Journeyman Electrician Exam (NEC)Control Devices and MotorsMedium
A variable frequency drive (VFD) is used to control a 25 hp, 480V, three-phase motor. The VFD's output conductors to the motor are being sized. The motor has a nameplate FLA of 34A. According to NEC Article 430.122(A), what minimum ampacity is required for the conductors between the VFD and the motor?
- A47.6A
- B38A
- C34A
- D42.5A
Show answer & explanationAnswer & explanation
Correct answer: D. 42.5A
NEC 430.122(A) states that conductors connecting the output of a variable frequency drive to a motor shall have an ampacity not less than 125% of the motor's full-load current rating as determined by 430.6(A)(1) or 430.6(A)(2). In this case, the motor FLA is 34A. Therefore, 34A * 1.25 = 42.5A.
Why the other options are wrong
- A. This value (140% of FLA) is higher than the minimum required by NEC 430.122(A).
- B. This value is less than the required 125% of FLA.
- C. This is the motor's FLA, which is not sufficient for VFD output conductors.
VFD Output Conductor Sizing (NEC 430.122(A))
Conductors connecting the output of a variable frequency drive (VFD) to a motor must have an ampacity not less than 125% of the motor's full-load current rating.
- Applies to conductors between VFD and motor.
- Minimum ampacity is 125% of motor FLA.
- Different from VFD input conductor sizing.
- Accounts for potential harmonic distortion and motor heating.
Memory trick: VFD's output needs a 'quarter-extra' current capacity.