TN LLE Overcurrent Protection 2 — Questions and Answers
Question 1: Per NEC 240.4(B), when can the next standard size overcurrent device be used above the conductor ampacity?
- Always
- When the ampacity does not correspond to a standard device rating AND the circuit does not supply multioutlet receptacle branch circuits (Correct answer)
- Never — the OCPD must always be at or below conductor ampacity
- Only for motor circuits
Correct answer: When the ampacity does not correspond to a standard device rating AND the circuit does not supply multioutlet receptacle branch circuits
NEC 240.4(B) permits the next higher standard overcurrent device rating when the conductor ampacity doesn't match a standard size, provided the circuit isn't a multioutlet receptacle branch circuit.
NEC 240.4(B) allows rounding up to the next standard OCPD size (per 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, etc.) when: (1) The conductor ampacity doesn't match a standard device rating; (2) The device rating is 800A or less; and (3) The conductor is not part of a multioutlet receptacle branch circuit.
Question 2: What are the standard ampere ratings for fuses and circuit breakers per NEC 240.6(A)?
- Any value can be used
- 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 (and continuing) (Correct answer)
- Only multiples of 10
- Only multiples of 15
Correct answer: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 (and continuing)
NEC 240.6(A) lists the standard ampere ratings starting at 15 and including specific values up through 6,000 amperes.
NEC 240.6(A) establishes standard OCPD ratings: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes. Additional standard ratings for fuses include 1, 3, 6, and 10 amperes.
Question 3: Per NEC 240.21(B), what is the maximum length for a feeder tap conductor that is at least 1/3 the ampacity of the feeder OCPD?
- 10 feet
- 15 feet
- 25 feet (Correct answer)
- No length limit
Correct answer: 25 feet
NEC 240.21(B)(2) permits tap conductors up to 25 feet in length if they have ampacity not less than 1/3 of the feeder OCPD rating and terminate in a single OCPD.
NEC 240.21(B) provides tap conductor rules: The 10-foot tap (B)(1) requires ampacity not less than the combined calculated loads and not less than the rating of the downstream OCPD. The 25-foot tap (B)(2) requires ampacity not less than 1/3 of the rating of the feeder OCPD, physical protection from damage, and termination in a single OCPD that limits current to the tap conductor ampacity.
Question 4: What is the difference between an inverse-time circuit breaker and an instantaneous-trip circuit breaker?
- They are the same device
- Inverse-time provides both overload and short-circuit protection; instantaneous-trip provides only short-circuit protection (Correct answer)
- Inverse-time is for AC; instantaneous is for DC
- Instantaneous-trip is faster and always preferred
Correct answer: Inverse-time provides both overload and short-circuit protection; instantaneous-trip provides only short-circuit protection
Inverse-time breakers have a time-delay element for overload protection plus instantaneous trip for short circuits. Instantaneous-trip breakers respond only to short-circuit level faults.
Inverse-time circuit breakers have two trip mechanisms: (1) A thermal element that provides time-delayed tripping for overloads — the higher the overload, the faster it trips (inverse-time relationship); (2) A magnetic element that provides instantaneous tripping for short circuits. Instantaneous-trip (magnetic-only) breakers have no thermal element and only trip on high-magnitude short-circuit currents. Per NEC 430.52(C)(3), instantaneous-trip breakers can only be used for motor branch-circuit protection as part of a listed combination motor controller.
Question 5: Per NEC 240.24(A), what accessibility requirement applies to overcurrent devices?
- They must be in a locked room
- They must be readily accessible (Correct answer)
- They must be at eye level
- No specific accessibility requirement
Correct answer: They must be readily accessible
NEC 240.24(A) requires overcurrent devices to be readily accessible — meaning they can be reached without climbing, removing obstacles, or using portable tools.
NEC 240.24(A) requires overcurrent devices to be readily accessible, defined in Article 100 as 'capable of being reached quickly for operation, renewal, or inspections without requiring those to whom ready access is requisite to take actions such as to use tools, climb over or under, remove obstacles, or resort to portable ladders.' The center of the operating handle must be no more than 6 feet 7 inches above the floor per 240.24(A).
Question 6: Per NEC 240.15, what requirement applies when a circuit breaker is used as a switch for normal load control?
- Any breaker can be used as a switch
- The breaker must be listed as suitable for switching duty (SWD or HID rated) (Correct answer)
- Breakers cannot be used as switches
- Only 20A breakers can be used as switches
Correct answer: The breaker must be listed as suitable for switching duty (SWD or HID rated)
NEC 240.15 requires circuit breakers used for switching 120V or 277V fluorescent lighting circuits to be listed and marked SWD or HID.
NEC 240.15(B)(1) and (2) address circuit breakers used for switching. When used to switch 120V or 277V fluorescent lighting, the breaker must be listed and marked 'SWD' (Switching Duty). For high-intensity discharge (HID) lighting, it must be marked 'HID.' Standard circuit breakers are designed for infrequent operation and may fail prematurely if used for daily switching. SWD-rated breakers are tested for 6,000 switching operations.
TN LLE Overcurrent Protection 2
Per NEC 240.4(B), when can the next standard size overcurrent device be used above the conductor ampacity?