NEC - National Electrical Code Grounding and Bonding Questions and Answers

Question 1

According to NEC Article 250, which of the following is a primary purpose of bonding?

Accepted Answer

To provide a low-impedance path for fault current to facilitate the operation of overcurrent devices.

Answer

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?

Accepted Answer

An equipment grounding conductor run with the feeder and a separate grounding electrode system.

Answer

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:

Accepted Answer

25 ohms or less

Answer

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?

Accepted Answer

At the same point where the grounding electrode conductor is connected.

Answer

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)?

Accepted Answer

It must be increased in size proportionately to the circular mil area increase of the phase conductors.

Answer

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.

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NEC - National Electrical Code Grounding and Bonding Questions and Answers