IEC - Independent Electrical Contractors Overcurrent Protection Questions and Answers — Questions and Answers
Question 1: According to NEC® 240.21, where must an overcurrent protective device (OCPD) be located in a circuit?
- At the point where the conductor to be protected receives its supply. (Correct answer)
- At the midpoint of the branch circuit.
- Within 3 feet of the load.
- Only within the main service panel.
Correct answer: At the point where the conductor to be protected receives its supply.
NEC® 240.21 generally requires that overcurrent protection be provided in each ungrounded conductor at the point where the conductors receive their supply, with some specific exceptions for taps and transformer secondaries.
Question 2: A feeder is protected by a 300A circuit breaker. Under the '25-foot tap rule,' what is the minimum required ampacity of a tap conductor connected to this feeder?
- 75A
- 150A
- 100A (Correct answer)
- 30A
Correct answer: 100A
The '25-foot tap rule' in NEC® 240.21(B)(2) requires that the ampacity of the tap conductors be not less than one-third (1/3) of the rating of the overcurrent device protecting the feeder conductors. Therefore, 300A / 3 = 100A.
Question 3: Which of the following is NOT a standard ampere rating for a fuse or inverse time circuit breaker according to NEC® 240.6(A)?
- 90A
- 110A
- 120A (Correct answer)
- 150A
Correct answer: 120A
NEC® Table 240.6(A) lists the standard ampere ratings for fuses and fixed-trip circuit breakers. The standard sizes include 90A, 110A, and 150A, but 120A is not listed as a standard size.
Question 4: In a motor branch circuit, the short-circuit and ground-fault protective device is sized larger than the conductor's ampacity. What is the primary reason for this allowance?
- To account for voltage drop over the length of the circuit.
- To allow for the high inrush current when the motor starts. (Correct answer)
- To simplify the selection of overcurrent devices.
- To provide overload protection for the motor.
Correct answer: To allow for the high inrush current when the motor starts.
NEC® Article 430 allows the branch-circuit short-circuit and ground-fault protective device to be sized higher than the conductor's ampacity to prevent tripping during the motor's starting period, which involves a high inrush current. Overload protection is a separate device.
Question 5: An electrician is installing a new branch circuit for a continuous load of 18A using THHN copper conductors. What is the minimum required rating for the overcurrent protection device?
- 20A
- 25A (Correct answer)
- 18A
- 30A
Correct answer: 25A
For continuous loads, the overcurrent device must be sized to be no less than 125% of the load. 18A * 1.25 = 22.5A. Since 22.5A is not a standard OCPD size, the 'next size up' rule from NEC® 240.4(B) permits using the next standard size, which is 25A.
Question 6: Which of the following BEST describes the function of 'supplementary overcurrent protection' as defined by the NEC®?
- It is the primary protection for an entire branch circuit.
- It is used in addition to a branch-circuit OCPD for specific equipment protection. (Correct answer)
- It must always be readily accessible.
- It can be used in place of feeder protection.
Correct answer: It is used in addition to a branch-circuit OCPD for specific equipment protection.
Supplementary overcurrent protection, as described in NEC® 240.10, is used for luminaires, appliances, and other equipment to provide additional protection beyond the main branch-circuit OCPD. It is not a substitute for branch-circuit protection and is not required to be readily accessible.
IEC - Independent Electrical Contractors Overcurrent Protection Questions and Answers
According to NEC® 240.21, where must an overcurrent protective device (OCPD) be located in a circuit?