Mike Dolt Enterprises
Mike Dolt Enterprises
User: nayan_r, Location: Tennessee
Asked on: Friday, March 25, 2011 at 5:26 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?
Group Motor Installation per NEC 430-53
NEC 430-53 applies to motor installations controlled by a single controller, such as a contactor or VFD. In your case, since VFDs are being used, the code applies.
Main Breaker Sizing
According to NEC 430-53(B)(1), the main breaker for a group motor installation shall be sized for the sum of the motor nameplate full-load currents (FLCs), multiplied by 1.25 (fuses) or 1.4 (circuit breakers).In your example: Motor FLC (1 HP motor) = 2.2 A Number of motors = 3 Main breaker size = (3 2.2 A) * 1.4 = 9.24 A
You would need to select the next standard breaker size, which is 10 A.
Motor Branch Circuit Protection
NEC 430-52(A)(1) requires branch-circuit protection for each motor. Since VFDs provide overload protection, a manual motor protector (MMP) can be used to serve as the disconnect and provide short-circuit/ground fault protection.
MMP Sizing
The MMP should be sized for the motor FLC, not the VFD requirement. In this case, the MMPs should be rated for 2.2 A. Flag for review