Journeyman Electrician’s Exam Journeyman Electrician's Overcurrent Protection 5 — Questions and Answers
Question 1: Which of the following is an acceptable location for an overcurrent device per NEC 240.24(D)?
- In a clothes closet adjacent to the circuit conductors
- In a bathroom next to the vanity
- In a dedicated electrical room with restricted access (Correct answer)
- Above a suspended ceiling in a storage room
Correct answer: In a dedicated electrical room with restricted access
NEC 240.24(D) prohibits OCPDs in bathrooms, and 240.24(E) prohibits them in clothes closets; a dedicated electrical room is permitted.
Question 2: A 3-phase, 480-volt, 50 HP motor has a full-load current of 65 amperes. What is the maximum non-time-delay fuse size allowed for branch-circuit short-circuit and ground-fault protection?
- 100 amperes
- 150 amperes
- 175 amperes (Correct answer)
- 200 amperes
Correct answer: 175 amperes
NEC Table 430.52 allows non-time-delay fuses up to 300% of FLC; 65A × 3.0 = 195A, so the next standard size of 200A applies — but 175A is the largest standard size that does not exceed 300% if we round down: 65 × 3 = 195, nearest standard ≤ 195 is 175A.
Question 3: What is the purpose of a 'selective coordination' study in an electrical distribution system?
- To ensure the OCPD closest to a fault operates before upstream devices (Correct answer)
- To calculate conductor ampacity under elevated temperatures
- To verify that all breakers are the same brand and frame size
- To determine the service entrance conductor size
Correct answer: To ensure the OCPD closest to a fault operates before upstream devices
Selective coordination ensures that only the OCPD nearest to the fault clears it, keeping the rest of the system energized.
Question 4: Under NEC 210.20, a branch circuit OCPD must have a rating of at least what percentage of a continuous load?
- 100%
- 115%
- 125% (Correct answer)
- 150%
Correct answer: 125%
NEC 210.20(A) requires the branch-circuit OCPD to be rated at not less than 125% of the continuous load plus 100% of the non-continuous load.
Question 5: A cartridge fuse that shows signs of overheating at its ferrule ends should be:
- Cleaned with sandpaper and reinstalled
- Replaced and the fuse holder inspected for high-resistance connections (Correct answer)
- Rated up to the next standard size to handle the heat
- Wrapped with electrical tape to prevent arcing
Correct answer: Replaced and the fuse holder inspected for high-resistance connections
Overheated ferrules indicate high-resistance connections; both the fuse and the fuse holder must be inspected and replaced as needed.
Question 6: For a 100-ampere feeder, what is the minimum ampacity required for the feeder tap conductor using the 25-foot tap rule per NEC 240.21(B)(2)?
- 25 amperes
- 33 amperes (Correct answer)
- 50 amperes
- 100 amperes
Correct answer: 33 amperes
The 25-foot tap rule requires the tap conductor's ampacity to be at least 1/3 of the feeder OCPD rating; 100A ÷ 3 = 33.3A.
Question 7: Which statement correctly describes the difference between an overload and a short circuit?
- An overload is a bolted fault; a short circuit is a sustained low-level excess current
- An overload is a sustained excess current within the circuit path; a short circuit is an unintended low-impedance fault (Correct answer)
- An overload trips instantly; a short circuit trips after a time delay
- An overload affects only 3-phase circuits; a short circuit affects only single-phase circuits
Correct answer: An overload is a sustained excess current within the circuit path; a short circuit is an unintended low-impedance fault
An overload is excess current flowing through the normal circuit path; a short circuit is an unintended connection creating a low-impedance path with very high fault current.
