Free Journeyman Electrician’s Overcurrent Protection Questions and Answers — Questions and Answers
Question 1: According to NEC 240.4(D), what is the maximum standard overcurrent protective device (OCPD) rating for 10 AWG copper conductors that are not part of a multi-outlet branch circuit supplying receptacles?
- 20A
- 25A
- 30A (Correct answer)
- 40A
Correct answer: 30A
NEC Section 240.4(D)(7) specifies that unless specifically permitted elsewhere in the Code, the overcurrent protection for a 10 AWG copper conductor shall not exceed 30 amperes.
Question 2: An electrician is installing a feeder tap that is 15 feet long. The feeder is protected by a 200A circuit breaker. What is the minimum required ampacity for these tap conductors?
- 20A
- 67A (Correct answer)
- 100A
- 200A
Correct answer: 67A
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 of the rating of the overcurrent device protecting the feeder conductors. Therefore, 200A / 3 = 66.67A, which requires an ampacity of at least 67A.
Question 3: Which of the following locations is explicitly prohibited for the installation of overcurrent devices, according to NEC 240.24?
- In a readily accessible basement
- Above the ceiling in a commercial kitchen
- In the bathrooms of dwelling units (Correct answer)
- Within a locked electrical room
Correct answer: In the bathrooms of dwelling units
NEC 240.24(E) states that overcurrent devices shall not be located in the bathrooms of dwelling units, guest rooms, or guest suites.
Question 4: When is it permissible to use the next higher standard rating of an overcurrent protective device if the conductor's ampacity does not correspond to a standard size OCPD?
- Only when the OCPD is rated 1000A or less
- When the circuit supplies a single motor
- When the OCPD is rated 800A or less and other specific conditions are met (Correct answer)
- It is never permissible to use the next higher standard rating
Correct answer: When the OCPD is rated 800A or less and other specific conditions are met
NEC 240.4(B) permits using the next higher standard overcurrent device rating, but only if the OCPD is rated 800A or less, the conductor is not part of a multi-outlet branch circuit serving cord-and-plug-connected portable loads, and the conductor's ampacity is not less than the continuous and noncontinuous loads.
Question 5: A 480V to 208/120V, three-phase transformer has a primary full-load current of 60A and a secondary full-load current of 167A. If both primary and secondary protection is provided, what is the maximum standard size overcurrent device permitted on the primary side?
- 60A
- 125A
- 70A
- 150A (Correct answer)
Correct answer: 150A
According to NEC Table 450.3(B), for a transformer with a primary current of 9 amperes or more and with both primary and secondary protection, the primary overcurrent device can be rated at a maximum of 250% of the primary full-load current. 60A x 2.50 = 150A. Since 150A is a standard size OCPD listed in 240.6(A), it is the maximum permitted rating.
Question 6: For a continuous-duty motor with a nameplate full-load ampere (FLA) rating of 40A, a service factor of 1.15, and separate overload protection, what is the maximum rating for the overload device?
- 40A
- 46A
- 50A (Correct answer)
- 60A
Correct answer: 50A
According to NEC 430.32(A)(1), for motors with a service factor of 1.15 or greater, the separate overload device shall be selected to trip at no more than 125% of the motor's nameplate full-load current rating. Therefore, 40A x 1.25 = 50A.
