Licensed Electrical Contractor Wiring Methods and Materials Questions and Answers 1 — Questions and Answers
Question 1: An electrical contractor is installing conductors in a raceway located on a rooftop where the ambient temperature is expected to reach 125°F (52°C). The design requires using 4/0 AWG THHN copper conductors. According to NEC Table 310.16, the ampacity of 4/0 THHN is 260A at 30°C. What is the corrected ampacity of these conductors after applying the required temperature correction factor from Table 310.15(B)(1)?
- 260A
- 213.2A
- 189.8A (Correct answer)
- 236.6A
Correct answer: 189.8A
According to NEC Table 310.15(B)(1), the correction factor for a 90°C rated conductor (like THHN) in an ambient temperature of 51-55°C (123-131°F) is 0.73. To find the corrected ampacity, you multiply the conductor's ampacity from Table 310.16 by this correction factor: 260A * 0.73 = 189.8A.
Question 2: When installing a multiconductor Type NM cable through a series of bored holes in wood studs, what is the minimum distance the edge of the hole must be from the nearest edge of the wood member to avoid the need for a protective steel plate?
- 3/4 inch
- 1 inch
- 1 1/4 inches (Correct answer)
- 1 1/2 inches
Correct answer: 1 1/4 inches
NEC Section 300.4(A)(1) requires that where cables are installed through bored holes in wood members, the holes shall be bored so that the edge of the hole is not less than 1 1/4 inches from the nearest edge of the wood member. If this distance cannot be maintained, a steel plate or bushing at least 1/16 inch thick must be installed to protect the cable.
Question 3: According to NEC Chapter 9, Table 1, what is the maximum permitted conduit fill percentage for a raceway containing three or more conductors?
- 31%
- 53%
- 60%
- 40% (Correct answer)
Correct answer: 40%
NEC Chapter 9, Table 1 specifies the allowable percentage of cross-sectional area of a conduit or tubing for conductors. For one conductor, the limit is 53%; for two conductors, it's 31%; and for over two conductors, the maximum permitted fill is 40%.
Question 4: A contractor is wiring a Class I, Division 1 hazardous location. Which of the following wiring methods is generally required to contain a potential explosion?
- Electrical Metallic Tubing (EMT) with compression fittings
- Type MC cable with standard connectors
- Threaded Rigid Metal Conduit (RMC) with explosion-proof fittings (Correct answer)
- PVC conduit sealed at all joints
Correct answer: Threaded Rigid Metal Conduit (RMC) with explosion-proof fittings
NEC Article 501 specifies the requirements for Class I locations. In a Division 1 location, where flammable gases or vapors can exist under normal operating conditions, the wiring method must be able to contain an explosion. Threaded Rigid Metal Conduit (RMC) with appropriate explosion-proof fittings is the standard and required method to achieve this containment.
Question 5: Which of the following must be bonded together to create an effective ground-fault current path in a commercial installation using metallic cable trays?
- Only the sections of the cable tray containing power conductors
- All metallic cable tray sections, but not necessarily to the grounding conductor
- All metallic cable tray sections, fittings, and the equipment grounding conductor system (Correct answer)
- Only the supporting hardware for the cable tray
Correct answer: All metallic cable tray sections, fittings, and the equipment grounding conductor system
According to NEC Article 250 and Article 392, metallic cable tray systems must be bonded to ensure electrical continuity. This involves bonding all metallic sections and fittings together to form a continuous path and connecting this system to the equipment grounding conductor to provide an effective ground-fault current path, which is essential for the operation of overcurrent protective devices.
Question 6: A contractor needs to run conductors in parallel in separate raceways. According to NEC 310.10(G), which of the following conditions must be met for conductors of size 1/0 AWG and larger?
- The conductors must be of different lengths to balance impedance.
- The conductors must be of the same length, material, cross-sectional area, and insulation type. (Correct answer)
- The conductors can be of different materials if the ampacity is the same.
- The conductors must be terminated using different methods to ensure secure connections.
Correct answer: The conductors must be of the same length, material, cross-sectional area, and insulation type.
NEC 310.10(G) outlines the requirements for conductors in parallel. To ensure the current divides equally among the conductors, it mandates that all parallel conductors in each phase, neutral, or grounded conductor must be the same length, made of the same material, have the same circular mil area, have the same insulation type, and be terminated in the same manner.
Licensed Electrical Contractor Wiring Methods and Materials Questions and Answers 1
An electrical contractor is installing conductors in a raceway located on a rooftop where the ambient temperature is expected to reach 125°F (52°C).
The design requires using 4/0 AWG THHN copper conductors.
According to NEC Table 310.16, the ampacity of 4/0 THHN is 260A at 30°C.
What is the corrected ampacity of these conductors after applying the required temperature correction factor from Table 310.15(B)(1)?