Journeyman Electrician Exam (NEC)Control Devices and MotorsMedium
A 20 hp, 230V, three-phase Squirrel Cage Induction Motor with a service factor of 1.0 is being installed. The motor branch-circuit conductors are sized for 125% of the motor's full-load current (FLA). According to NEC Table 430.250, the motor's FLA is 54A. What is the minimum ampacity required for the branch-circuit conductors?
- A67.5 A
- B70 A
- C54 A
- D81 A
Show answer & explanationAnswer & explanation
Correct answer: A. 67.5 A
According to NEC 430.22(A), branch-circuit conductors supplying a single motor used in a continuous duty application must have an ampacity not less than 125% of the motor's full-load current (FLA). Therefore, 54A * 1.25 = 67.5A.
Why the other options are wrong
- B. This is a standard conductor ampacity, but not the calculated minimum based on 125% of FLA.
- C. This is the motor's FLA, not the required conductor ampacity.
- D. This is 150% of the motor's FLA, which is higher than the minimum required.
Motor Branch-Circuit Conductor Sizing
NEC 430.22(A) requires that motor branch-circuit conductors for continuous duty motors have an ampacity not less than 125% of the motor's full-load current (FLA) to ensure they can safely carry the motor's running current and accommodate minor overloads.
- Conductor ampacity is calculated as 125% of the motor's FLA for continuous duty motors.
- This sizing accounts for normal operating conditions and minor, sustained overloads.
- The FLA is typically obtained from the motor nameplate or NEC Tables 430.247 through 430.250.
Memory trick: For Continuous Duty, Conductors need a Quarter More power.