PC - Professional Electrical Contractor Grounding and Bonding Questions and Answers 1 — Questions and Answers
Question 1: An electrical service is supplied by 3/0 AWG copper ungrounded conductors. The grounding electrode system consists solely of two 8-foot ground rods spaced 10 feet apart. According to NEC Article 250, what is the minimum size copper grounding electrode conductor (GEC) required for this installation?
- 4 AWG
- 2 AWG
- 6 AWG (Correct answer)
- 1/0 AWG
Correct answer: 6 AWG
According to NEC 250.66(A), if a grounding electrode conductor is connected only to rod, pipe, or plate electrodes, the conductor is not required to be larger than 6 AWG copper. Even though Table 250.66 would otherwise require a 4 AWG copper conductor based on the 3/0 AWG service conductors, the specific rule for ground rods limits the required size.
Question 2: A 400A feeder circuit's ungrounded conductors are increased in size from 500 kcmil copper to 750 kcmil copper to account for voltage drop. Per NEC Table 250.122, the minimum size copper equipment grounding conductor (EGC) for a 400A circuit is 3 AWG. What is the minimum size copper EGC required for this modified circuit?
- 3 AWG
- 1/0 AWG (Correct answer)
- 2 AWG
- 1 AWG
Correct answer: 1/0 AWG
According to NEC 250.122(B), when ungrounded conductors are increased in size from the minimum size that has sufficient ampacity for the intended installation, the equipment grounding conductor must be increased in size proportionately. The circular mil area of 500 kcmil is 500,000 and 750 kcmil is 750,000, a 1.5 ratio. The circular mil area of a 3 AWG conductor (52,620) must be multiplied by 1.5, resulting in 78,930 circular mils. The next standard size conductor with at least this circular mil area is 1/0 AWG.
Question 3: Which of the following best describes the primary purpose of bonding in an electrical system?
- To stabilize the system voltage to ground during normal operation.
- To provide a direct connection to the earth for lightning protection.
- To limit voltage surges from the utility.
- To provide a low-impedance path for fault current to facilitate the operation of overcurrent devices. (Correct answer)
Correct answer: To provide a low-impedance path for fault current to facilitate the operation of overcurrent devices.
The primary purpose of bonding is to ensure electrical continuity and create a low-impedance, effective ground-fault current path. This path allows a large enough current to flow during a ground fault to quickly trip the circuit breaker or blow the fuse, clearing the fault. Grounding to earth stabilizes voltage, but bonding ensures the fault-clearing mechanism works correctly.
Question 4: A contractor installs a single ground rod as the grounding electrode for a new 200A service. A resistance measurement to earth shows a value of 30 ohms. According to the NEC, what is the appropriate next step?
- The installation is compliant as only one electrode is required.
- Drive the single rod deeper until it reaches 25 ohms or less.
- Install a second grounding electrode and bond it to the first. (Correct answer)
- Replace the ground rod with a concrete-encased electrode.
Correct answer: Install a second grounding electrode and bond it to the first.
Per NEC 250.53(A)(2), if a single rod, pipe, or plate electrode has a resistance to earth of 25 ohms or greater, it must be supplemented by an additional electrode of a type specified in 250.52. The supplemental electrode must be installed no closer than 6 feet from the first. When a supplemental electrode is installed, the 25-ohm requirement is waived.
Question 5: In a commercial building, a metal underground water pipe is used as the primary grounding electrode. Which of the following statements is correct regarding this installation?
- The metal water pipe is sufficient and no other electrodes are needed.
- A supplemental grounding electrode is required. (Correct answer)
- The grounding electrode conductor must be bonded to the pipe within 10 feet of where it enters the building.
- The water pipe must be in direct contact with the earth for at least 8 feet.
Correct answer: A supplemental grounding electrode is required.
According to NEC 250.53(D)(2), a metal underground water pipe electrode must be supplemented by an additional electrode. This is because the metal water pipe is subject to damage or replacement (e.g., with plastic pipe), which would result in the loss of the grounding electrode for the system.
Question 6: What is the primary function of the Main Bonding Jumper (MBJ) in a service panel?
- To connect the grounding electrode conductor to the panel enclosure.
- To connect the equipment grounding conductors to the neutral bar.
- To bond the service neutral conductor and the equipment grounding conductor(s) to the service disconnect enclosure. (Correct answer)
- To provide a terminal for connecting bonding jumpers from other systems like telephone and CATV.
Correct answer: To bond the service neutral conductor and the equipment grounding conductor(s) to the service disconnect enclosure.
The Main Bonding Jumper (MBJ), as defined in NEC Article 100 and required by 250.24(C) and 250.28, provides the critical connection between the grounded conductor (service neutral), the equipment grounding conductors, and the service disconnect enclosure. This connection creates the effective ground-fault current path back to the source.
PC - Professional Electrical Contractor Grounding and Bonding Questions and Answers 1
An electrical service is supplied by 3/0 AWG copper ungrounded conductors.
The grounding electrode system consists solely of two 8-foot ground rods spaced 10 feet apart.
According to NEC Article 250, what is the minimum size copper grounding electrode conductor (GEC) required for this installation?