Nightmare waiting to happen?

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dunk76

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I am contracted to a mining facility to maintain electrical equipment. My problem is a new installation that I believe to be incorrect. This is what I have found. Overhead 4160vac 3ph 3w primary to a 500kva xfrmr fused at what size I don't know. Secondary 460vac 3ph 3w to a 800amp main cb motor control panel. The feeder cable is 6 paralleled 500kcmil conductors and 1 size 1/0 ground in a single 4" conduit. The two motors are 150hp 460vac 3ph 1800rpm 168fla with VFD,s on each. The wire feeding the motors are 3 size 1/0 with a size 4 ground for each in a 1-1/2" conduit. I contend that this installation does not meet code because of derating of paralleled feeder cable in a single conduit and motor cables are undersized. Am I correct?
 

Jraef

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You didn't state the type of wire, but nevertheless I think you may have found some problems.

As I see it, the feeders from the transformer secondary to the main CB should have been derated to 80% for having 6 current carrying conductors in a raceway. Even if it is 90deg.C rated copper THHN or similar, that would be 430A/ conductor, x .8 = 344A per conductor, 688A per phase, protected by an 800A circuit breaker. Your conduit fill is OK, albeit marginal, for the conductors installed, but when you size them properly it will not fit, you will need to run another conduit.

The only caveat is that if those 2 motors are the entire load, then you can make a case that the transformer secondary is a dedicated motor circuit for a 336FLA load, in which case then the only problem is that the MCCB is too big. 336 x 1.25 = 420A, they should have used a 500A CB at the most, in which case the conductors are fine. Are you sure it isn't just an 800A frame with a 500A or even 450A trip setting?

The motor leads should have been at the very least based on 168 x 1.25 = 210A, so again assuming THHN, should have been 3/0. It appears as though they didn't apply Article 430.22 for upsizing the conductors to 125%; 1/0 THHN is rated for 170A. Although not required, I always recommend sizing the motor leads to the max. amp output rating of the VFD, just in case someday someone replaces the motor. So if the VFD is 150HP, probably rated 196A max output, 196 x 1.25 = 245A, I would have suggested using 4/0 THHN.
 
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infinity

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As I see it, the feeders from the transformer secondary to the main CB should have been derated to 80% for having 6 current carrying conductors in a raceway. Even if it is 90deg.C rated copper THHN or similar, that would be 430A/ conductor, x .8 = 344A per conductor, 688A per phase, protected by an 800A circuit breaker. Your conduit fill is OK, albeit marginal, for the conductors installed, but when you size them properly it will not fit, you will need to run another conduit.


If the maximum VFD loads are less than your adjusted ampacity then I don't see a problem with you going up to the next standard sized OCPD which would be 700 amps. The 800 amp CB is too large.
 

Bob NH

Senior Member
Let's start with the idea that the original installer might have known what they were doing.

500,000 VA/(460x1.732)=628 Amps.

So the conductors at 430 Amps, derated to 80% = 344 Amps, appear to be OK. They handle the full capacity of the transformer.

The breaker should be 700 Amps as someone pointed out earlier.

The motor conductors may be small but the motors may be oversized for the load and the capacity of the VFD, which is not known from the original post.
 
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don_resqcapt19

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Trevor,
then I don't see a problem with you going up to the next standard sized OCPD which would be 700 amps.
You can't use the "round-up" rule here.
(C) Transformer Secondary Conductors Each set of conductors feeding separate loads shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.
Don
 

infinity

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don_resqcapt19 said:
Trevor,

You can't use the "round-up" rule here.

Don

You're absolutely right. I totally missed the part about this being a transformer secondary in the OP.
 

Jraef

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That's OK, I completely missed the issue of calculating the transformer capacity.
D'oh!

So it still might possibly be an 800AF 700AT breaker, in which case everything there would be copacetic.

On the motor lead topic, he said the MOTORS are 150HP, not the VFDs. Even if they are "over sized for the load", you cannot use that as a rule to downsize the cables. Cable sizes must be based on the motor nameplate FLA at a minimum. Even if he had, for instance, a 100HP VFD connected to a 150HP motor, there is no rule in the section 430 (that I know of) to allow one to size the cables for the VFD capacity if it is lower than the FLA of the motor. I had to do this once for an emergency retrofit where the VFD and cables were existing but we couldn't get the correct motor in time. We put in the next size up, figuring that the VFD would limit the current to the motor and it would not run at full capacity anyway. It ddn't fly with the AHJ, even though it was temporary. If I'm wrong about that I'd like to know the exception to point to if I get RFd in the future, because I did on that project!

The only valid resizing exercise would be, as I said, to oversize the cables based on the maximum motor FLA that can be connected to the VFD in order to allow for future changes to the motor without having to pull new cables, as long as it would be more than the motor FLA.
 
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dunk76

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In answer to above questions. The drives are rated for 200hp at 240amps each. The wire is all thhn 90c but code calls for using 75c table. The cb does have a 700amp plug in a 800amp frame.

Thank You all.
 

iwire

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dunk76 said:
In answer to above questions. The drives are rated for 200hp at 240amps each.
That really throws a wrench in this.

1/0 is at best rated 150 amps.

430.2 requires VFDs to have incoming conductors rated for the drives input current regardless of the motor current.

These drives require a minimum of a 240 amp conductor, a 250 CU or 350 AL or even two 1/0 ALs.
 

Jraef

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Huh. I had an AHJ interpret that as the input current when adjusted for the connected motor nameplate FLA load, not the maximum nameplate current on the VFD. The reason he said was to account for the losses in the VFD when selecting the feeder.

Now that I read it again, I think maybe he was wrong (and I along with him).
430.2 Adjustable-Speed Drive Systems. The incoming
branch circuit or feeder to power conversion equipment
included as a part of an adjustable-speed drive system shall
be based on the rated input to the power conversion equipment.
...
Don't know why or how I missed the word "rated" in there. I guess I dodged a bullet that time! As it turned out they swapped out the motor and we had to change condustors anyway, that's when I came up with my standard of putting in cables sized for the drive nameplate. Maybe somewhere in the back of my brain I did register the correct wording, just subconsciously chose to ignore it that first time.
 

dunk76

Member
My NEC 2005 addition Article 430.2 reads.
Adjustable-Speed Drive System. An interconnected combination of equipment that provides a means of adjusting the speed of a mechanical load coupled to a motor. A drive system typically consists of an adjustable speed drive and auxiliary electrical apparatus.
I do not see a reference to rated input anywhere.
Article 430.6 Ampacity and Motor Rating Determination. The size of conductors supplying equipment covered by Article 430 shall be selected from the allowable ampacity tables in accordance with 310.15(B) or shall be calculated in accordance with 310.15(C). Where flexible cord is used, the size of the conductor shall be selected in accordance with 400.5. The required ampacity and motor ratings shall be determined as specified in 430.6(A),(B), and (C).
 

infinity

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Jraef said:
Huh. I had an AHJ interpret that as the input current when adjusted for the connected motor nameplate FLA load, not the maximum nameplate current on the VFD. The reason he said was to account for the losses in the VFD when selecting the feeder.

Now that I read it again, I think maybe he was wrong (and I along with him).
Don't know why or how I missed the word "rated" in there. I guess I dodged a bullet that time! As it turned out they swapped out the motor and we had to change condustors anyway, that's when I came up with my standard of putting in cables sized for the drive nameplate. Maybe somewhere in the back of my brain I did register the correct wording, just subconsciously chose to ignore it that first time.

Your quote of 430.2 is from the 2002 NEC. I don't see this in the 2005 NEC. Has this section been moved?

So it still might possibly be an 800AF 700AT breaker, in which case everything there would be copacetic.

You need to read Don's post again. A 700 amp OCPD is too large for the secondary conductors in the OP.
 

iwire

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dunk76 said:
I do not see a reference to rated input anywhere.

Sorry folks, I had been working out of my 2002 NEC.

The section in the 2005 NEC is now 430.122(A) and it requires that the feeder or branch circuit conductors supplying a 'drive' be rated at least 125% of the input rating of the power conversion equipment.

From the drive to the motor you may size the conductors based on the motor HP.
 

kingpb

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By having a CB installed in the drive, that has a lower current rating then the MAXIMUM drive capability, would effectively reduce the rating of the drive, and therefore allow conductors to be run to the drive smaller then those required by 430.122(A).

After re-reading 430.122(A). I do not see where it requires the conductors to be sized for the fully rated value of the drive. it simply says that they have to be 125% of the rated input to the drive.

I interpret this to mean that a 20Hp drive, running a 10 Hp motor has to be sized for the 125% of the input to the drive, e.g. the 10Hp, NOT the full 20Hp. No different then say 200A breaker feeding a 600A rarted MCC. Certainly don't need 600A+ worth of cable.
 
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iwire

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kingpb said:
By having a CB installed in the drive, that has a lower current rating then the MAXIMUM drive capability, would effectively reduce the rating of the drive, and therefore allow conductors to be run to the drive smaller then those required by 430.122(A).

We can not change the rating of the drive in the field to suit our needs.

If you could get the factory to change the rating of the drive than you may be able to do what you suggest.

Also notice that 430.122(A) requires the conductors to rated 125% of the conversion equipment not any over current device that may or may not be included with the drive.

I don't know the reason that we are we required to use the rating of the drive but I have no doubt what you suggest if done in the field is a violation.
 

don_resqcapt19

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King,
The "rated input" is what is marked on the drive nameplate by the manfacturer. Just because you choose to use an over sized drive does not let you change the "rated input" current.
Don
 

Jraef

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don_resqcapt19 said:
King,
The "rated input" is what is marked on the drive nameplate by the manufacturer. Just because you choose to use an over sized drive does not let you change the "rated input" current.
Don
I now believe this to be the correct reading, but as I said, the first time I came up against it, the AHJ interpreted it the same as kingpb just did. I'd be willing to bet however that I would not get away lucky like that again.

And thanks to iwire for the 2005 reference. I am in California and we are not yet using 2005, still on 2002.
 

kingpb

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I understand where you are coming from, but let's take that logic of "Rated Input" one step further.

Then, why do I not have to use the fully rated input on an adjustable trip circuit breaker.

I take a 200A rated adjustable breaker, turn in down to 100A trip. I provide a feeder to it rated for 125% of 100A. I do not have to provide 200A+ worth of cable.

How is that different then buying a drive with a built in breaker, with a breaker rated for half the drive rating. The breaker is an integral part of the power conversion equipment, just as the adjustable trip is part of the breaker.

Logic would say you do not have to run fully rated cable to the drive. I think the intent is clear, although obviously a little vague, and maybe on purpose to leave it up to the AHJ.
 
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