Parallel MC Cables between 750 HP motor and VFD

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walter Knox

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I am looking at installation of a 750 HP, 575V motor on a VFD. The motor has a nameplate of 770 FLA, and the VFD supplier says that software will limit the VFD output to 770 A. I would like to use MC cable between the VFD and motor, and am looking at using one that is type MC-HL, even though it is not a hazardous location. I'm looking at two possible cable types, both are continuously welded corrugated aluminum sheath, with a rubber jacket. I would use (6) parallel cables, size 2/0.

I calculate the minimum ampacity required as 770x1.25 = 962.5 A. Using six parallel cables, I need 161 Amps per cable, and 2/0 is good for 175 A.

The problem is the equipment grounding. The cable I want to use has (3) #10 ground wires and (3) 2/0 conductors. I figure that the combination of the ground wires with the aluminum sheath is good as long as the ground fault protection is set at 200 Amps or less. This is some specialized equipment, and the equipment supplier has been trying to get information out of their VFD maker (in Europe) on the ground fault protection. They had assumed that the ground fault setting would be well under 200A, but had not been able to tell me an actual number.

Today, I saw a copy of the generic installation manual from this VFD maker. It contains the statement "Earth fault protection--In case of earth fault in motor or motor cable, only the inverter is protected". I am trying to get some clarification on this.

My other option is to get a similar cable, but with (4) 2/0 conductors and (1) #6 ground. I could use the 4th 2/0 as the new ground, which should be good for overcurrent protection up to 1000A. The color code in the cable is black-red-blue-orange plus green #6. Can I re-identify the 4th 2/0 wire with green tape, and cut off and not use the #6 ground?
 
Which code cycles are you working under?

250.122(F) was changed in the 2008 NEC to require a full sized EGC to be run with each paralleled cable. So if you have an 800 amp breaker supplying the VFD then each cable would be required to have a 1/0 copper or 3/0 aluminum EGC. The allowance for the use of ground fault protection was removed from 250.122 for the 2008 NEC.

Chris
 
Southwire does make a MC feeder cable with over sized grounds such as a 350-4 with a 1/0 bare or 500-3 with a 2/0 bare
Other possibilities might be available.
 
raider1,

This is under the 2008 NEC.

With regards to the conductors between the motor and VFD, I was using 250.122(D)(1) for motor circuits, "based on the rating of the branch-circuit short-circuit and ground-fault protective device." I am considering MC cable due to space constraints.

With regards to the feeder to the VFD, the VFD has an active front end (AFE), with a nameplate of 920A. The breaker is 1200A, based on 430.122: 920A x 1.25 = 1150. The feeder to the VFD will be (4) sets of 350 kcmil, with 3/0 copper ground, in rigid conduit.

Since the AFE converts the 575V AC to a DC bus, I don't think that a ground fault at the motor or in the motor cable has much to do with the 1200A breaker supplying the VFD. Is it your opinion that I would need to have 3/0 grounds in my MC cable? What is your opinion on re-purposing a 4th 2/0conductor as a ground? Would 250.122(A) "but in no case shall they be required to be larger than the circuit conductors supplying the equipment" save me on this??

Thanks,
Walter
 
walter Knox said:
raider1,

This is under the 2008 NEC.

With regards to the conductors between the motor and VFD, I was using 250.122(D)(1) for motor circuits, "based on the rating of the branch-circuit short-circuit and ground-fault protective device." I am considering MC cable due to space constraints.
Click to expand...

250.122(D)(1) tells us that we use the branch circuit short circuit and ground fault protective device to size the EGC for a branch circuit to a motor. If that branch circuit happens to be supplied by parallel conductors then 250.122(F) would come into play.

With regards to the feeder to the VFD, the VFD has an active front end (AFE), with a nameplate of 920A. The breaker is 1200A, based on 430.122: 920A x 1.25 = 1150. The feeder to the VFD will be (4) sets of 350 kcmil, with 3/0 copper ground, in rigid conduit.

Since the AFE converts the 575V AC to a DC bus, I don't think that a ground fault at the motor or in the motor cable has much to do with the 1200A breaker supplying the VFD.
Click to expand...

What is providing your short circuit and ground fault protection for the motor?

Is it your opinion that I would need to have 3/0 grounds in my MC cable?
Click to expand...

No that was just part of an example I was using if the motor short circuit and ground fault protective device was an 800 amp breaker.
What is your opinion on re-purposing a 4th 2/0conductor as a ground? Would 250.122(A) "but in no case shall they be required to be larger than the circuit conductors supplying the equipment" save me on this??

Thanks,
Walter
Click to expand...

No, 250.122(A) would not save you with a a 4th 2/0 conductor in the paralleld cable as the circuit conductors would be the combined paralleled conductors.

Again 250.122(F) is clear that the EGC for each cable must be full size in each cable based on the motor short circuit and ground fault protective device for the motor.

Chris
 
raider1 wrote:
"What is providing your short circuit and ground fault protection for the motor?"

The VFD should be providing this. The VFD inverter output is supposed to have a nameplate output of 820A, and is limited by software to 770A, which is the motor nameplate rating. One manual says that the inverter in the VFD provides "overcurrent", "overload", and "earth fault" protection. I don't have any data on what these trip levels are. The supplier of this special equipment is buying a foreign made VFD, and is going to have the system listed by CSA.

Let's say that I do use the 1200A circuit supplying the VFD input as a basis for selecting the equipment ground size in the MC cables between the motor and the VFD. The minimum size of the conductors between the VFD and motor are determined by 430.22(A) as 1.25 x 770 FLA = 963A. If I use (5) parallel sets of 4-conductor 3/0 cables, there is enough ampacity. If I use the 4th conductor in each cable as a re-identified ground wire, the 3/0 size would be good for an overcurrent device up to 1200A.

The question in that case is, am I allowed to cut off the supplied #4 ground wire and use the re-identified 4th 3/0 conductor as the new ground?

Thanks,
Walter
 
The question in that case is, am I allowed to cut off the supplied #4 ground wire and use the re-identified 4th 3/0 conductor as the new ground?
Click to expand...

Yes, that would be acceptable.

Chris
 
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