Integral Unit (Chiller w/ VFD) Input Conductor Sizing

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NoVA Comms Power

Senior Member
Location
Alexandria, VA
Hello all:

I've been an "active lurker" and infrequent poster for many years (and have attended Mike's seminars in person)

My question deals with a VFD and motor integrated into a single chiller by a large, well-known, and reputable chiller manufacturer

All amperage values are approximate as the actual values are irrelevant to my actual question.

Equipment Detail:
Motor Nameplate FLA: 392 A
VFD Nameplate Input: 780 A

(Note that both nameplates are mounted on the unit)

One school of design thought is that the branch conductors led to the VFD should have an ampacity of 975A (1.25 x the VFD's "nameplate" 780) per NEC 430.122.

However, the manufacturer insists -- after repeated queries -- that the ampacity of branch conductors need only be 490A (1.25 * 392) based on their interpretation of NEC 440.33.

The manufacturer has also declined to "remove" the nameplate of the VFD (i.e. so have single "common" nameplate for the entire integrated chiller)

The branch conductor design is at a bit of a quandary over which way to proceed.

Q: What IS the "right" interpretation?

Q: Specifically, does specific guidance by the manufacturer of an integrated unit take precedence over the individual "unintegrated" component nameplates?

I hope that we might obtain some true "3rd party" interpretation of the NEC from this forum.

Looking forward to any and all replies and to the discussion this question might generate.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Why would they have two nameplates?

Can you post pictures of them?

However, I would suggest they could well be right based on 440.33.

440.33 Motor-Compressor(s) With or Without Additional
Motor Loads. Conductors supplying one or more
motor-compressor(s) with or without an additional load(s)
shall have an ampacity not less than the sum of the rated load
or branch-circuit selection current ratings, whichever
is larger, of all the motor-compressors plus the full-load
currents of the other motors, plus 25 percent of the highest
motor or motor-compressor rating in the group.

This provision says the ampacity required is based on the motor FLA.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Hello all:

I've been an "active lurker" and infrequent poster for many years (and have attended Mike's seminars in person)

My question deals with a VFD and motor integrated into a single chiller by a large, well-known, and reputable chiller manufacturer

All amperage values are approximate as the actual values are irrelevant to my actual question.

Equipment Detail:
Motor Nameplate FLA: 392 A
VFD Nameplate Input: 780 A

(Note that both nameplates are mounted on the unit)

One school of design thought is that the branch conductors led to the VFD should have an ampacity of 975A (1.25 x the VFD's "nameplate" 780) per NEC 430.122.

However, the manufacturer insists -- after repeated queries -- that the ampacity of branch conductors need only be 490A (1.25 * 392) based on their interpretation of NEC 440.33.

The manufacturer has also declined to "remove" the nameplate of the VFD (i.e. so have single "common" nameplate for the entire integrated chiller)

The branch conductor design is at a bit of a quandary over which way to proceed.

Q: What IS the "right" interpretation?

Q: Specifically, does specific guidance by the manufacturer of an integrated unit take precedence over the individual "unintegrated" component nameplates?

I hope that we might obtain some true "3rd party" interpretation of the NEC from this forum.

Looking forward to any and all replies and to the discussion this question might generate.
I don't have my code with me and 440 is not something I know well. But is there something in that section that says it supersedes all other code requirements? If not, 430.22 would prevail, because it has a VFD. I would think that if they were really concerned, they would hide the VFD nameplate inside of the chiller and use an equipment nameplate on the outside that unequivocally gives you the MCA and MOCP values so that you can use those and have good cause. That's what most chiller mfrs do by the way.

Or, if they did do that but you looked deeper into it, then you may have created your own dilemma here.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I don't have my code with me and 440 is not something I know well. But is there something in that section that says it supersedes all other code requirements? If not, 430.22 would prevail, because it has a VFD. I would think that if they were really concerned, they would hide the VFD nameplate inside of the chiller and use an equipment nameplate on the outside that unequivocally gives you the MCA and MOCP values so that you can use those and have good cause. That's what most chiller mfrs do by the way.

Or, if they did do that but you looked deeper into it, then you may have created your own dilemma here.

440.1 Scope. The provisions of this article apply to electric
motor-driven air-conditioning and refrigerating equipment and
to the branch circuits and controllers for such equipment. It
provides for the special considerations necessary for circuits
supplying hermetic refrigerant motor-compressors and for any
air-conditioning or refrigerating equipment that is supplied
from a branch circuit that supplies a hermetic refrigerant
motor-compressor.

It appears to me that they can take advantage of 440.33, even though it seems a bit odd.
 

NoVA Comms Power

Senior Member
Location
Alexandria, VA
Some Amplification

Some Amplification

Thanks to all who have replied so far:

I suspected that this topic might generate some "active discussion"
;)


I'll try to get some nameplate pics posted later today.


RE:
"... It appears to me that they can take advantage of 440.33, even though it seems a bit odd... "

For amplification ... in THIS particular application, the chiller uses a semi-hermetically-sealed motor and compressor assembly ... (meaning that the motor casing, while normally sealed and refrigerant-cooled while in operation, can be opened and repaired by unbolting flanges)

... and I'd like keep this discussion focused on this particular problem.

(That said, depending on how this thread pans-out I might start a future thread discussing the same question as it applies to non-hermetically sealed chillers.)

 

NoVA Comms Power

Senior Member
Location
Alexandria, VA
Other Articles

Other Articles

Re: "... But is there something in that section that says it supersedes all other code requirements? If not, 430.22 would prevail, because it has a VFD ... "

NEC 440.3 (Other Articles)
(A) These provisions are in addition to, or amendatory of, the provisions of Article 430 and other articles in this Code, which apply except as modified in this article.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Re: "... But is there something in that section that says it supersedes all other code requirements? If not, 430.22 would prevail, because it has a VFD ... "

NEC 440.3 (Other Articles)
(A) These provisions are in addition to, or amendatory of, the provisions of Article 430 and other articles in this Code, which apply except as modified in this article.
Yeah, I just read that now that I'm back in my office. Looks like they are right.
Also, the NEC goes on to make it VERY clear in 440.6 (A);
(A) Hermetic Refrigerant Motor-Compressor. For a hermetic
refrigerant motor-compressor, the rated-load current
marked on the nameplate of the equipment
in which the
motor-compressor is employed shall be used in determining
the rating or ampacity of the disconnecting means, the
branch-circuit conductors, the controller, the branch-circuit
short-circuit and ground-fault protection, and the separate
motor overload protection
. Where no rated-load current is
shown on the equipment nameplate, the rated-load current
shown on the compressor nameplate shall be used.
Learned something new today.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Why is the VFD nameplate twice the motor nameplate? I can't believe they used a VFD twice as large as necessary on a motor that size.

I assume everything is rated at 480 volts?

IMO, its a packaged unit, and you should use the unit nameplate. The VFD nameplate would only apply if the VFD were installed as an individual piece of equipment.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I am guessing that the supplier got lazy and didn't provide a nameplate at all. They provided a collection of equipment each with its own nameplate.

Something to investigate: is the motor 'supercharged' or whatever the slang term is, where a 230/460 motor is connected as 230V, but supplied from a 480V inverter to permit operation at higher than 60Hz? If this is the case, then the inverter is _not_ oversized, but the motor would be expected to use current at the 230V levels...

-Jon
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Why is the VFD nameplate twice the motor nameplate? I can't believe they used a VFD twice as large as necessary on a motor that size.

I assume everything is rated at 480 volts?

IMO, its a packaged unit, and you should use the unit nameplate. The VFD nameplate would only apply if the VFD were installed as an individual piece of equipment.
As long as it really is a unit nameplate and not just a copy of the motor nameplate....
 

Besoeker

Senior Member
Location
UK
Why is the VFD nameplate twice the motor nameplate? I can't believe they used a VFD twice as large as necessary on a motor that size.
I wondered about that too. Not only that, it's VFD input current that's given. The rated output current is usually significantly higher than that. So more than twice the motor rating.
 

Knightryder12

Senior Member
Location
Clearwater, FL - USA
Occupation
Sr. Electrical Designer/Project Manager
Why is the VFD nameplate twice the motor nameplate? I can't believe they used a VFD twice as large as necessary on a motor that size.

I assume everything is rated at 480 volts?

IMO, its a packaged unit, and you should use the unit nameplate. The VFD nameplate would only apply if the VFD were installed as an individual piece of equipment.

I would suspect it would because the manufacturer of the chiller only wants to stock a limited supply of sizes of VFD's. I would be under the impression that since it is a package unit then you design around the equipment's nameplate and not the VFD's nameplate as there is no way that they could upsize the motors on the chiller to get more capacity, which I believe is the reason why we design around the VFD's rated input when the motor and the VFD are separate items.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
I wondered about that too. Not only that, it's VFD input current that's given. The rated output current is usually significantly higher than that. So more than twice the motor rating.

Its almost like the VFD input voltage is 208V, and the output voltage (and motor voltage) are 480V.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
I wondered about that too. Not only that, it's VFD input current that's given. The rated output current is usually significantly higher than that. So more than twice the motor rating.

Its almost like the VFD input voltage is 208V, and the output voltage (and motor voltage) are 480V.

Are there VFD's that can take either 208 or 480V as the input, and provide the correct 480V output for the motor?
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Its almost like the VFD input voltage is 208V, and the output voltage (and motor voltage) are 480V.

Are there VFD's that can take either 208 or 480V as the input, and provide the correct 480V output for the motor?
No, so far you cannot create voltage that is not there to start with without a transformer, at least not above about 1HP (using a voltage doubler circuit on the front end of the drive).

The other way (480V input to a drive to run a 208V motor)? Yes, although its a really bad idea.

Several of the Japanese mfrs, Mitsubishi for one, really only make a single drive that is a "400V class", which accepts anywhere from 380-480V input. But when they get them UL listed, they interpret the rules (or don't want to pay extra) to mean that it must be listed for a particular HP regardless of the voltage rating. So the drives end up looking over sized for our motors, which can cause issues with conductor sizing.

But this is too much of a difference to be that issue. I agree with Knightryder12, it must be that the OEM wants to stock one VFD for any voltage, rated for the highest voltage, sized for the lowest. It only makes sense if you are intent on trading convenience for component cost.
 

Besoeker

Senior Member
Location
UK
No, so far you cannot create voltage that is not there to start with without a transformer, at least not above about 1HP (using a voltage doubler circuit on the front end of the drive).
Sorry to disagree, but yes you can.
And we have.

My basic equations for step up choppers:

Stepupchopper01_zpsf57870f3.jpg

We have used these on VSDs up to several MW.

Specials of course.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Sorry to disagree, but yes you can.
And we have.

My basic equations for step up choppers:

View attachment 11746

We have used these on VSDs up to several MW.

Specials of course.
Yes, but what you do is beyond the realm of what most people experienc day to day as electricians. And that is really difficult to find. I recently needed a boost converter for 30HP motors where the only available input was 240V single phase, but the only motors that could do the job were 480V servos (our VFDs can run servos). It was for an OEM who would buy about 100/year. I tried dozens of sources, no takers. I should have thought of contacting you...

They ended up finding a third-tier servo company in China that would make them a 240V servo motor that big. If it ends up failing and they come back to me, I'll get hold of you...
 

tesi1

Member
Location
florida
chiller vfd mocp-mca

chiller vfd mocp-mca

we have done a number of water cooled chillers & the newer frictionless motor chillers with a vsd or vfd factory mounted controllers, they always put the chiller
fla & mca tag on one end, and a different tag for the vfd on the vfd on the other end, it always seem like the vfd's are way larger than needed for the motor which
they serve which leaves you in a problem with the circuit and breaker selection. the one thing we have found is even though the internal vfd disconnect-breaker
have always been of the adjustable type with the breaker's setting, set or programed at the factory, if not then the chillers factory tech doing start up should catch this
during his pre startup check list, & then reset & reseal the breaker to the proper settings as per code.. we always seem to end up fighting the the consulting engineer & owner over this.:
 
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