Free Licensed Electrical Contractor Grounding and Bonding Questions and Answers — Questions and Answers
Question 1: An electrical service is supplied by 4/0 AWG copper service-entrance conductors. According to NEC Table 250.66, what is the minimum size copper Grounding Electrode Conductor (GEC) required to connect to a metal underground water pipe electrode?
- 4 AWG
- 6 AWG
- 2 AWG (Correct answer)
- 1/0 AWG
Correct answer: 2 AWG
According to NEC Table 250.66, when the largest ungrounded service-entrance conductor is over 3/0 AWG through 350 kcmil copper, the minimum size for the copper Grounding Electrode Conductor is 2 AWG.
Question 2: What is the primary purpose of bonding all non-current-carrying metal parts of an electrical installation together?
- To stabilize the system voltage to ground during normal operation.
- To provide a low-impedance path for fault current to facilitate the operation of overcurrent devices. (Correct answer)
- To connect the electrical system to the earth to dissipate lightning strikes.
- To reduce the effects of electromagnetic interference (EMI) on sensitive equipment.
Correct answer: To provide a low-impedance path for fault current to facilitate the operation of overcurrent devices.
The primary purpose of bonding is safety. It creates a continuous, low-impedance path for ground-fault current to return to the source, ensuring the current is high enough to trip a circuit breaker or blow a fuse, thereby clearing the fault. While grounding helps stabilize voltage and dissipate lightning, bonding specifically deals with creating an effective ground-fault current path.
Question 3: According to NEC 250.52(B), which of the following is expressly NOT permitted to be used as a grounding electrode?
- The metal frame of a building that is effectively grounded.
- A concrete-encased electrode with at least 20 feet of bare copper conductor.
- A metal underground gas piping system. (Correct answer)
- A ground ring encircling the building or structure.
Correct answer: A metal underground gas piping system.
NEC Section 250.52(B)(1) explicitly prohibits the use of metal underground gas piping systems as grounding electrodes due to the inherent safety risks, such as the potential for a fault to ignite the gas. The other options are all permissible types of grounding electrodes described in NEC 250.52(A).
Question 4: A contractor is installing a branch circuit protected by a 60-amp circuit breaker. What is the minimum size copper Equipment Grounding Conductor (EGC) required for this circuit?
- 12 AWG
- 8 AWG
- 6 AWG
- 10 AWG (Correct answer)
Correct answer: 10 AWG
The Equipment Grounding Conductor (EGC) is sized based on the rating of the overcurrent protective device, not the load. According to NEC Table 250.122, for a circuit protected by a 60-amp device, the minimum required size for a copper EGC is 10 AWG.
Question 5: When installing a separately derived system, such as a dry-type transformer, which component is required by NEC 250.30 to connect the system's grounded conductor (neutral) to the equipment grounding conductor(s)?
- A ground rod installed within 5 feet of the transformer.
- A supply-side bonding jumper.
- A system bonding jumper. (Correct answer)
- An intersystem bonding termination.
Correct answer: A system bonding jumper.
For a separately derived system, a system bonding jumper is required to be installed at the source. Its purpose is to connect the grounded conductor to the equipment grounding conductor and the normally non-current-carrying metal parts of the system, establishing the necessary low-impedance path for fault current.
Question 6: If an interior metal water piping system is not being used as a grounding electrode, but is required to be bonded, where must the bonding jumper be connected?
- Only at the main water meter.
- To any of the grounding electrodes, the service enclosure, or the grounded service conductor. (Correct answer)
- To the nearest branch circuit equipment grounding conductor.
- Within 5 feet of where the water pipe enters the building.
Correct answer: To any of the grounding electrodes, the service enclosure, or the grounded service conductor.
NEC 250.104(A)(1) requires interior metal water piping to be bonded to the electrical system to ensure equipotentiality. Acceptable connection points include the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor, or one of the grounding electrodes. The rule requiring connection within 5 feet of entry applies when the pipe is used as a grounding electrode, not just for bonding.
Free Licensed Electrical Contractor Grounding and Bonding Questions and Answers
An electrical service is supplied by 4/0 AWG copper service-entrance conductors.
According to NEC Table 250.66, what is the minimum size copper Grounding Electrode Conductor (GEC) required to connect to a metal underground water pipe electrode?