Group Motor Installation with IP65 VFDs

[nayan_r]

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?

 

[augie47]

oops Dennis, I closed the duplicate so I reopened this.

 

[Dennis Alwon]

oops Dennis, I closed the duplicate so I reopened this.

I wondered what was going on. :grin:

 

[skeshesh]

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?

I would specify the VFD with factory-installed disconnects using circuit breakers which would take care of your disconnect and SC/GP requirements and overload protection is internal to the VFD and then size the conductor to the sum of VFD inputs x 1.25 - The OCPD protecting this conductor can be sized as a regular feeder since there's no in-rush effect to account for.

 

[nayan_r]

I would specify the VFD with factory-installed disconnects using circuit breakers which would take care of your disconnect and SC/GP requirements and overload protection is internal to the VFD and then size the conductor to the sum of VFD inputs x 1.25 - The OCPD protecting this conductor can be sized as a regular feeder since there's no in-rush effect to account for.

So you're saying I can size the circuit breaker to protect all the drives, without the need of an MMP?

 

[skeshesh]

So you're saying I can size the circuit breaker to protect all the drives, without the need of an MMP?

I don't see any reason why you need to. You already have motor starting and overload protection means in your VFD. The disconnect means and SC/GR protection you can satisfy with breakers easily. In fact even if you did not have a VFD you could still do so. You seemed really occupied with having to use a "MMP", why is that?

 

[nayan_r]

I don't see any reason why you need to. You already have motor starting and overload protection means in your VFD. The disconnect means and SC/GR protection you can satisfy with breakers easily. In fact even if you did not have a VFD you could still do so. You seemed really occupied with having to use a "MMP", why is that?

I must admit my understanding is piecemeal about this. I've seen examples online of Group Motor Installation GMI (note: showing contactors not VFDs) and each one had an MMP. I thought this would be necessary for the SC protection for each VFD.

Maybe you can clarify a few things.
Assume we have 5 motors each 1 HP. The VFD recommends a 10A circuit breaker.

So the VFD basically protects the motor from overload. We just need to size the circuit breaker and the conductor for protecting the VFDs.

The conductor would be sized for 50Ax1.25?
The breaker would be sized also 50Ax1.25

 

[Jraef]

For EACH VFD circuit you must have a branch circuit protective device and locking disconnect means. The VFD provides the motor thermal overload protection and the motor short circuit protection, so the upstream devices are only to protect the conductors feeding it.

If you are in the US, you cannot use the MMPs as branch circuit protection ahead of VFDs. You can only use them directly ahead of motors, but you would be required to use ANOTHER listed BOCP device ahead of the MMP for each VFD circuit, rendering the MMP pointless. Those "Group Fusing" applications are for when you DON'T have another device between the motor starter and the motor, because the MMP is ONLY listed and a motor starter.

You must size the conductors feeding the VFD for 125% of the VFD's max input amp rating, not the motor's. Then you size the circuit breaker ahead of it to protect conductors.

If those VFDs are UL listed to require a 10A maximum circuit breaker, you must use that or a 10A fuse. The problem is, 10A CBs are a little hard to find, most start at 15A. If they just recommend a 10A circuit breaker, take into consideration that most systems require 14ga minimum wiring size anyway, so a 15A breaker would be fine. Don't worry about protecting the VFD with the smaller breaker, no breaker on the planet can act fast enough to protect the VFD from damage, their purpose is to protect OTHER parts of the system from a bad VFD.

But as skeshesh suggested, if you have multiple distributed drives like this I would look at the VFDs that have the disconnect (and usually fuses) built-in to the VFD package. Much simpler installation.

 

[skeshesh]

... I would look at the VFDs that have the disconnect (and usually fuses) built-in to the VFD package. Much simpler installation.

Do you usually specify fused disconnects? Is there a reason why it's a better idea to use a fuse in this application?

 

[Jraef]

Do you usually specify fused disconnects? Is there a reason why it's a better idea to use a fuse in this application?

It's not really that I specify it, I'm just stating the reality of whats available out there.

Some people think that high speed (i.e. Class T) fuses will protect the front-end of a VFD, but I have never seen that to be the case. But one thing fuses are better at is current limiting of faults. Because of that, a lot of VFD mfrs (especially those who are not also CB mfrs) don't bother to have their VFDs UL listed with circuit breakers in front of them, they just opt for fuses. It has more to do with the UL listing process; it's easier to attain an SCCR rating with fuses than breakers. Mfrs who make both VFDs and CBs, like Siemens, Schneider, ABB etc. have a vested interest in paying for the package testing, but those who don't just settle on using fuses.

So for example of you were a VFD mfr and you wanted to sell the combo package, using a CB would mean a destructive test ($20K+) for each and every combination possibility of breaker and drive. If you don't do all those expensive tests, the SCCR rating you end up with is 5kA and that then becomes a problem for the installer to deal with, which limits your sales. But if you use fuses, you can often get away with a maximum frame size test and a good spreadsheet showing the let-through of each size fuse you might use on that frame size. Lower cost, higher SCCR, fewer impediments to sales, better ROI for the VFD mfr.

 

[augie47]

You might want to keep in mind the requirements at the end users location.
Were this piece of equipment you are designing end up at a location in some areas (like TN) and it were to be inspected, a NRTL label would be needed.
The methods you use would, in that case, be dictated by the listing agency, not by NEC.

 

[nayan_r]

Ok, I'm now seeing in the documentation that the VFD does have built in short circuit protection. So it does make sense to use the VFD with the built in disconnect.

Though, I'm confused about the "recommended" circuit breaker/fuse size. The manual says for a 1/2HP motor it "recommends" 14AWG with 10A breaker. It also says the main current to the VFD is 1.5A. Why would they recommend such a large breaker? Is it for inrush current?

So when Jraef refers to VFD's max input amp rating he's talking about the 1.5A not 10A?

 

[skeshesh]

Though, I'm confused about the "recommended" circuit breaker/fuse size. The manual says for a 1/2HP motor it "recommends" 14AWG with 10A breaker. It also says the main current to the VFD is 1.5A. Why would they recommend such a large breaker? Is it for inrush current?

So when Jraef refers to VFD's max input amp rating he's talking about the 1.5A not 10A?

Jraef is talking about the maximum input to the VFD as defined by the manufacturer which would be the the maximum amps output plus the VFD internal losses.
The VFD likely has overload protection against exceeding the 1.5A rating. I believe the recommended breaker is branch circuit protection for the 14AWG wire. Don't confuse the branch circuit protection breaker with the disconnect means breaker or fused disconnect we were discussing earlier. There would be no significant in-rush current at the input of the VFD which is one of the great advantages of using these drives.

 

[@VFDMan]

Dont Forget

Dont Forget

Make sure you do some research on VFD cable as well. With all the harmonics coming off the cable, you'll want to make sure you properly ground the shield. Also, part of the reason most VFD cables are rated at 2000V is due to the mini-voltage spikes inherent with VFD's - use a cable that is rated at 2000V.