IEC - Independent Electrical Contractors Overcurrent Protection Questions and Answers

Question 1

According to NEC® 240.21, where must an overcurrent protective device (OCPD) be located in a circuit?

Accepted Answer

At the point where the conductor to be protected receives its supply.

Answer

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?

Accepted Answer

100A

Answer

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)?

Accepted Answer

120A

Answer

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?

Accepted Answer

To allow for the high inrush current when the motor starts.

Answer

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?

Accepted Answer

25A

Answer

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.

IEC - Independent Electrical Contractors Practice Test

IEC - Independent Electrical Contractors Practice Test

IEC - Independent Electrical Contractors Overcurrent Protection Questions and Answers