Free NEC Grounding and Bonding Questions and Answers — Questions and Answers
Question 1: An electrician is installing a new 480/277V, 3-phase separately derived system from a transformer inside a building. According to NEC 250.30(A), which of the following must be used as the grounding electrode for this new system?
- The nearest available metal water pipe
- Two new 8-foot ground rods driven 6 feet apart
- The existing building or structure grounding electrode system (Correct answer)
- An isolated, dedicated ground plate buried near the transformer
Correct answer: The existing building or structure grounding electrode system
NEC Section 250.30(A)(4) mandates that the existing building or structure grounding electrode system shall be used as the grounding electrode for a separately derived system. This ensures all systems within a building are bonded to a common electrode system, preventing potential differences between them.
Question 2: When installing an equipotential bonding grid for a permanently installed swimming pool, what is the MINIMUM size solid copper conductor required by NEC Article 680?
- 10 AWG
- 8 AWG (Correct answer)
- 6 AWG
- 12 AWG
Correct answer: 8 AWG
According to NEC Section 680.26(B), the conductors used for equipotential bonding in and around swimming pools must be solid copper and not smaller than 8 AWG. Aluminum conductors are not permitted for this application due to corrosion concerns.
Question 3: A service is supplied by 500 kcmil copper ungrounded service-entrance conductors. If the only connection to the grounding electrode system is to two 8-foot ground rods, what is the MINIMUM size copper grounding electrode conductor (GEC) required?
- 1/0 AWG
- 4 AWG
- 2 AWG
- 6 AWG (Correct answer)
Correct answer: 6 AWG
NEC Section 250.66(A) provides an exception to the general sizing rules in Table 250.66. It states that if a grounding electrode conductor connects only to rod, pipe, or plate electrodes, the conductor is not required to be larger than 6 AWG copper.
Question 4: A main bonding jumper is being installed in a service panel. Its size is determined based on which of the following?
- The rating of the main overcurrent protection device
- The size of the grounding electrode conductor
- The size of the ungrounded service-entrance conductors (Correct answer)
- The available fault current at the service
Correct answer: The size of the ungrounded service-entrance conductors
According to NEC 250.28(D) and Table 250.102(C)(1), the main bonding jumper shall be sized based on the size of the ungrounded service-entrance conductors. This ensures the jumper can safely carry any fault current it might be subjected to.
Question 5: Which of the following locations is a required point of attachment for bonding an interior metal water piping system in a building?
- To the nearest metal structural frame of the building
- To the equipment grounding conductor of the largest branch circuit
- Within 10 feet of where the water pipe enters the building
- The service equipment enclosure (Correct answer)
Correct answer: The service equipment enclosure
NEC Section 250.104(A)(1) requires the metal water piping system to be bonded to the service equipment enclosure, the grounded service conductor, the grounding electrode conductor, or one of the grounding electrodes used. The service equipment enclosure is one of the specifically listed and most common points of attachment.
Question 6: When bonding the structural steel of a building that is used as part of the grounding electrode system, the connection must be permanent, electrically continuous, and often achieved through which of the following means?
- A listed pressure connector tightened to a painted column
- A single clamp attached to a loose metal deck
- Embedded anchor bolts or reinforcing bars tied to a concrete-encased electrode (Correct answer)
- A self-tapping screw into a steel stud
Correct answer: Embedded anchor bolts or reinforcing bars tied to a concrete-encased electrode
For structural steel to be considered part of the grounding electrode system per NEC 250.52(A)(2), it must have a direct and permanent connection to the earth. This is commonly achieved through its anchor bolts being in contact with a concrete-encased electrode (Ufer ground) or similar foundational electrode.
Free NEC Grounding and Bonding Questions and Answers
An electrician is installing a new 480/277V, 3-phase separately derived system from a transformer inside a building.
According to NEC 250.30(A), which of the following must be used as the grounding electrode for this new system?