Free TN-LLE Fault Current & OCPD Ratings Questions and Answers 1 — Questions and Answers
Question 1: The marking 'AIC' on a circuit breaker or fuse stands for:
- Amperes in Continuous-duty
- Ampere Interrupting Capacity (Correct answer)
- Alternating Inductive Current
- Available Instantaneous Current
Correct answer: Ampere Interrupting Capacity
AIC stands for Ampere Interrupting Capacity, also known as the Interrupting Rating (IR). This rating indicates the maximum amount of fault current that the overcurrent protective device (OCPD) can safely interrupt without failing or causing a hazard.
Question 2: According to NEC 110.9, an overcurrent protective device's interrupting rating must be:
- Less than the available fault current.
- Equal to the circuit's continuous load.
- Equal to or greater than the available fault current. (Correct answer)
- Exactly 125% of the conductor ampacity.
Correct answer: Equal to or greater than the available fault current.
NEC 110.9 is a fundamental safety rule stating that equipment intended to interrupt current at fault levels must have an interrupting rating sufficient for the current that is available at the line terminals of the equipment. This means the device's rating must be equal to or greater than the available fault current.
Question 3: Which of the following actions would most likely INCREASE the available fault current at a panelboard?
- Increasing the length of the feeder conductors
- Replacing the upstream transformer with a larger kVA unit (Correct answer)
- Using smaller gauge feeder conductors
- Adding more non-continuous loads to the panel
Correct answer: Replacing the upstream transformer with a larger kVA unit
The size of the upstream transformer is a primary factor in determining available fault current; a larger kVA transformer has a lower internal impedance and can deliver more current during a fault. Longer or smaller conductors would add impedance and decrease fault current.
Question 4: A panelboard is marked "Suitable for use as service equipment only when not more than six main circuit breakers are installed." This is an example of what NEC rule?
- The Tap Rule (240.21)
- The Six Disconnect Rule (230.71) (Correct answer)
- The Series Rating Rule (240.86)
- The Interrupting Rating Rule (110.9)
Correct answer: The Six Disconnect Rule (230.71)
This marking relates directly to NEC 230.71, which permits up to six switches or circuit breakers to serve as the service disconnecting means. The panelboard's listing and marking ensure it is used in compliance with this 'six-handle rule' for service equipment.
Question 5: What is required for a 'series rated' overcurrent protection system to be compliant with the NEC?
- Both devices must be from the same voltage class.
- The combination must be specifically tested and listed for series use. (Correct answer)
- The load-side breaker must have at least half the AIC of the line-side breaker.
- The system must be designed by a licensed professional engineer.
Correct answer: The combination must be specifically tested and listed for series use.
NEC 240.86 allows for series ratings where a line-side device (like a main breaker) with a high AIC rating can protect a load-side device (like a branch breaker) with a lower AIC rating. For this to be compliant, the combination of devices must be specifically tested and listed by the manufacturer for that purpose.
Question 6: The general rule in NEC 240.21 requires that overcurrent protection for a conductor be located:
- At the midpoint of the circuit run.
- At the load end, nearest the equipment.
- At the point where the conductor receives its supply. (Correct answer)
- Within a readily accessible junction box.
Correct answer: At the point where the conductor receives its supply.
NEC 240.21(A) establishes the fundamental rule for overcurrent protection. It states that protection shall be provided in each ungrounded conductor at the point where the conductors receive their supply, preventing the entire length of the conductor from being overloaded.
Free TN-LLE Fault Current & OCPD Ratings Questions and Answers 1
The marking 'AIC' on a circuit breaker or fuse stands for: