Fuse Sizing for 125 HP VFD Compressor 460V 3 phase

[branchedout]

Ingersoll Rand will not give specific answers on recommended sizing for the breaker or fuses even though they require Class J fast acting fuses to be installed on any VFD compressor.

Here's the details
-460V
-3 phase
-125 HP
-total package amps 186

Just need some help as I keep getting vague answers from IR.

 

[zbang]

Is this a VFD compressor? More specifics would help, and a photo of the nameplate would be great.

 

[branchedout]

It is a VFD compressor. Ingersoll Rand RS90n-A145. What other info would help?

 

[branchedout]

https://flic.kr/p/2qbmEB1

 

[Macbeth]

I see the panel has a SCCR of 5K. Good luck with that.. They may be saying you need "J" fuse knowing is difficult to have a 250a feeder circuit to limit your available Fault Current. Looking this up I see the lowest let through on a Class J is 6k, 1k to much. Nope can't use "J"...

1. Ask if the need for the "J" fuse is a UL requirement (This would also include if the VFD manufacturer requires it) or for supplemental warranty coverage were they are just trying to protect the product for reduced failure.

If it is supplementary protection without UL or VFD manufacture requirement any fuse would work as long as it is Not your primary Feeder Circuit Protector. Assuming there is a CB "OCPD". I would use 150% to 300% since it is a VFD staying to 150% should be fine..

If they a requiring it as need by UL or VFD manufacture they must supply you with

"shall be marked with the type and size of feeder circuit protection required to be installed in the field. This marking shall be included as part of the marking"

 

[petersonra]

You cannot use the let thru rating of a fuse for determining available short circuit current.

It is marked as 5kA SCCR so that is what it is. You have to find a circuit somehow that can feed a 125 HP compressor with less than 5kA of available short circuit current. Good luck with that.

 

[ptonsparky]

Line reactor and enough wire length of minimum size.

 

[Macbeth]

You cannot use the let thru rating of a fuse for determining available short circuit current.

This is true. But if the manufacture supplies additional markings that include additional field devices to raise the SCCR, then the Panel SCCR can be raised with the additional Field Devices. I don't see the marking, so it can not be raised.
I was wondering if they forgot to update their name plate..

 

[augie47]

Unless a higher SCCR was mandated when the unit was purchase you have an all to common situation.
5k is often the manufacturers standard resulting in the problem being on the customer/installer shoulder.

 

[petersonra]

This is true. But if the manufacture supplies additional markings that include additional field devices to raise the SCCR, then the Panel SCCR can be raised with the additional Field Devices. I don't see the marking, so it can not be raised.
I was wondering if they forgot to update their name plate..

Unless it is marked that you get a higher SCCR rating with certain external fuses, it does not count. And you can't field Mark it.

 

[Jraef]

This is true. But if the manufacture supplies additional markings that include additional field devices to raise the SCCR, then the Panel SCCR can be raised with the additional Field Devices. I don't see the marking, so it can not be raised.
I was wondering if they forgot to update their name plate..

They didn't think it through that far. 5kA is the "courtesy" SCCR value that UL allows someone to put on the nameplate without even a tiny amount of mental effort put to it. Attaining MUCH higher values is not in the least bit difficult, they are just not interested in trying to make life easier for their customers. Shows a lack of respect IMHO...

By the way, I recently fount this site that provides the max. fault currents for different sizes of transformers. On a 480V system, in order to keep the Available Fault Current at 5kA, your transformer can't be larger than 75kVA. But a 75kVA transformer cannot possibly run a 125HP compressor! So they created an impossible situation.

branchedout,
Part of the UL listing of VFDs requires that they be tested with OCPDs and the type and maximum size of those OCPDs be listed in their instructions. So SOMEWHERE, IR has that information available. Find out what brand of VFD they are using, then download the technical data on that unit to get the values it is listed with. It's not going to help your impossible SCCR situation, but that would at least answer your specific initial question.

For comparison purposes though, a Rockwell drive of that size (186A) can have fuses that are MINIMUM 250A, max. 400A, and a circuit breaker that is Max. 500A. But just to warn you, a lot of VFD manufacturers do NOT list their VFDs behind circuit breakers (it's expensive and many do not make circuit breakers). It sounds as though might be the case here: Class J fuses are pretty common when they don't. So you still CAN use a breaker, as long as you ALSO have the right fuses.