Free TN LLE Overcurrent Protection Questions and Answers 1 — Questions and Answers
Question 1: An electrician installs a 12 AWG copper branch circuit in a residential home. According to NEC 240.4(D), what is the MAXIMUM standard size overcurrent protective device that can be used for this circuit?
- 15 A
- 20 A (Correct answer)
- 25 A
- 30 A
Correct answer: 20 A
NEC Section 240.4(D) provides specific overcurrent protection limits for small conductors to prevent overheating. For a 12 AWG copper conductor, the maximum permitted overcurrent protective device is 20 amperes.
Question 2: According to NEC Article 240, an overcurrent protective device (OCPD) must generally be located at the point where the conductors receive their supply. Which of the following is a permitted exception to this rule?
- When using supplementary overcurrent protection.
- When the circuit is located in a damp location.
- When installing feeder tap conductors under specific conditions. (Correct answer)
- When the load on the circuit is continuous.
Correct answer: When installing feeder tap conductors under specific conditions.
NEC 240.21 requires OCPDs to be located where conductors receive their supply. However, it provides several exceptions, including specific rules for feeder taps found in 240.21(B). These rules allow a conductor to be tapped from a feeder without an OCPD at the point of the tap, provided conditions like length and ampacity are met.
Question 3: An electrical circuit has a calculated load of 28 amps. The conductors have an ampacity of 35 amps. Per NEC 240.4(B), the 'next size up' rule is being applied. Which is the highest standard size overcurrent device from the list below that can be used?
- 25 A
- 30 A
- 35 A
- 40 A (Correct answer)
Correct answer: 40 A
NEC 240.4(B) allows the use of the next higher standard overcurrent device rating (from 240.6) above the ampacity of the conductors, provided the rating does not exceed 800A and other conditions are met. Since the conductor ampacity is 35A, and 35A is a standard size, you could use a 35A breaker. However, the rule allows going to the *next higher* standard size. The next standard size up from 35A is 40A.
Question 4: Which of the following scenarios describes an 'overload' condition rather than a short circuit or ground fault?
- An ungrounded 'hot' conductor accidentally touches a metal junction box.
- A power tool's cord is damaged, causing the hot and neutral wires to make contact.
- Insulation between two conductors fails, allowing them to touch.
- A circuit continues to operate, but with excessive current drawn by equipment connected to it, causing the conductor to heat up. (Correct answer)
Correct answer: A circuit continues to operate, but with excessive current drawn by equipment connected to it, causing the conductor to heat up.
An overload is a type of overcurrent that occurs when excessive current is confined to the normal current-carrying path, often due to connecting too many loads or faulty equipment. This causes overheating. A short circuit or ground fault involves current flowing outside of its intended path.
Question 5: A feeder is protected by a 200A circuit breaker. An electrician needs to make a 10-foot feeder tap from this circuit to supply a panelboard. What is the minimum required ampacity of the tap conductors?
- 10% of the feeder conductor ampacity.
- 1/3 of the feeder overcurrent device rating.
- 10% of the feeder overcurrent device rating. (Correct answer)
- The rating of the panelboard's main breaker.
Correct answer: 10% of the feeder overcurrent device rating.
According to the 10-foot tap rule in NEC 240.21(B)(1), if the tap conductors leave the enclosure, their ampacity must be at least 10% of the rating of the overcurrent device protecting the feeder. In this case, 10% of 200A is 20A. The tap conductors must also have an ampacity not less than the calculated load and the rating of the device they terminate on.
Question 6: Supplementary overcurrent protective devices, such as a small fuse found inside an appliance, are permitted to be used for what purpose?
- To serve as the required branch-circuit overcurrent protection.
- To protect conductors against short-circuits on a feeder.
- To provide limited, additional protection for specific appliances or luminaires. (Correct answer)
- To replace the need for a main service disconnect.
Correct answer: To provide limited, additional protection for specific appliances or luminaires.
NEC 240.10 states that supplementary overcurrent protection shall not be used as a substitute for required branch-circuit overcurrent devices. Its purpose is to provide limited, additional protection for specific equipment, like appliances or luminaires, in addition to the primary branch-circuit protection.
