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480 VAC VFD Power - Do I need an Enclosure & Disconnect?

Question asked by pwk@kraemerkoating.com on Sep 26, 2020
Latest reply on Oct 3, 2020 by jmueller245

Hi,

 

I work for an equipment manufacturer.  For a machine we are building, I have a VFD for an Exhaust Blower that requires 480 VAC 3-phase 6 amps.  The VFD is designed so that it can be mounted on a wall without having to be inside an electrical enclosure.  Typically, the exhaust blower is located inside a building.  In these cases, I provide a small electrical enclosure with a disconnect switch that I mount on the framework that supports the blower.  I mount the VFD externally on the side of the enclosure.  Incoming power is run into the enclosure and landed at the disconnect.  After the disconnect, power passes through fuses and goes out of the enclosure to the VFD.  Aside from the 480 VAC fuses and the disconnect inside the enclosure, I also have one additional fuse, a relay, and a few terminal blocks to support some 24 VDC I/O related to the Exhaust Blower controls.  Being the enclosure is so close to the blower, I do not use an additional disconnect between the VFD and the Blower Motor. 

 

On this new project, the exhaust blower will be located outside.  Rather than having to obtain a weather-proof enclosure, my customer has told me they will have a climate controlled room within 10-15 feet of the blower location in which I can put my VFD and any other components related to the Exhaust Blower.  In addition to my Exhaust Blower controls, this room will contain other controls that support unrelated equipment.  Being this room might typically be locked and because it is 10-15 away from the Exhaust Blower, I am planning to provide a disconnect at the blower location to make/break the VFD output power to the Blower Motor.

 

With the knowledge that this equipment will be in the United States (New Jersey), I have looked through NFPA 79 (Electrical Standard for Industrial Machinery), and I am still not sure how to answer the following questions:

 

  1. Am I required to have a disconnect switch for the incoming power circuit (480 VAC)?  Or is it allowable to simply land the incoming power circuit at finger safe fuses, relying on the fuses as the means by which someone would remove power from the VFD?
  2. Do the 480 VAC fuses (and the disconnect if I need one) have to be in an enclosure?  Or is it allowable to mount these and my 24 VDC components (fuse, relay, and terminal blocks) on an open, wall-mounted panel?

 

Any advice will be greatly appreciated.

 

Thanks and best regards,
Paul

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