Master Electrician Exam (NEC)Overcurrent ProtectionHard
An industrial facility is being designed with a 480V, 3-phase electrical system. To comply with NEC 240.21(B)(2), what is the maximum length of a tap conductor that is protected by an overcurrent device rated at 300% of the tap conductor's ampacity, and terminates in a single circuit breaker or set of fuses?
- A25 feet
- B100 feet
- C10 feet
- D50 feet
Show answer & explanationAnswer & explanation
Correct answer: A. 25 feet
NEC 240.21(B)(2) permits tap conductors up to 25 feet long under specific conditions. These conditions include the tap conductor having an ampacity not less than one-third the rating of the overcurrent device protecting the feeder, terminating in a single OCPD that limits the load current to the tap conductor's ampacity, and being protected from physical damage.
Why the other options are wrong
- B. 100 feet is a specific tap rule, but requires different conditions (e.g., for high-bay manufacturing buildings).
- C. 10 feet is a specific tap rule, but not the one described by the 300% OCPD protection condition.
- D. 50 feet is a specific tap rule, but requires different conditions (e.g., for high-bay manufacturing buildings or outdoor installations).
25-Foot Tap Rule
The NEC 240.21(B)(2) 25-foot tap rule permits tap conductors up to 25 feet long, provided their ampacity is at least one-third of the feeder OCPD rating, they terminate in an OCPD, and are protected from physical damage.
- Tap conductor length limited to 25 feet.
- Tap conductor ampacity must be at least 1/3 of the feeder OCPD.
- Tap conductors must terminate in an OCPD sized for the tap's ampacity.
- Must be protected from physical damage.
Memory trick: Taps are like 'short threads', '25 feet' is the limit for the '1/3' rule.