NEC - National Electrical Code Grounding and Bonding Questions and Answers 1 — Questions and Answers
Question 1: According to NEC Article 250, which of the following is a primary purpose of bonding?
- To limit voltage imposed by lightning.
- To stabilize voltage to earth 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 itself.
Correct answer: To provide a low-impedance path for fault current to facilitate the operation of overcurrent devices.
NEC 250.4(A)(4) and 250.4(A)(5) clarify that bonding of electrical equipment is performed to create a low-impedance path for ground-fault current. This path allows a large enough current to flow during a fault to quickly open the circuit breaker or fuse. While grounding connects the system to the earth to handle lightning and stabilize voltage, bonding specifically connects metallic parts to ensure an effective fault current path.
Question 2: A feeder is run to a detached garage from a main building. Per NEC 250.32, which of the following is generally required at the detached structure?
- A connection between the neutral conductor and the grounding electrode.
- Reliance solely on the main building's grounding electrode system.
- An equipment grounding conductor run with the feeder and a separate grounding electrode system. (Correct answer)
- A ground fault circuit interrupter for the entire feeder.
Correct answer: An equipment grounding conductor run with the feeder and a separate grounding electrode system.
NEC 250.32(A) requires a separate building or structure supplied by a feeder or branch circuit to have its own grounding electrode system. Furthermore, 250.32(B)(1) mandates that an equipment grounding conductor (EGC) be run with the feeder conductors to the separate building for grounding non-current-carrying metal parts.
Question 3: When installing a single ground rod electrode, it must be supplemented by an additional electrode unless the single rod has a resistance to earth of:
- 10 ohms or less
- 50 ohms or less
- 100 ohms or less
- 25 ohms or less (Correct answer)
Correct answer: 25 ohms or less
According to NEC 250.53(A)(2), a single rod, pipe, or plate electrode must be supplemented by an additional electrode unless it can be demonstrated that its resistance to earth is 25 ohms or less. If a supplemental electrode is required, it must be installed no less than 6 feet from the first electrode per 250.53(A)(3).
Question 4: In a separately derived system, such as a transformer, where must the system bonding jumper be connected?
- At the first outlet supplied by the system.
- At the same point where the grounding electrode conductor is connected. (Correct answer)
- Between the primary and secondary windings.
- Only within the main service panel for the building.
Correct answer: At the same point where the grounding electrode conductor is connected.
NEC 250.30(A)(5) requires the grounding electrode conductor for a separately derived system to connect the grounded conductor of the derived system to the grounding electrode. This connection must be made at the same point on the separately derived system where the system bonding jumper is connected. This ensures a low-impedance path and prevents objectionable current.
Question 5: An electrician is installing a 200-amp feeder using 3/0 AWG copper ungrounded conductors. Due to voltage drop concerns over a long distance, they decide to increase the ungrounded conductors to 250 kcmil copper. According to NEC 250.122(B), what must be done with the wire-type equipment grounding conductor (EGC)?
- It can remain the size specified in Table 250.122 for a 200A device.
- It must be increased in size proportionately to the circular mil area increase of the phase conductors. (Correct answer)
- It must be one standard trade size larger than the original EGC.
- It is no longer required if the raceway is metal.
Correct answer: It must be increased in size proportionately to the circular mil area increase of the phase conductors.
NEC 250.122(B) states that if ungrounded conductors are increased in size from the minimum required for the load (e.g., for voltage drop), the equipment grounding conductor must also be increased in size proportionately based on the circular mil area. This ensures the EGC remains effective over the longer distance.
Question 6: Which of the following is required for the equipotential bonding of a permanently installed swimming pool?
- Bonding only the pump motor housing with a #12 AWG copper wire.
- A solid copper conductor, not smaller than #8 AWG, bonding specified metal parts. (Correct answer)
- Connecting all metal parts to the neutral conductor of the pump circuit.
- Insulating all metal components within 10 feet of the pool.
Correct answer: A solid copper conductor, not smaller than #8 AWG, bonding specified metal parts.
NEC 680.26(B) requires that various parts of a swimming pool, including conductive shells, reinforcing steel, metal fittings, and electrical equipment, be bonded together with a solid copper conductor, not smaller than 8 AWG. This creates an equipotential grid to prevent dangerous voltage gradients in and around the pool.
NEC - National Electrical Code Grounding and Bonding Questions and Answers 1
According to NEC Article 250, which of the following is a primary purpose of bonding?