AC equiptment and disconnecting means

[ox110]

I have self contained AC unit with a FLA of 85 amps. This unit is supplied from 100a three pole breaker in the cellar level two floors below the AC unit.
There was a 100a non fused switch installed within sight of the unit. This switch was removed and replaced with a VFD that has its own on/off switch.
The VFD now feeds the self contained AC unit, however the VFD is only supposed to control the fan and NOT the compressor. One idea is to tap the line side of the VFD with full size wire and feed a new switch that will be mounted on the unit and feed the compressor portion of the unit. We will rework the internals to seperate the feeds. My concern is that now there will be two service switches. The only simultanious disconnect will be from the 100a breaker located in the cellar. The VFD and new switch will be upto 10 feet away from eachother so I certainly would not consider them grouped.
Would this instlation be acceptable?

 

[Caesium]

My gut says no. You should have a switch before the VFD.

If you do still have a switch on the unit I would have a sign pointing back to the disconnect in sight of the unit... (Hope that makes any since)

 

[ox110]

caesium- I dont think you are understanding the question. The question is referring to the fact that I will have two disconnect switches suppling the same peice of equiptment with the only simultanious means for disconnect would be a 100a breaker two floors away. Also the VFD controller is allowed to serve as a service for the unit.

 

[kwired]

I bet that was one expensive drive compared to the one that was needed.

entire unit 85 amps - blower only, probably 10 amps or less

Why not re-install the removed 100 amp switch ahead of everything and then you would have a single disconnect for everything?

Reading through art 440 Part II multiple disconnects serving one unit is not mentioned. It is not specifically prohibited either. All that is mentioned is disconnecting means is to be provided does not even indicate whether disconnecting means is or can be singular or plural.

 

[iwire]

I bet that was one expensive drive compared to the one that was needed.

Ditto on that.

Not to mention that a self contained unit that has fan motors and compressors will usually have relays, contactors, transformers and maybe a PC board, none of which can be 'legally' supplied from the load side of a VFD.

I think kwireds suggestion is the way to go.

Why not re-install the removed 100 amp switch ahead of everything and then you would have a single disconnect for everything?

 

[GlennH]

I have a question about what is controlling the VFD and who's idea is it to do this and why?

Reducing air flow across a DX coil is like running the unit with clogged filters isn't it?

Just curious.

 

[iwire]

I have a question about what is controlling the VFD and who's idea is it to do this and why?

Reducing air flow across a DX coil is like running the unit with clogged filters isn't it?

Good questions, VFDs are used more and more to control fan speeds of HVAC equipment but of course the person doing the programing must be aware of the limits of doing so.

You cannot slow the fan down to much without risk of overheating a heat exchanger or freezing up an evaporator coil. (Or maybe even allowing liquid refrigerant to make its way back to the compressor which is very bad.)

It sure sounds like this installation is being planed by someone that may not know everything they should.

 

[Mgraw]

I would guess the manufacturer of the AC unit calls for one disconnect. They usually do on package units. Look at the wiring diagram for the unit. Also as others have said lowering the speed of the blower motor is not a good idea.

 

[kwired]

I have a question about what is controlling the VFD and who's idea is it to do this and why?

Reducing air flow across a DX coil is like running the unit with clogged filters isn't it?

Just curious.

It would really depend on the entire setup, if you are running a blower continuously for whatever reason, you could slow it down when there is no call for heat or cooling, or you could vary the speed depending on multiple stages of heating or cooling if that is an option.

I have not seen a speed control on an outdoor coil but could see the use of one with the speed driven by the coil temperature. I have seen outdoor coils with multiple fans with each fan controlled by coil temperature and each one set at different temp so that only one runs if that is all that is needed or they all run if necessary.

 

[ox110]

The VFD is only rated for 25a and the fan is 16a. IT was installed as a mistake
and we did not pick up on this until the AC shows up to look into starting the units (there are actually two units in question). Space is so limited in this 80 year old mechanical room. We will pull the feed coming from the VFD off the block as well as the fan wires and splice them direct. But now need to refeed the unit which will creat the second means of disconnect. One would have to be in this room to see what I am working with. So the Question about the two disconnecting means stiil stands.

 

[iwire]

I work on some large RTUs that control fan speed, but they also have at least four stages of cooling and two stages of heating.

If the demand for heat is low they will run only one stage of heat and slow the fan.

Same in the summer for better humidity control, they will run as few stages of cooling as they can get away with and adjust fan speed to match.

In these cases the fan speed is controlled entirely by programing perimeters.

In other cases in large buildings fan speed will be controlled by static pressure and as the VAV boxes throughout the building open and close the fan speed will change to maintain the static pressure within the programed perimeters.

 

[Mgraw]

Good questions, VFDs are used more and more to control fan speeds of HVAC equipment but of course the person doing the programing must be aware of the limits of doing so.


It sure sounds like this installation is being planed by someone that may not know everything they should.

I agree. Those type units are designed for variable air flow. They could have two speed compressors or multiple compressors or valves on the compressors heads to control the amount of cooling as the demand changes. Just changing a motors speed without the compressor being designed to handle the lower demand could damage the compressor.

 

[ox110]

I am the installer on this job and am not designing anything for them, the GC has there engineers for that. They are trying to fix their screwup and I am just trying to make sure the fix is proper. They also slove things from a desk and are not on the job to see the room and the space in question plus they do not install electrical work so they do not know anything about how much work is involved.

 

[kwired]

The VFD is only rated for 25a and the fan is 16a. IT was installed as a mistake
.

No one caught this when trying to terminate an 85 amp conductor on a 25 amp terminal?

 

[iwire]

We will pull the feed coming from the VFD off the block as well as the fan wires and splice them direct. But now need to refeed the unit which will creat the second means of disconnect. One would have to be in this room to see what I am working with. So the Question about the two disconnecting means stiil stands.

I am not sure the NEC allows more than one for this type of equipment.

For appliances it is directly addressed.

III. Disconnecting Means
422.30 General. A means shall be provided to disconnect each appliance from all ungrounded conductors in accordance with the following sections of Part III. If an appliance is supplied by more than one source, the disconnecting means shall be grouped and identified.

I see nothing in Article 430 or 440 that is similar.

If it was me I would make sure I had one clearly marked disconnecting means within sight of the equipment.

 

[Pierre C Belarge]

It sure sounds like this installation is being planed by someone that may not know everything they should.

I agree with Bob's comment above. I think that someone who knows VFD's, AC wiring and the NEC/codes needs to get involved with this installation. I bet there is more to this than is being explained, and it sounds just plain old wrong to me.

 

[ox110]

The #4 awg actually fit under the termination points. There is no lables on the inside of the cover with the only label being one the is on the side of a contactor and the writing is so small one needs a magnifiying glass to read it plus it is upside down and mostly blocked by its own wiring and plastic housing. Also no paperwork supplied in the carton it came in. So we did what the plans state to do, which is to inslled the VFD that were supplied to us by the mechanical contractor.

 

[Mgraw]

Sounds like someone screwed up. Somebody needs to bite the bullet and spend some money. Put the disconnect where it was and install the VFD by the unit. It does not matter how small the room is it still needs to be done right. You also need to look at what kind of OCPD the VFD may need. I don't understand the need for a VFD on a package unit nor do I understand why there would be a package unit in a mechanical room. I hope there is alot of ventilation in there.

 

[ox110]

Mgraw-ofcourse it needs to be done right thats why the question. The solution is the engineers idea not mine so I am just checking. Yes I agree that the VFD needs its own OCPD. The units are existing and were operating
prior to this renovation. I wonder if the VFD has its own internal OCPD inside, I will check Monday.

 

[Smart $]

The #4 awg actually fit under the termination points. There is no lables on the inside of the cover with the only label being one the is on the side of a contactor and the writing is so small one needs a magnifiying glass to read it plus it is upside down and mostly blocked by its own wiring and plastic housing. Also no paperwork supplied in the carton it came in. So we did what the plans state to do, which is to inslled the VFD that were supplied to us by the mechanical contractor.

My line of thinking is this installation, without the blessing of the ACU manufacturer, would void the warranty.

If manufacturer blessed, I would think they would have a good deal to say about the equipment and the way it is wired in (and controlled). If I designed such a unit option, the VFD would obtain its feed from inside the controller section, ideally with ocpd already in place. That would put the unit back to one feed, one disconnecting means.