PC - Professional Electrical Contractor Wiring Methods and Protection Questions and Answers 1 — Questions and Answers
Question 1: According to NEC Article 240, what is the standard maximum overcurrent protection permitted for a 12 AWG copper conductor used in a residential branch circuit?
- 15 amperes
- 20 amperes (Correct answer)
- 25 amperes
- 30 amperes
Correct answer: 20 amperes
NEC Section 240.4(D)(5) specifically limits the overcurrent protection for 12 AWG copper conductors to 20 amperes. This rule is in place to prevent overheating of conductors commonly used in residential branch circuits and applies regardless of the conductor's insulation temperature rating.
Question 2: An electrician is installing a feeder in a commercial building. The conductors will be run in a raceway where all conductors of the same circuit must be grouped together. Which NEC Article provides the general requirements for this installation practice?
- Article 240
- Article 250
- Article 300 (Correct answer)
- Article 310
Correct answer: Article 300
NEC Article 300 covers the general requirements for wiring methods and materials. Specifically, Section 300.3(B) mandates that all conductors of the same circuit, including the equipment grounding conductor and neutral conductor, must be contained within the same raceway, cable, or trench to minimize inductive heating and maintain circuit integrity.
Question 3: A new installation requires running Type UF cable underground to supply an outdoor receptacle. The circuit is 120V and protected by a 20A GFCI circuit breaker at its source. What is the minimum required burial depth for the Type UF cable under a residential lawn?
- 6 inches
- 24 inches
- 18 inches
- 12 inches (Correct answer)
Correct answer: 12 inches
According to NEC Table 300.5, Column 4, for residential branch circuits rated 120 volts or less with GFCI protection and a maximum overcurrent protection of 20 amperes, the minimum cover requirement for Type UF cable is 12 inches.
Question 4: Which of the following wiring methods is NOT permitted for use in a Class I, Division 1 hazardous location?
- Threaded rigid metal conduit (RMC)
- Threaded intermediate metal conduit (IMC)
- Type MI cable with listed fittings
- Electrical metallic tubing (EMT) (Correct answer)
Correct answer: Electrical metallic tubing (EMT)
NEC Article 501.10(A) lists the approved wiring methods for Class I, Division 1 locations. These include threaded rigid metal conduit, threaded steel intermediate metal conduit, and Type MI cable. Electrical Metallic Tubing (EMT) is not permitted in these highly hazardous areas due to its non-threaded fittings, which cannot reliably prevent the passage of hot gases in an explosion.
Question 5: In a commercial installation, a 400A feeder is tapped to supply a 100A panelboard. The tap conductors are 15 feet long and are installed in a raceway. According to the NEC feeder tap rules, what is the minimum required ampacity for these tap conductors?
- 100A
- 40A (10% of feeder OCPD)
- 133A (1/3 of feeder OCPD) (Correct answer)
- 200A (50% of feeder OCPD)
Correct answer: 133A (1/3 of feeder OCPD)
Per NEC 240.21(B)(2), for feeder taps not over 25 feet long, the tap conductors must have an ampacity of not less than 1/3 of the rating of the overcurrent protective device protecting the feeder conductors. In this case, 400A / 3 = 133.3A. The tap conductors must also have an ampacity not less than the rating of the termination device (100A). Since 133.3A is greater than 100A, the minimum ampacity for the tap conductors is 133.3A, which would necessitate a conductor size with an ampacity meeting or exceeding this value.
Question 6: When installing conductors in parallel, which of the following is a requirement according to NEC 310.10(G)?
- The conductors must be of different sizes to accommodate voltage drop.
- The conductors can be made of different materials, such as copper and aluminum.
- The conductors must be the same length, material, and size, and terminated in the same manner. (Correct answer)
- The conductors are only required to be the same length and material.
Correct answer: The conductors must be the same length, material, and size, and terminated in the same manner.
NEC 310.10(G) requires that when conductors are run in parallel, they must be the same length, made of the same conductor material, have the same circular mil area (size), have the same insulation type, and be terminated in the same manner. This ensures that the current divides evenly between the conductors.
PC - Professional Electrical Contractor Wiring Methods and Protection Questions and Answers 1
According to NEC Article 240, what is the standard maximum overcurrent protection permitted for a 12 AWG copper conductor used in a residential branch circuit?