Overcurrent Protection Flashcards

An inverse-time circuit breaker trips faster when the overcurrent is:

Answer: Much larger than its rated amperage

Inverse-time breakers have a trip time that decreases as the magnitude of overcurrent increases.

Per NEC 240.85, a circuit breaker marked '120/240V, 2-pole' may be used on which system?

Answer: 120/240V single-phase systems

A breaker must be rated for the voltage of the system it protects; a 120/240V 2-pole breaker is rated for 120/240V single-phase systems.

Which NEC section covers the location requirements for overcurrent devices in dwelling units?

Answer: NEC 240.24

NEC 240.24 specifies where overcurrent devices shall and shall not be located, including dwelling unit requirements.

A 20A, 120V branch circuit supplies a single receptacle. The cord-and-plug-connected load must not exceed what amperage for continuous operation?

Answer: 16A

Continuous loads on a branch circuit must not exceed 80% of the circuit rating (20A × 80% = 16A).

What is the primary difference between a Type T and a Type W Class K fuse?

Answer: Interrupting rating and let-through current

Class K fuses are subdivided by let-through current and interrupting ratings; Type W has higher let-through than Type T.

Per NEC 240.50, plug fuses of the Edison-base type shall only be used as a replacement in existing installations where there is no evidence of:

Answer: Overfusing

Edison-base fuses may only replace existing fuses where there is no evidence of overfusing or tampering.

A service disconnect rated 200A feeds a panel. Per NEC 230.90, the service overcurrent device must be rated at or below:

Answer: 200A

NEC 230.90 requires the service overcurrent device to be rated not more than the ampacity of the service conductors, which here supports 200A.