4.7 Services and Grounding Case Lab

Case-lab method

A journeyman exam question rarely says, Please apply Article 250. It describes a house, store, transformer, panel, meter, or raceway and expects you to find the Code path. The best way to handle these questions is to mark the electrical system like a one-line diagram. Label the source, service point, service conductors, service disconnect, overcurrent devices, grounded conductor, equipment grounding conductors, grounding electrode conductor, bonding jumpers, and downstream feeders.

Use the following method on every mixed service and grounding question:

1. Identify the source: utility, transformer, generator, inverter, or other system.
2. Identify whether the conductors are service, feeder, branch circuit, or secondary conductors.
3. Locate the first disconnect and overcurrent protection.
4. Mark the grounded conductor bond point.
5. Identify all metal enclosures and raceways that need bonding.
6. Identify all grounding electrodes that are present and qualifying.
7. Choose the Code article and table only after the roles are known.

Code-navigation table

Case 1: service upgrade at a dwelling

A one-family dwelling is upgraded from 100 amps to 200 amps. The utility overhead conductors terminate at the weatherhead, the service-entrance conductors run down the mast to a meter-main, and the meter-main contains the service disconnect. A feeder runs to an interior panel. The installer drives two ground rods, connects the GEC to the meter-main, and leaves the bonding screw installed in the interior panel.

The service point appears to be at the weatherhead splice if the utility and local service rules make that the premises boundary. The meter-main is service equipment. The interior panel is feeder-supplied equipment. The grounding electrode conductor belongs at the service equipment or permitted connection point, and the interior feeder panel must keep neutral conductors isolated from the cabinet and equipment grounding conductor. The bonding screw in the interior panel is the defect.

The exam answer would likely say remove the bonding screw or isolate the grounded conductor in the feeder panel. A tempting answer might say add another ground rod. That may not address the actual defect. Ground rods do not fix objectionable neutral current on the feeder equipment grounding path.

Case 2: small commercial service with water pipe

A small office has a 400 amp service disconnect inside nearest the service conductor entrance. The electrician installs a copper GEC to building steel and two rods. The building also has a metal underground water pipe in contact with earth for the required distance. The water pipe is not bonded because the installer says the rods are enough.

The missing step is to identify all present qualifying electrodes. If the metal underground water pipe qualifies as an electrode, it must be part of the grounding electrode system and is usually supplemented. Building steel and rods do not let the installer ignore it. If the pipe does not qualify as an electrode, interior metal water piping may still need bonding under a separate rule. The question wording will decide which issue is being tested.

Case 3: transformer-fed panel

A 480 volt feeder supplies a 30 kVA dry-type transformer. The transformer secondary supplies a 208Y/120 volt panel. The neutral is bonded in the transformer, a grounding electrode conductor connects to building steel, and the secondary panel has isolated neutral and bonded equipment grounding bars. The primary feeder includes an equipment grounding conductor.

This arrangement describes a separately derived system with the system bonding jumper at the transformer. The secondary panel is not service equipment and not the SDS bonding point, so its neutral stays isolated. The primary equipment grounding conductor bonds the transformer case back to the 480 volt source. The SDS grounding electrode conductor connects the derived system to the grounding electrode system. Each conductor has a different job.

Now change one fact: the secondary panel also has its bonding screw installed. That creates a second neutral-to-case bond and a parallel path for neutral current. The correction is not to remove the grounding electrode conductor. The correction is to keep one permitted system bonding point and isolate the neutral elsewhere.

Case 4: service calculation with equipment result

A tenant space calculation produces 182 amps at 120/240 volts single phase after all demand factors and continuous-load adjustments. The available answer choices are 150 amp, 175 amp, 200 amp, and 225 amp service disconnects. The service conductors are copper with 75 degree C terminations, and the question asks for minimum service disconnect rating.

Do not jump to conductor size. The calculated load exceeds 175 amps, so the next suitable standard rating in the choices is 200 amps, assuming the applicable standard-size and service rules permit it. If the question instead asked for conductor ampacity, you would navigate to conductor tables, terminal temperature limits, adjustment and correction factors, and any service conductor rules. Read the final sentence before choosing.

Case 5: blocked switchboard

A 1200 amp service switchboard is installed in a maintenance room. The electrical contractor completed the work correctly, but the owner later stores boxes and a rolling cart in front of it. A chilled-water line is also installed above the switchboard in the dedicated equipment space.

This is an Article 110 problem as much as a service problem. Working space must remain clear, and dedicated equipment space protects the equipment area from unrelated systems. On the exam, do not be distracted by the service ampacity unless the question asks for it. The defect is access and dedicated space.

Final exam workflow

For Chapter 4 questions, use this compact decision tree:

The exam is open book, but the ICC bulletin warns that candidates will not have time to look up every answer from scratch. Use the book to confirm rules, not to discover the subject for the first time. Your speed comes from recognizing the case type, naming each conductor correctly, and knowing which table or article comes next.