Full Load Amps for VFD or Motor

[Stubby78]

Hello all,

My company designs custom HVAC units that are designed to be used primarily for data centers. We have various designs of different products but for the most part, these units consist of a few motor pumps that pump water throughout some coils, and some EC fans that pull the air across the coils to chill the water inside the coils. There are several damper actuators and other sensors involved as well but those aren't really in question right now.

I am posting this question as this has got our Electrical engineering department divided right now. It is over the topic of how we are calculating the FLA of a custom HVAC unit. My question is, which FLA do we need to use in our load calculations for these units, the FLA of the VFD, or the FLA of the motor itself?

Traditionally, I have always calculated the FLA based on the nameplate data from the motor itself. The OCPD upstream of the VFD and the wire gauge is determined by the FLA of the VFD on the line side. However, some of the engineers I work with are stating that the FLA of the VFD is what should be used in the load calculations. The FLA then helps determine the MCA and the MOP of the units which is required by UL.

My concern with this is that we could potentially oversize the MCA and MOP and leave our electrical components susceptible to damage.

I have been told by a few of our engineers that supposedly this is a new standard to use the FLA of the “conversion device” (the VFD in this case) and not the FLA of the Motor. However, when I question them on this, they can not find the specific standard that states this. In all fairness, I can’t find anywhere that states the other side of the argument either.

If there is anything in the NEC, UL508A, UL1995, or 60335 that state the FLA of the VFD’s should be used, please help point me in the right direction so that we can reference one of these standards moving forward.

 

[charlie b]

If I'm the design engineer and you are the equipment supplier, I need you to give me the information I need to size the circuit components (i.e., the circuit breaker and the branch circuit conductors). The first thing they touch is the VFD. So I will side with those who would do the calculations based on the VFD.

 

[charlie b]

Traditionally, I have always calculated the FLA based on the nameplate data from the motor itself.

Please allow me to mention that we do not use motor nameplate information in our calculations. Instead, we use tables somewhere around 230.250 or so - I don't have my NEC handy.

 

[Stubby78]

Is there any standard that makes you side with the VFD?
I understand that the OCPD, and the wire size connecting to the VFD should be sized per the VFD. The question is based on the loads. Is the VFD consuming the Load or is the motor? Which of those FLAs gets calculated in the unit design.

Please allow me to mention that we do not use motor nameplate information in our calculations. Instead, we use tables somewhere around 230.250 or so - I don't have my NEC handy.

I think you may be looking at this from a standpoint of correctly sizing the conductors and upstream protection to the VFD. I am in total agreement that those must be sized by the data on the VFD.
On the load side of the VFD, I also understand that the conductors connecting to the motor, must be sized using the FLC tables in the NEC.
But where I disagree with your statement above, is the actual load of that specific motors FLA, must be used to calculate the total load of the system.
That is where my question lies. The accurate interpretation of the FLA of the unit will help accurately calculate the MOP and the MCA of the units. if the FLA isn't right, the other two values aren't right.

 

[ptonsparky]

MCA, I'm inclined to 125% of the largest single load plus 100% of the rest for each unit. Use Kw if need be.

The MOP for a drive is in excess of anything we would have used for an ATL starter.

 

[petersonra]

220.40 tells you how to calculate feeder loads.

As a practical matter, the actual amps of the motor will always be more than what the VFD is drawing at any specific point in time because the input to the VFD is at a PF of unity and the power factor out to the motor is inductive.

IIRC, the code actually tells you to use the motor fla to determine the feeder size. That's ok because it will just be more conservative.

Also keep in mind the VFD provides over current protection to the motor, just like an overload does if it was an atl starter. The ocpd feeding the VFD only provides short circuit protection.

 

[Jraef]

IIRC, the code actually tells you to use the motor fla to determine the feeder size. That's ok because it will just be more conservative.

When you are determining the feeder for a control panel with multiple VFDs in it, this is true. Everything is based on the MOTOR size. The VFD is not a “load”, in the same way that a motor starter is not factored in as a load.

The confusion likely stems from the fact that for an INDIVIDUAL drive, 430.122 requires that the minimum conductor size for feeding a VFD is determined by the VFD rated INPUT amps, which is usually lower than the motor FLA, but not always. So if the VFD is oversized for some reason, that can become an issue. But barring that, when determining the feeder for a group of drives, you inherently end up with a conductor size larger than required for an individual drive anyway.

The individual VFD branch OCPD is determined by the MOTOR size, but must also follow the listing information from the VFD installation data, which may list a maximum and a minimum, as well as whether the branch OCPD must be specific fuse or can be a Circuit Breaker or Motor Protector. Rule changes that recently took place make that more important, especially when it comes to SCCR.

 

[eric stromberg]

430.122 states that the conductors feeding the VFD have to be 125% of the RATED input current. Interesting that they do not use "Nameplate" or "FLA." The informational note further states that power electronics (VFDs) can have multiple input ratings, depending on the load. If you can get input rating from the manufacturer, you can use these ratings. For example, if you use a 40 hp motor on a 50 horsepower VFD, and you get the information from the manufacturer on the rating of the 50hp vfd while using a 40 hp motor, you can use 125% of the 40hp rating in order to size the conductor feeding the VFD.

 

[zbang]

Perhaps I'm being dense today, but how does the NEC reach into a piece of equipment? If we're talking about a packaged unit of motors, pumps, controls, etc, the NEC stops with the nameplate of the package (if it says MOP 55 amp, that's what you use). The way I read the question is how does the manufacturer calculate the MCA/MOP for the nameplate, it looks like the method ought to come from UL1995, not directly from the NEC (even if they're similar or identical).

And strangely enough, this has come up before....

Sizing MCA per UL 1995 - Fan Powered HVAC units

Hi everyone, Long time observer, first time poster. I'm in the middle of a discussion with an HVAC rep regarding the MCA and MOCP values for their units. During the design we were not given the MCA and MOCP values so I had to calculate them myself. Now that the submittals are coming in, I'm...

forums.mikeholt.com

There's also

https://colmacwaterheat.com/wp-content/uploads/2020/04/Explanation-of-Electrical-Terms-920817-0001.pdf

and even an old edition of UL1995-

https://law.resource.org/pub/us/cfr/ibr/006/ul.1995.1999.pdf