# Connecting VFD Cable grounding conductors inside JB's



## KennyW (Aug 31, 2013)

There is some debate on an installation where the cables connecting a VFD to a motor change from MC to flex cable in the middle of the run. So, the cables are connected inside a JB with terminal blocks. The armor of the MC and shield of the flexible are grounded to the JB via listed connectors, at both ends. 

Here's the issue- they have also connected the equipment grounding conductors using terminal blocks, with no connection to the JB backpan. 

The argument for this is that the VFD manufacturers reccommend that the EGC go directly back to the PE terminal on the VFD.

Seems like a fine suggestion if the cable were a single run, but something about it doesn't sit right with me, I feel like each ground conductor in each "segment" of cable need to be connected to ground at each end where the cable lands in a metallic enclosure. So in this case, replacing the normal terminal blocks with listed "grounding" terminal blocks *the green ones that bite the DIN rail, which is subsequently grounded to the enclosure which has a dedicated bonding jumper to the building structure.


My code reference for why this is so is 250.148, but the argument could be made for 250-146(d) that it is an isolated ground for the purpose of noise. 

Any ideas? Personally I have always just grounded the heck out of everything, everywhere.... everything works as is so as long as it is code legal we will leave it, but if not, it's an easy fix.


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## papaotis (Jun 8, 2013)

isnt evrything between here and there connected to each other? what difference does it make if the wire goes straight through. its still all connected, right?


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## KennyW (Aug 31, 2013)

papaotis said:


> isnt evrything between here and there connected to each other? what difference does it make if the wire goes straight through. its still all connected, right?


You mean because the motor has a bonding jumper to ground and the VFD has a bonding jumper to ground, both separate to the EGC's in the cable itself. in reality these are not EGC's at all, they're just bare copper wires that carry induced currents from the phase conductors? As in, they happen to be bare copper wires, and happen to be connected to ground, but they actually contribute nothing to the system from a functional grounding/safety standpoint and do not need to serve such a purpose, so it doesn't matter?

This is true, there are jumpers in place at the motor and drive adequate to carry actual fault current. and the cable run in bonded cable tray also listed to act as an EGC.


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## John Valdes (May 17, 2007)

I am with the drive folks on this one. Should the ground connection at the motor become compromised, the drive will still think its solid if its bonded/grounded in the first J-box.
I say leave it like it is.
The EGC goes from VFD to motor unbroken.


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## Rockyd (Apr 22, 2007)

John Valdes said:


> I am with the drive folks on this one. Should the ground connection at the motor become compromised, the drive will still think its solid if its bonded/grounded in the first J-box.
> I say leave it like it is.
> The EGC goes from VFD to motor unbroken.


110.3(B) is the rule. bonding is to trip the ocpd. 250.4(A) explains the diff in grounding vs bonding.


*(2) Grounding of Electrical Equipment. 
*Normally non–
current-carrying conductive materials enclosing electrical
conductors or equipment, or forming part of such equipment,
shall be connected to earth so as to limit the voltage to
ground on these materials.​​*(3) Bonding of Electrical Equipment. *​*
*​*
*Normally non–
current-carrying conductive materials enclosing electrical
conductors or equipment, or forming part of such equipment,
shall be connected together and to the electrical supply
source in a manner that establishes an effective​ground-fault current path.

Faults do not take the path of least resistance....faults take all paths of resistance back to the source!
​


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## sparkywannabee (Jan 29, 2013)

If you are working industrial what do you care about the nec, cec whatver, do what you have to do make your equipment run right and don't kill anybody in the process.


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## KennyW (Aug 31, 2013)

sparkywannabee said:


> If you are working industrial what do you care about the nec, cec whatver, do what you have to do make your equipment run right and don't kill anybody in the process.


Not nearly that simple unfortunately- Code compliance is a contractual requirement of the project. 

John, I know what you're saying and I agree in principle, but the problem is that the below says ANY EGC's associated with the circuit conductors should be connected to the box when it is metallic (Per (C)), I'm not sure if the fact that the box is already bonded via a separate bonding jumper allows us to skip this. What's funny is that we could skip it if the box was fiberglass.:whistling2::laughing: 

NEC: 250.148 Continuity and Attachment of Equipment

Grounding Conductors to Boxes. Where circuit conductors are spliced within a box, or terminated on equipment
within or supported by a box, *any equipment grounding conductor(s) *associated with those circuit conductors shall be connected within the box or to the box with devices suitable for
the use in accordance with 250.148(A) through (E).

…

*(C) Metal Boxes .A connection shall be made between the
one or more equipment grounding conductors and a metal
box by means of a grounding screw that shall be used for
no other purpose, equipment listed for grounding, or a
listed grounding device.*




The exception is 250.146(D) but it only talks about isolated receptacles. The principle would be exactly the same though.


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## John Valdes (May 17, 2007)

If you must follow the code, then make and install the jumper. Its not going to interfere with the operation.

I do not see any remarkable reason not to add the jumper, other than the manufacturer warns against it.
The drive cable should have never been cut to start with. Transitioning from one raceway to another is no reason to cut and splice or in your case land the conductors on a terminal strip.

Once this cable was cut and spliced, IMO the characteristics of the cable have become compromised and it is no better than THHN run out to the motor.

I for one have never used any drive cable as it had just come out when I left this business. We always used THHN and an over sized EGC with no issues that we knew about.


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## KennyW (Aug 31, 2013)

John Valdes said:


> If you must follow the code, then make and install the jumper. Its not going to interfere with the operation.
> 
> I do not see any remarkable reason not to add the jumper, other than the manufacturer warns against it.
> The drive cable should have never been cut to start with. Transitioning from one raceway to another is no reason to cut and splice or in your case land the conductors on a terminal strip.
> ...


Agreed - Really where they screwed up was not running a single home run from the motor to the drive. Then it would be to code and to the manufacturers recommendations. I think there were delivery issues with get VFD cable that was both tray rated and flexible enough for the flexing part of the run. 

And I also agree that since the armor and shielding is already grounded mid-span, it probably won't make difference in terms of performance. The noise aspect is already compromised.

Thanks for the insight guys!


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