TN LLE Motors and Motor Circuits 2 — Questions and Answers
Question 1: Per NEC Table 430.248, what is the full-load current of a 5 HP, single-phase, 230V motor?
- 15 amperes
- 28 amperes (Correct answer)
- 40 amperes
- 56 amperes
Correct answer: 28 amperes
NEC Table 430.248 lists the full-load current of a 5 HP, single-phase, 230V motor at 28 amperes.
NEC Table 430.248 provides full-load current values for single-phase AC motors. For a 5 HP, 230V motor, the listed FLC is 28 amperes. It is important to use Table 430.248 values (not the motor nameplate) for sizing conductors (430.22), branch-circuit protection (430.52), and feeder sizing (430.24). The nameplate current is used only for sizing the overload protection (430.32). This distinction is critical for the TN LLE exam — using the wrong current value for the wrong calculation is a common error.
Question 2: Per NEC 430.22, what is the minimum conductor ampacity for a single motor branch circuit?
- 100% of motor FLC
- 115% of motor FLC
- 125% of motor FLC (Correct answer)
- 150% of motor FLC
Correct answer: 125% of motor FLC
NEC 430.22(A) requires branch-circuit conductors supplying a single motor to have an ampacity of not less than 125% of the motor full-load current from the applicable table.
NEC 430.22(A) requires motor branch-circuit conductors to have an ampacity of at least 125% of the motor FLC from the applicable NEC table (430.247 through 430.250). For the 5 HP, 230V single-phase motor: 28A x 1.25 = 35A minimum conductor ampacity. This would require 8 AWG copper THHN (40A from Table 310.16). The 125% factor accounts for the continuous duty nature of most motor loads and the heat generated during operation.
Question 3: What is the maximum size inverse-time circuit breaker permitted for branch-circuit protection of a single-phase motor per NEC 430.52?
- 125% of FLC
- 150% of FLC
- 250% of FLC (Correct answer)
- 300% of FLC
Correct answer: 250% of FLC
NEC Table 430.52 permits inverse-time circuit breakers up to 250% of the motor FLC for standard motor branch-circuit short-circuit and ground-fault protection.
NEC Table 430.52 lists maximum ratings for motor branch-circuit short-circuit and ground-fault protective devices. For inverse-time circuit breakers, the maximum is 250% of motor FLC. For the 28A motor: 28 x 2.5 = 70A, so a 70A breaker is the maximum. If 250% doesn't correspond to a standard size, 430.52(C)(1) Exception 1 permits the next standard size up. For dual-element time-delay fuses, the maximum is 175%. These higher percentages accommodate motor starting inrush current, which can be 6-8 times the full-load current.
Question 4: Per NEC 430.32, what is the maximum overload protection for a motor with a service factor of 1.15 or greater?
- 100% of nameplate current
- 115% of nameplate current
- 125% of nameplate current (Correct answer)
- 140% of nameplate current
Correct answer: 125% of nameplate current
NEC 430.32(A)(1) limits overload protection to 125% of the motor nameplate full-load current for motors with a service factor of 1.15 or greater.
NEC 430.32(A)(1) sizes motor overload protection based on the nameplate full-load current (not the NEC table value). For motors marked with a service factor of 1.15 or greater, or with a marked temperature rise of 40 degrees C or less, the overload device can be up to 125% of nameplate FLA. For all other motors, the limit is 115% of nameplate FLA. If these values don't trip during starting, 430.32(C) permits increasing to 140% (for SF >= 1.15) or 130% (for others).
Question 5: Per NEC 430.102, a motor disconnecting means must be located where relative to the motor?
- Within 10 feet
- Within sight from the motor location and the controller (Correct answer)
- At the main panel only
- Anywhere on the same floor
Correct answer: Within sight from the motor location and the controller
NEC 430.102(A) and (B) require the motor disconnect to be within sight from the controller location, and 430.102(B) requires a disconnect within sight from the motor.
NEC 430.102 has two requirements: (A) Each controller must have a disconnect within sight from the controller. (B) A disconnecting means must be located within sight from the motor location and the driven machinery location. 'Within sight' means visible and not more than 50 feet distant. If a disconnect is not within sight of the motor, NEC 430.102(B) Exception permits the controller disconnect to serve both purposes if it can be locked in the open position with a permanently installed lock.
Question 6: What is the purpose of motor overload protection separate from the branch circuit breaker?
- To protect against short circuits
- To protect the motor windings from sustained overcurrent that would cause overheating (Correct answer)
- To protect against ground faults
- To control motor speed
Correct answer: To protect the motor windings from sustained overcurrent that would cause overheating
Motor overload protection specifically protects the motor windings from damage due to sustained overcurrent conditions that are below the branch-circuit breaker's trip point.
Motor overload protection serves a distinct purpose from branch-circuit protection. Branch-circuit breakers (sized at 250% of FLC) protect against short circuits and ground faults — high-magnitude, short-duration events. Overload relays (sized at 115-125% of FLC) protect against sustained overcurrent from mechanical overload, locked rotor conditions, phase loss, or voltage imbalance that would overheat motor windings. Without overload protection, a motor could draw 150% of rated current (below the breaker trip point) continuously, destroying the windings.
TN LLE Motors and Motor Circuits 2
Per NEC Table 430.248, what is the full-load current of a 5 HP, single-phase, 230V motor?