Journeyman Electrician Exam (NEC)Control Devices and MotorsHard
A variable frequency drive (VFD) is controlling a 20 hp, 460V, three-phase motor. The VFD's input power is supplied by a branch circuit. According to NEC 430.122(A), what is the minimum required ampacity for the conductors supplying the VFD, assuming the VFD is rated for the motor's full-load current (FLA) of 25A?
- A43.75 A
- B25 A
- C37.5 A
- D31.25 A
Show answer & explanationAnswer & explanation
Correct answer: D. 31.25 A
According to NEC 430.122(A), conductors supplying a variable frequency drive (VFD) must have an ampacity not less than 125% of the VFD's input current rating. Since the VFD is rated for the motor's FLA of 25A, we use 25A as the basis for calculation. Therefore, 25A * 1.25 = 31.25A.
Why the other options are wrong
- A. This is 175% of the VFD's input current, which is significantly higher than the minimum required.
- B. This is the motor's FLA/VFD's rated input current, not the required conductor ampacity.
- C. This would be 150% of the VFD's input current, which is higher than the minimum required.
VFD Input Conductor Sizing (NEC 430.122(A))
NEC 430.122(A) specifies that conductors supplying the input side of a variable frequency drive (VFD) must have an ampacity not less than 125% of the VFD's rated input current, to account for harmonics and continuous operation.
- The 125% rule applies to the VFD's rated input current, not directly to the motor's FLA.
- This sizing accounts for harmonic currents generated by the VFD.
- The VFD's input current rating is typically found on its nameplate or documentation.
Memory trick: VFD input: A Quarter more power for its fancy dance.