Mike Dolt Enterprises
Mike Dolt Enterprises
User: nayan_r
Asked on: Friday, March 25, 2011 at 5:31 pm EDT
Answered on: Tuesday, May 14, 2024 at 3:17 am EDT
Question: I am designing an application with field mountable IP65 VFDs. I'm looking at using a Group Motor Installation configuration per NEC 430-53. Does this code only apply to Contactors as the motor controlling device or would VFDs apply as well? If it does, how would you size that main breaker.e.g. I have three 1HP motors and VFDs on field. The VFD recommends a 10A breaker. Would the main breaker be 30A? Or would I size it according to the motor FLA?In front of every VFD I was thinking of having an Enclosed MMP (Manual Motor Protector) to cover the requirements for 1) Disconnect, 2) Short Circuit/Ground Protection, 3) Overload (redundant since VFD is overload protected). Again should this be sized by the 10A VFD requirement or the motor FLA?Any thoughts/suggestions on doing a decentralized VFD design that allows the power to be daisy changed from VFD to VFD?
NEC 430-53 Group Motor Installation
NEC 430-53 applies to any motor controller, including contactors and VFDs. It allows multiple motors to be connected to a single branch circuit provided the following conditions are met: All motors are on the same branch circuit. All motors are of the same horsepower rating. The sum of the nameplate current ratings of all the motors does not exceed the branch circuit rating.
Main Breaker Sizing
For a group motor installation with VFDs, the main breaker would be sized based on the nameplate current ratings of all the motors, not the VFD current ratings.In your example, you have three 1HP motors. Assuming they have a nameplate current rating of 4.5A each, the main breaker would be sized at 13.5A (4.5A x 3).
Enclosed MMPs (Manual Motor Protectors)
The Enclosed MMPs must meet the following requirements: Disconnect
Serve as a means to disconnect the VFD in case of maintenance or emergency.
Short Circuit/Ground Protection
Provide protection against short circuits and ground faults. This can be achieved through a circuit breaker or fuse integral to the MMP.
Overload
Can be redundant since VFDs typically have built-in overload protection. However, it is recommended to have the MMP provide overload protection as a backup.The MMP should be sized based on the
nameplate current rating of the motor, not the VFD current rating.
Decentralized VFD Design
Daisy chaining power from VFD to VFD is not recommended as it can create ground loops and other electrical hazards. Instead, each VFD should be powered separately from a dedicated branch circuit.
Additional Considerations Ensure that the VFDs are installed in a field-rated enclosure and protected from the elements (IP65). Use a shielded motor cable to prevent electromagnetic interference (EMI). Provide appropriate ventilation for the VFDs to prevent overheating. Consult a qualified electrician to design and install the VFD system according to applicable codes and standards. Flag for review