pkardok
New member
- Location
- norwell, ma, USA
I have a 50A AC motor driven by a VFD with a 150A rating. What size should the breaker feeding the VFD be? 150A or 60A? What are the code instructions?
430.122 Conductors ? Minimum Size and Ampacity.
(A) Branch/Feeder Circuit Conductors. Circuit conductors
supplying power conversion equipment included as
part of an adjustable-speed drive system shall have an ampacity
not less than 125 percent of the rated input current to
the power conversion equipment.
From the 2011 NEC
430.122 Conductors ? Minimum Size and Ampacity.
(A) Branch/Feeder Circuit Conductors. Circuit conductors
supplying power conversion equipment included as
part of an adjustable-speed drive system shall have an ampacity
not less than 125 percent of the rated input current to
the power conversion equipment.
So does this assume that the OCPD is sized according to the conductor size? The way I read it only the conductors need to be sized according to the ampacity of the VFD. How would you size the OCPD?
430.128 Disconnecting Means. The disconnecting means
shall be permitted to be in the incoming line to the conversion
equipment and shall have a rating not less than
115 percent of the rated input current of the conversion
unit.
The OCPD should be sized per 430.52, the same as other motors.
430.120 say "The installation provisions of Part I through Part IX are applicable unless modified or supplemented by Part X." Like Bob, I don't see anything in Part X that modifies the earlier provisions.
I agree but find it odd that both the conductors and the disconnect must be sized per input current yet the breaker sized per the motor.
I am missing the point. :?
I agree but find it odd that both the conductors and the disconnect must be sized per input current yet the breaker sized per the motor.
I am missing the point. :?
What we have feeding the drive needs to have the wire sized and overcurrent protection selected based upon what the drive requires. "Requires" as what it might call for within the limits of the drive. If your drive is oversize (say, a 10-hp drive controlling a 3-hp motor), you still need to supply the drive according to its' nameplate.
Do you have a code citation to support that? It appears to me that Article 430 requires that the overcurrent protection be based on the motor horsepower, per 430.52.
I would suggest it could possibly be a 110.3(B) issue depending on the inverter instructions.