid

• Draw source, service disconnect, derived system, equipment, and electrode locations before sizing conductors.
• Mark each neutral-to-ground bond and confirm there is only one where the system design requires one.
• Use fault-current-path logic to check whether each metal enclosure can clear a fault.
• Record the final correction as a field directive, not just an article citation.