Overcurrent Protection & Controls Flashcards
7 cards from real NEC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Overcurrent Protection & Controls flashcards as text
Per NEC 240.21, overcurrent protection is generally required at what point of a conductor?
Answer: The point where the conductor receives its supply
NEC 240.21 requires overcurrent protection at the point where the conductor receives its supply, except for specified tap rules.
Under the 10-foot tap rule of NEC 240.21(B)(1), the tap conductor ampacity must be at least what percentage of the device or conductor it supplies?
Answer: 10 percent of the rating of the OCPD supplying the feeder
The 10-ft tap rule requires the tap conductor ampacity to be not less than 10% of the rating of the overcurrent device protecting the feeder.
Under the 25-foot tap rule of NEC 240.21(B)(2), the tap conductors must have an ampacity at least what fraction of the feeder overcurrent device rating?
Answer: One-third
The 25-ft tap rule requires tap conductors to have ampacity not less than one-third of the feeder OCPD rating.
Per NEC 240.21(C), transformer secondary conductors are generally protected based on what?
Answer: The primary overcurrent device only with specific multiplier conditions
NEC 240.21(C) governs transformer secondary conductors, with protection tied to primary protection and specified tap conditions.
A motor branch-circuit short-circuit and ground-fault protective device is sized per Article 430, and per 240.4(G) it may exceed the conductor ampacity because what protection is provided separately?
Answer: Motor overload protection
Motor circuits use separate overload protection, so the branch-circuit short-circuit device per Article 430 may exceed conductor ampacity under 240.4(G).
Per NEC 240.21(B)(5), outside taps of unlimited length are permitted when the conductors are located outside and which condition is met?
Answer: The overcurrent device is located at the point the conductors enter the building
Outside taps of unlimited length require the overcurrent device at the point the conductors enter a building or structure.
Per NEC 240.4(F), what protects secondary conductors of a single-phase 2-wire transformer?
Answer: Primary overcurrent protection sized per the secondary-to-primary voltage ratio
NEC 240.4(F) allows primary protection to protect 2-wire secondary conductors when sized accounting for the voltage ratio.