Multiple VFD feeds from Single Disconnect

[JimmyDeanxs]

Hey All,

Looking for a gut check on the a specific installation that came through. There is (1) main disconnect for a piece of equipment that provides OCPD and feeds (2) VFDs downstream. The VFDs load side is connected to a disconnect switch, which provides OCPD to a distribution block that feeds manual motor protectors. Sizing the conductors for the VFD feeder is straightforward per 430.122. However, it seems the waters get muddy when it comes to the OCPD of this disconnect. I understand that the manufacturer provides a minimum and maximum for their drives, but for this specific application there doesn't seem to be any guidance. My knee-jerk is to look at 430.131 which refers me to 430.53, but the OCPD would then be based on motor load at that point. This results in a conductor size on the line side of the VFD and a seemingly insufficient OCPD rating for the VFD. I have seen a couple of instances where the feeder OCPD (disconnect OCPD upstream of the line side of the VFDs) is determined by taking the FLA of one VFD and multiplying by 1.75 and adding the FLA of the other VFD (ex. 100HP -> 124FLA, 124*1.75+124A, as a maximum).

Looking for any advice or past experiences.

 

[petersonra]

What does the VFD manufacturer say?

A lot of VFDs seem to suggest a maximum OCPD rating of about 250% of the input current.

The code does not preclude having two VFDs fed from the same OCPD as long as the rating of the OCPD does not exceed the rating allowed by code or the manufacturer's instructions.

Here is a typical PF525 chart showing what you can do. Note that 140Ms can be used as SC and OL protection for the drive or you can use a specified fuse with the specified maximum rating. In my opinion, the disconnect if a feeder and the feeder size is not determined by the VFD input rating. You can make whatever you want as long as it meets the feeder minimum ratings found in the code, and the tap rules.

For instance you could have a 100 A OCPD feeding a couple of branches of 5 Amp fuses that protects an individual VFD. The 100A feeder would need conductors rated for 100 A and the taps to the 5 AMP fuses would have to meet the tap rules of either 10% or 33% ampacity of the 100 A OCPD.

 

[JimmyDeanxs]

Thanks for the response. These specific drives are Allen Bradley 753 drives and unfortunately they do not provide guidance on what should be done without first having a contract #, which can be difficult to provide if this product is being ordered through a separate controls contractor. For the record, the VFDs will have fused disconnects that are sized by Rockwell, but it still doesn't help for the situation upstream in my opinion per code. This specific application would be (2) 100HP drives under one disconnect.

 

[petersonra]

That would be a feeder circuit. There are rules about sizing feeder circuits, but the basic rule is you can make them as big as you want as long as you abide by the tap rules and protect the conductors.

I would be inclined to put in a feeder OCPD of 300 Amps and run (2) 1/0 conductors per phase. You could use a properly rated OCPD and run a single conductor, but 1/0 conductors tend to give you the most ampacity per $.

AB is not going to tell you what to do with your feeder.

 

[Jraef]

430.110 addresses sizing of disconnects when there are multiple loads behind one disconnect:

(2) Ampere Rating. The ampere rating of the disconnecting
means shall not be less than 115 percent of the sum of all
currents at the full-load condition determined in accordance
with 430.110(C)(1).

I have to say though that this part of your original statement bothers me:

The VFDs load side is connected to a disconnect switch, which provides OCPD to a distribution block that feeds manual motor protectors.

You should not have a disconnect on the load side if it can be avoided, and likewise unless there are multiple motors behind each VFD, the manual motor protectors would be unnecessary and unwise.

 

[JimmyDeanxs]

430.110 addresses sizing of disconnects when there are multiple loads behind one disconnect:



I have to say though that this part of your original statement bothers me:

You should not have a disconnect on the load side if it can be avoided, and likewise unless there are multiple motors behind each VFD, the manual motor protectors would be unnecessary and unwise.

That is a customer specification item driving the redundant disconnects and typically we include manual motor protectors because 95% of the time there are 4-6 fans being controlled by a VFD.

 

[Jraef]

…there are 4-6 fans being controlled by a VFD

Ah, well that’s totally legit then. The disconnect not so much, but as long as you use an aux contact to shut down the VFD before the contacts open, it should be fine.