IEC Overcurrent Protection 2 — Questions and Answers
Question 1: What is the maximum standard ampere rating for a fuse per NEC 240.6(A)?
- 400A
- 600A (Correct answer)
- 800A
- 1200A
Correct answer: 600A
NEC 240.6(A) lists 600A as a standard ampere rating for overcurrent protective devices.
Question 2: Which type of fuse is designed to open on sustained overloads but NOT on normal motor starting currents?
- Type S fuse
- Time-delay fuse (Correct answer)
- Cartridge fuse
- One-time fuse
Correct answer: Time-delay fuse
Time-delay (dual-element) fuses tolerate short-duration inrush currents while still protecting against sustained overloads.
Question 3: Per NEC 240.4(D), a 14 AWG copper conductor used for general-purpose branch circuits must be protected at no more than:
- 20A
- 15A (Correct answer)
- 25A
- 30A
Correct answer: 15A
NEC 240.4(D) limits overcurrent protection for 14 AWG copper conductors to 15A.
Question 4: A fusible disconnect that opens all ungrounded conductors simultaneously is described as which type of fuse?
- Single-pole
- Multi-pole (Correct answer)
- Dual-element
- Current-limiting
Correct answer: Multi-pole
Multi-pole (multi-element) fuses interrupt all ungrounded conductors at once, preventing single-phasing on three-phase equipment.
Question 5: Where must an overcurrent device NOT be located per NEC 240.24(E)?
- At the service entrance
- In bathrooms of dwellings (Correct answer)
- In an outdoor panelboard
- Adjacent to the load it protects
Correct answer: In bathrooms of dwellings
NEC 240.24(E) prohibits overcurrent devices in bathrooms of dwelling units, dormitories, and guest rooms.
Question 6: The NEC term for a protective device that limits the peak let-through current during a fault is:
- Inverse-time breaker
- Current-limiting fuse (Correct answer)
- GFCI breaker
- Thermal-magnetic breaker
Correct answer: Current-limiting fuse
Current-limiting fuses open fast enough to limit the peak let-through current and reduce fault energy on the system.
Question 7: Per NEC 240.21, a tap conductor may be up to 25 feet long without overcurrent protection at the tap point provided it meets which condition?
- It is enclosed in a raceway and terminates in a single OCPD (Correct answer)
- It is larger than the feeder conductors
- It is run in free air only
- It is protected by a GFCI device
Correct answer: It is enclosed in a raceway and terminates in a single OCPD
The 25-foot tap rule requires the tap conductors to be in a raceway and terminate in a single overcurrent protective device.
IEC Overcurrent Protection 2
What is the maximum standard ampere rating for a fuse per NEC 240.6(A)?