Mike Dolt Enterprises
Mike Dolt Enterprises
User: nayan_r, Location: Chapel Hill, NC
Asked on: Friday, March 25, 2011 at 5:25 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?
NEC 430-53 Group Motor Installation Configuration
NEC 430-53 allows for the use of a single disconnecting means and branch-circuit short-circuit and ground-fault protection for a group of motors that are installed in a single location and driven by a single power source. This code applies to both contactors and VFDs used as motor controlling devices.
Sizing of Main Breaker
In a group motor installation, the main breaker must be sized to protect the entire load, including the running current of all the motors and the starting current of the largest motor.In your example, with three 1HP motors and VFDs, each VFD is recommended to have a 10A breaker. However, the main breaker should be sized based on the motor FLA, which is typically higher than the VFD's recommended breaker size.Assuming the motor FLA is 5A, the main breaker should be sized as follows: Running current: 3 x 5A = 15A Starting current (largest motor): 6.5A (assuming 200% FLA)Total current
15A + 6.5A =
21.5A
Therefore, the main breaker should be sized at 25A.
Motor Manual Protectors (MMPs)
NEC 430-52(A)(1) requires that motors be protected against running overloads by an automatic protective device that will interrupt the motor circuit on a sustained overload. While VFDs typically incorporate overload protection, it is not a substitute for a separate overload protection device.Therefore, an enclosed MMP should be installed in front of each VFD to provide the following functions: Disconnect switch Short-circuit/ground-fault protection Overload protection (redundant)Sizing of MMP
The MMP should be sized to protect the motor, not the VFD. In this case, the MMP should be sized for 5A, which is the FLA of the motor.
Additional Considerations The group motor installation must be properly designed and installed in accordance with NEC 430-53, including the use of proper grounding and bonding. The VFDs must be suitable for group motor installations and have features such as common trip functionality. The MMPs must be designed for use in a wet location and meet the requirements of NEC 430-52(A)(1). Flag for review