Industrial Factory- Motor Bonding Question

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JOEZ06

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I have a new construction scenario and some debate has ensued among old school electricians that I would like to run by folks. Apologies if terminology is not NEC standard.

SES is 480/3P secondary from the utility, with a main isolation breaker, attached to the SES is the branch feeder panel that supplies several MCPs (motor control panels). Brand new, state of the art stuff....

The building slab (200' x 100') is 1/2" rebar enforced and the rebar is tied via "J" connectors to the all steel building. In the building we have several large towers (12x8" ibeam). The building, the steel towers and large conveyors (8" steel channel frame) are all connected via welded bar stock that this thicker than A 250 requirements for bonding jumpers, etc.

Induction Motors and AC Drives in the plant are either bolted to the conveyor ends, or bolted to frames that are welded to the steel towers. Care was taken to remove paint from mounting structures that are bolted, but supplemental tack welds put in place to ensure a connectivity path. Motor conduit is rigid metallic, with exception of the last 2 feet or so to seal tight rigid flex, for ease of routing.

I am impressed on our bonding and grounding, in so far as a welder can ground off pretty much anywhere he wants and weld a great distances to his work. If there is such thing as zero reference everywhere, I probably have a good example of it. ;-)

We plan to do a more formal ground/bond testing on every motor, but preliminary tests show 000.0 ohm fluke readings from every motor case to the cat walk, which is also bonded. The latest debate considers 3 options... 1) the need to run any ground wire from the motor pecker head at all since everything is bonded 2) running a short green jumper (12-24") from the motor head to the steel beem mount which is already bonded to the tower/building/conveyor 3) taking another ground wire back to the transformer along with circuit conductors (via feeder MCPs). I am speaking figuratively speaking, not literally routing the ground wire.

Summary... The question under discussion is the need for running a ground wire to every motor because "we have always done it that way"? Ultimately we will be under MSHA for inspection, and I plan on running this by our local inspector before we start up in the next few weeks but until now it has been our own electricians under my mandate to follow the NEC.

Great Forum... Thanks for any input. I just purchased Mike's book on grounding and bonding, and I hope it helps solve our discussions also.

R,

Joez06
 
At the very least you will have to bond around the flexible conduits. The rigid metal conduits are permitted to be used as the EGC. None of the other paths count as EGCs because they are of a higher impedance do to the seperation of the return path from the supply path. This seperation sets up inductive reactane with AC systems and that limits the current flow on the remote path. Most industrial facilities that I have worked in specify an EGC in the conduits.
Don
 
You must have an EGC per the code. What you have described so far doesn't meet the EGC code requirement. A ground wire would accomplish this (as your electricians have pointed out), but you can also use conduit as well as other types of conductors. See NEC 250.4, 250.102, 250.118, and then 250.122.

Note if you use conduit, you need to be sure it will be well maintained.
 
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Thanks...

Thanks...

It's a grounded system. Secondary is 480 Y with a neutral that can carry 277, but the plant is largely 480V and no phase to neutral power is routed.

I am still not convinced. What do you think about 250-136-a? The motors in question are installed on a 45' tower structure that is a grounded metal support for the 40 motors in question. The tower structures sole purpose, is to support these motors. It does not need to rely on the building ground, although we had intentions and discussions of bonding the inside tower to the building structure for overall bonding purposes. The tower is NOT part of the building structure, although they share the same concrete foundation. I feel like I am meeting he intention here. The tower is probably composed of about 100 tons of structural steel (all I beam) and it is bonded to cement rebar foundation in a manner that would meet code intention for grounding in it's own right.

It would seem to me that if I run a 1/0 to the X0 of the transformer from that tower structure, I meet the intention of the EGC.

I wish I could show you a picture of this.... Part of me can not accept that a 3/4" steel conduit, or flex, or a 14 ga copper wire can provide a more complete ground fault path for a phase to case motor short, than what is probably 60 square inches of I beam cross sectional area, effectively grounded, and to which my motors mounts are bolted or welded too....
 
For 250.136(A) to apply, the structure would have to be grounded with a 250.134 means.

In 250.134, you have two choices. The (B), with circuit conductors, is what you're trying to avoid for whatever reason. That leaves you with (A) .... permitted by 250.118

Your tower structure doesn't seem to have an EGC that's listed in 250.118

You probably do have a superior grounding system. I just can't find anything in the code that makes it okay. I think that the mines safety people will hammer you on this.
 
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Joe,
Part of me can not accept that a 3/4" steel conduit, or flex, or a 14 ga copper wire can provide a more complete ground fault path for a phase to case motor short, than what is probably 60 square inches of I beam cross sectional area, effectively grounded, and to which my motors mounts are bolted or welded too....
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As long as the motors are 200 hp or less, that sould be a reliable fault clearing path, but it is not code compliant. With larger motors and circuits, the inductive reactance of a remote fault clearing path can limit the fault current enough to slow or prevent the operation of the OCPD.
Don
 
Pierre... I think you are taking the sentence out of context. I am not relying on the BUILDING structure. The building is the frame, columns, steel joists, etc. I am relying on an independent structure (the motor stand tower) within the building that is effectively grounded in it's own right. It does not say that AC equipment, with an EGC dedicated to the support stand, is problematic. It's just trying to trap the case where folks just take a small motor stand, tie to the building beams and claim it's effectively grounded. I am not trying to do that. My tower stand probably weights 50 tons more than than building structure proper and can make the defend the valid point that if the building were to disappear, I am still effectively grounded as well as any building (1/2" rebar, in concrete of sufficient depth, length and continuity...).

Don- Correct- The motors range from 1/2 HP to 25HP and most are 1-5HP. There is no way these winding will create inductive breaking so as to limit a fault clearing path.... Certainly no more so than other EGC's allowed by code. Been 20 years since EE101, but I am pretty sure calcs won't be required for this one. I do soft start some motors/AC drives over 200HP outside, and those all have proper EGCs.

mdshunk.... RE: structure EGC... Correct... it does not have one at present. That is the point of the post/question. I want to bond the tower to the X0 (outside transformer) with an 1/0 copper wire EGC and not have to pull grounds wires to every small motor on the tower/stand.

Thanks for all the great ideas and constructive thoughts.

R, Joe
 
JOEZ06 said:
Pierre... I think you are taking the sentence out of context.
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I disagree Joe. Pierre is "spot on".

Joe, in your scenario, how is a ground fault going to get back to the transformer. It's through the building steel, or earth ground, or something other than a NEC approved method of grounding a motor. The NEC (as others have shown you) requires you to pull a ground with the conductors or use the conduit (where approved) the conductors are installed in to provide the grounding back to the OCP device.

I am curious, why is it you are arguing this point so hard? Is it the conduits are sized too small? I know copper has gone up, but I can't believe this will make that much of a cost difference on this project. So why fight this?
 
Joe,
There is no way these winding will create inductive breaking so as to limit a fault clearing path.... Certainly no more so than other EGC's allowed by code.
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It has nothing to do with the motor windings..only with the fact that the EGC is not run with the supply conductors. The seperation of the two current paths, the ungrounded conductor and the fault return path, is what causes the inductive reactactane that limits the current. You would have the supply current in the raceway and the fault return current on the structure...this is the problem.
I agree with Pierre, you can't use the structure as the EGC. You are required to use one of the EGC listed in 250.118
Don
 
Thanks guys!

Thanks guys!

Ouch... Lost my long reply...

Can someone elaborate on 250.136a is and what it's trying to do? Seems like if you say that 134 and 118 are required it does not do much... the question in my mind is one of semantics... It's not clear to me if it refers on the EGC to the equipment proper, or to the stand, which is what I am equating to my tower... It just holds up 40 motors, base mounted to virgin steel ibeam (no paint)...

Obviously I am tight on conduit, and also my EGC path from the MCP is very long compared to the short run from the leg of my internal tower (does not use the building structure) to the X0. It's a cost and speed issue.... one item on a checklist of many before we get into testing and production.

The OCPs at the MCP are all grounded back to the X0... it's only the load side segment of the circuit from the VFD (AC Drive Soft Starts) or smart starter (induction motors) at the MCP to the motor stand that are not EGC protected. The smart starters have all motor protection built in, FLA back to the PLC, smart diagnostics, etc.

Question? I have a junction box that is 75% of the way to motor clusters on the tower and between that box and the MCP I am all rigid steel conduit with no flex in between. Can I take credit for that portion using conduit as the EGC and then mount up ground lugs in that junction box to span the last 30 feet or to each motor? In other words, to avoid pulling a ground wire from every motor all the way back to the MCP?

Really appreciate the help.

Not sure I buy the theories on inductive reactance (phase to case short) as they apply to my specific case, but I agree to disagree and just want to move on to a code compliant soln regardless of theory. The 2 current paths being separate does not induce reactance in and of itself...does it? To generate EMF opposition (inductive reactance) you need coils, ampere-turns, right hand rule and all that stuff no? Could a single pass of 750-900mA (typcial 1HP motor) over a 14 gauge wire running parallel and 12 inches from the 12x8 Ibeam induce induction of any degree? Enough theory... I just want to be legal in the most painless way.... ;-) I would certainly rather be redundantly safe than not safe enough.

Regards, Joe
 
Joe,
The 2 current paths being separate does not induce reactance in and of itself...does it?
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Where the conductors of any AC circuit are seperated, the impedance goes up. As I said in an earlier post, it does not present any real problems until you have high current circuits.
Could a single pass of 750-900mA (typcial 1HP motor) over a 14 gauge wire running parallel and 12 inches from the 12x8 Ibeam induce induction of any degree?
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The problem is not with the normal circuit currents, but with the high ground fault current.
Can someone elaborate on 250.136a is and what it's trying to do?
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In my opinion that section is for installations of electrical equipment like panels, transformers, starters, ect. on a common metal rack.
Question? I have a junction box that is 75% of the way to motor clusters on the tower and between that box and the MCP I am all rigid steel conduit with no flex in between. Can I take credit for that portion using conduit as the EGC and then mount up ground lugs in that junction box to span the last 30 feet or to each motor? In other words, to avoid pulling a ground wire from every motor all the way back to the MCP?
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Yes, that would comply with the code rules. What is the wiring method between the junction box and the motors?
Don
 
JOEZ06 said:
Question? I have a junction box that is 75% of the way to motor clusters on the tower and between that box and the MCP I am all rigid steel conduit with no flex in between. Can I take credit for that portion using conduit as the EGC and then mount up ground lugs in that junction box to span the last 30 feet or to each motor? In other words, to avoid pulling a ground wire from every motor all the way back to the MCP?

Really appreciate the help.

Not sure I buy the theories on inductive reactance I would certainly rather be redundantly safe than not safe enough.

Regards, Joe
Click to expand...


Joe
Yes you can use the Rigid Metal Conduit as the EGC up to and including the box.250.118(2)
As far as blatantly saying yes to the rest, I will say with caution, that you can install a grounding bus in the metallic enclosure the RMC is entering and install individual EGC with the supplies to the different motor loads.

Another part you may not be too happy about.
If the enclosure you are installing the RMC to is supplied by a feeder, and you are "tapping" off to the different motors, the EGC to each motor will be sized based on the size of the overcurrent device supplying the feeder, not the size of the branch circuit to each individual motor,but never required to be larger than the size of the ungrounded conductor supplying the load (tap). 250.122(G)



In answer to the "inductance" thing, I think he may have meant Capacitance reactance. (separating two conductors)
 
Thanks Guys!

Thanks Guys!

Well it's better to be safe than sorry, so we will take credit for the having a conduit EGC and we did squeeze the copper ground wire, sized the same as the load conductor, to every motor load on the tower rack all the way back to the MCP... Lost a day doing it, but now I am headache free.

Thanks again for all the insight.
 
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