CME NEC Code and Theory Questions and Answers 1 — Questions and Answers
Question 1: According to NEC Article 500, which of the following best describes a Class I, Division 1 hazardous location?
- A location where ignitible fibers or flyings are handled, manufactured, or used.
- A location where combustible dust is normally in suspension in the air in quantities sufficient to produce explosive or ignitible mixtures.
- A location in which flammable gases or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitible mixtures under normal operating conditions. (Correct answer)
- A location where volatile flammable liquids or flammable gases are handled, but are normally confined within closed containers from which they can escape only in case of accidental rupture.
Correct answer: A location in which flammable gases or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitible mixtures under normal operating conditions.
NEC Section 500.5(B)(1) defines a Class I, Division 1 location as one where ignitible concentrations of flammable gases or vapors can exist under normal operating conditions; or in which they may exist frequently because of repair or maintenance operations or because of leakage; or in which breakdown or faulty operation of equipment might release ignitible concentrations.
Question 2: A 480V to 208Y/120V, 75 kVA transformer is installed indoors to create a new separately derived system. Per NEC 250.30(A), where must the system bonding jumper be installed?
- Only at the transformer's secondary neutral terminal.
- At both the transformer and the first disconnecting means.
- At either the source of the separately derived system or at the first system disconnecting means, but not at both locations. (Correct answer)
- Only within the enclosure for the first system disconnecting means or switchboard.
Correct answer: At either the source of the separately derived system or at the first system disconnecting means, but not at both locations.
NEC 250.30(A)(1) states that for a separately derived system, the system bonding jumper shall be installed at only one location. It can be installed at the source (the transformer) or at the first system disconnecting means. Installing it at both locations would create a dangerous parallel path for neutral current.
Question 3: An electrician is installing three sets of 500 kcmil THWN copper conductors in parallel for a 3-phase, 4-wire feeder. Which of the following is NOT a requirement for this parallel installation according to NEC 310.10?
- All conductors must be the same length.
- All conductors must be made of the same material.
- Each parallel conductor must have its own separate equipment grounding conductor. (Correct answer)
- All conductors must have the same circular mil area and insulation type.
Correct answer: Each parallel conductor must have its own separate equipment grounding conductor.
NEC 310.10(G) requires that paralleled conductors be the same length, material, circular mil area, and have the same insulation type. While an equipment grounding conductor is required where conductors are run in parallel in multiple raceways, the rule in 250.122(F) requires a single equipment grounding conductor to be run in each raceway, sized based on the feeder overcurrent device, not that each individual parallel conductor has its own EGC.
Question 4: A 400A feeder is protected by a 400A circuit breaker. Under the '25-foot tap rule' in NEC 240.21(B)(2), what is the minimum required ampacity for the tap conductors?
- 100A
- 40A
- 133A (Correct answer)
- 200A
Correct answer: 133A
According to NEC 240.21(B)(2), for feeder taps not over 25 feet long, the tap conductors must have an ampacity of not less than one-third (1/3) of the rating of the overcurrent protective device protecting the feeder conductors. Therefore, 400A / 3 = 133.33A. The tap conductors must have an ampacity of at least 133A.
Question 5: When grounding an indoor separately derived AC system, such as a dry-type transformer, which of the following must be used as the grounding electrode?
- The nearest available qualified metal water pipe.
- A new ground rod driven at the transformer location.
- The metal frame of the building, if effectively grounded.
- The building or structure grounding electrode system. (Correct answer)
Correct answer: The building or structure grounding electrode system.
NEC 250.30(A)(4) mandates that for an indoor separately derived system, the building or structure's existing grounding electrode system must be used as the grounding electrode for the new system. This ensures all grounded systems within the building are bonded together, minimizing potential differences.
Question 6: Which of the following conditions would classify a location as Class II, Division 1 according to the NEC?
- Where ignitible concentrations of flammable gases can exist under normal operating conditions.
- Where combustible dust is in the air under normal operating conditions in quantities sufficient to produce explosive or ignitible mixtures. (Correct answer)
- Where easily ignitible fibers are stored or handled, but not in the process of manufacture.
- Where combustible dust is not normally in the air but may be present as a result of infrequent malfunctioning of handling equipment.
Correct answer: Where combustible dust is in the air under normal operating conditions in quantities sufficient to produce explosive or ignitible mixtures.
NEC 500.5(C)(1) defines a Class II, Division 1 location as an area where combustible dust is in the air under normal operating conditions in sufficient quantities to be explosive or ignitible, or where malfunctioning equipment might create such conditions, or where combustible dusts of an electrically conductive nature may be present.
CME NEC Code and Theory Questions and Answers 1
According to NEC Article 500, which of the following best describes a Class I, Division 1 hazardous location?