IEC Overcurrent Protection 4 — Questions and Answers
Question 1: Per NEC 240.4(B), the next standard OCPD size above the conductor ampacity may be used only when the conductor ampacity does NOT exceed:
- 600A (Correct answer)
- 800A
- 400A
- 1000A
Correct answer: 600A
NEC 240.4(B) permits the next larger standard OCPD when the conductor ampacity is 600A or less and the OCPD is not for a motor or similar load.
Question 2: Which Class of fuse has the highest interrupting rating and is designed for current-limiting service at 600V or less?
- Class H
- Class R
- Class L (Correct answer)
- Class G
Correct answer: Class L
Class L fuses are rated 601A–6000A at 600V and carry a 200,000A interrupting rating, the highest available for standard fuses.
Question 3: Ground-fault protection of equipment (GFPE) per NEC 230.95 is required on solidly grounded wye services of more than 150V to ground but not exceeding 600V phase-to-phase when the service rating is at least:
- 400A
- 800A (Correct answer)
- 1000A
- 600A
Correct answer: 800A
NEC 230.95 requires GFPE on services rated 800A or more on solidly grounded wye systems meeting the voltage criteria.
Question 4: When a circuit breaker is used as a service disconnect, it must be capable of being closed and opened by:
- Remote operation only
- Manual operation (Correct answer)
- Automatic means only
- A licensed electrician only
Correct answer: Manual operation
Per NEC 230.76, service disconnects must be capable of being operated manually.
Question 5: Per NEC 240.4(G), which conductor type is NOT permitted to have its overcurrent protection sized using the next standard rating above its ampacity?
- THWN in a raceway
- Flexible cords (Correct answer)
- XHHW in conduit
- Type NM cable
Correct answer: Flexible cords
Flexible cords must be protected by an overcurrent device rated at or below the cord's ampacity per NEC 240.4(G).
Question 6: A 3-phase, 4-wire, 208Y/120V system with an 800A main breaker is required to have GFPE per NEC 230.95. The maximum GFPE trip setting allowed is:
- 300A
- 1200A
- 100A (Correct answer)
- 1000A
Correct answer: 100A
NEC 230.95(A) limits the maximum GFPE trip setting to 1200A and the maximum setting is 1200A, but trip current must not exceed 1200A; the maximum trip setting is 1200A — however, NEC 230.95 sets the maximum at 1200A with a maximum setting of 1200A, and the code maximum for the trip level is 1200A.
Question 7: A feeder tap conductor must have an ampacity of at least what fraction of the rating of the overcurrent device protecting the feeder, per the 10-foot tap rule?
- One-half
- One-third (Correct answer)
- One-quarter
- Two-thirds
Correct answer: One-third
The 10-foot tap rule per NEC 240.21(B)(1) requires tap conductors to have an ampacity not less than one-third of the rating of the upstream overcurrent device.
IEC Overcurrent Protection 4
Per NEC 240.4(B), the next standard OCPD size above the conductor ampacity may be used only when the conductor ampacity does NOT exceed: