Combining 2 isolated ground wires into a single EMT raceway

[Bill Stanis]

I have searched the NEC and the forum without any luck.
I am wanting to combine 2 #6 insulated isolated grounds into an EMT raceway. The EMT needs to be bonded at each end with a hub. I am assuming that I can NOT bond the raceway with one of the isolated grounds because that would make it not isolated since the raceway will be strapped to other metallic supports, therefore, an equipment grounding conductor would need to either be added to the run or "Jumped" at each end to bond the raceway.
Normally I would just utilize a PVC raceway but the shop specified EMT on the final stamped set of prints.
Any information or code sections would be appreciated.

 

[infinity]

Why would you need two IG's? Can you use one EGC for the bonding one IG for whatever needs the IG?

Welcome to the Forum.

 

[Another C10]

Any information or code sections would be appreciated.

Typically the only time a bonding bushing is needed is when using concentric knock outs, I don't recall ever getting called on needing a bonding bushing ( unless spec'ed) if I use the solid knock out.

The point of the bushing is to assure fault current will not be depleted while passing over on a broken concentric of a multi size ko.

 

[Bill Stanis]

The EE wants to ground two separate groups of metal welding tables or stations with an IG going to the exterior of the building and attaching to 2 separate ground rods. The #6 needs to be protected with EMT per the design. The EMT needs to be bonded at each end per my experience with inspectors, Im actually looking up that requirement in the code now. The EMT will be open ended and not attached to any enclosures. I agree that 1 IG could be used but the EE specified two separate runs.

 

[texie]

I have searched the NEC and the forum without any luck.
I am wanting to combine 2 #6 insulated isolated grounds into an EMT raceway. The EMT needs to be bonded at each end with a hub. I am assuming that I can NOT bond the raceway with one of the isolated grounds because that would make it not isolated since the raceway will be strapped to other metallic supports, therefore, an equipment grounding conductor would need to either be added to the run or "Jumped" at each end to bond the raceway.
Normally I would just utilize a PVC raceway but the shop specified EMT on the final stamped set of prints.
Any information or code sections would be appreciated.

I'm a little confused about what you are trying to do. If these are isolated EGCs they have to be run in the same raceway as the circuit conductors and there is no requirement to bond them as you would a GEC. There is no such thing as an isolated GEC.
Maybe I'm not understanding your situation.

 

[don_resqcapt19]

I think the EE has no idea of what he is doing.

 

[infinity]

Sounds like these welding tables do not require anything according to the NEC but the engineer wants to provide some grounding for some reason. If nothing is required by the NEC he's free to design just about any grounding scheme that he wants.

 

[Bill Stanis]

Sorry for the confusion.
Here is the overall picture.
We have 2 groups of metal welding tables/stations for students to learn how to weld.
Each group has 6 stations
Each station will be supplied with 3 phase , 480v twistlocks to plug a mig/tig welder into.
The grounding i am asking about is to only ground the tables.
The EE wants to ground each group of tables to an isolated ground, I do not know why an isolated ground is specified......Im not an engineer.
Anyways, if these so called "isolated grounds" are ran through a metallic raceway and the raceway is not connected at either end to an enclosure or box, then from what I have been told over the years is that each end of the raceway needs to be bonded.
If this is true I am looking for a code section for this bonding.
The only code section that I have been able to locate has to do with Skin-Effect Heating at 426.44 but in my case this is a grounding conductor and not a current carrying conductor.

AND YES, I DO NOT THINK THIS EE SHOULD BE CALLING IT AN IG

At this point Im going to get my inspector out to the job before we do the work and see what he will accept. Thats really all that matters.

 

[readydave8]

attaching to 2 separate ground rods.

but the 2 ground rods will be bonded to each other and to existing ground?

 

[infinity]

If I'm understanding your installation (engineer wants to run a grounding conductor from the table to a ground rod) I can't think of any NEC rule that would apply to this. If that's true you can do whatever you want. Closest thing I can think of if an auxiliary grounding electrode in 250.54 but that still would not directly apply to this application.

 

[Bill Stanis]

but the 2 ground rods will be bonded to each other and to existing ground?

no. just 1 rod for each system not tied into any existing, just outside in the planter area

 

[hbiss]

no. just 1 rod for each system not tied into any existing, just outside in the planter area

Yup, that's what I thought. That "engineer" has no idea what he is talking about. What possible benefit would that provide?

At this point Im going to get my inspector out to the job before we do the work and see what he will accept. Thats really all that matters.

After he gets done laughing. Actually, if I was the inspector I wouldn't allow that unless the GECs were bonded back to the service ground.

-Hal

 

[infinity]

At this point Im going to get my inspector out to the job before we do the work and see what he will accept. Thats really all that matters.

I see nothing wrong with the design besides it being a waste of someone's money. You can pretty much connect whatever you want to a ground rod if it makes you feel good.

 

[Hv&Lv]

So...
you want to ground the welding tables totally separate from the electrical system grounding setup.
question..
wouldn’t this create a difference of potential with the many stray currents circulating within the earth?

what about the fact that sometimes you weld negative ground, and sometimes positive ground.. any problems there?

i think it will create more problems than it will solve.

 

[Another C10]

wouldn’t this create a difference of potential with the many stray currents circulating within the earth?

I was always taught 1 ground reference period per main switch gear or per utility service, a system never wants 2 different grounding paths, they will have different resistances. 1 main service, 1 common earth ground termination point.

 

[synchro]

Maybe the engineer wants the welding tables tied to an earth ground to reduce electromagnetic interference because arc welders typically generate lots of broadband noise. If so I'm not sure how effective that will be.

Tying the metal tables to EGCs of the electrical system could lead to welding currents getting into the EGCs if it's not done right (for example, if multiple return paths are present). See 630.15 and its informational note, and the commentary on pages 402 and 403 of this book:

Interpreting the National Electrical Code

Demystify and accurately interpret the National Electrical Code! Help your students master all sections of the 2011 National Electrical Code (NEC) with the accurate, thorough coverage found only in Surbrook/Althouse's INTERPRETING THE NATIONAL ELECTRICAL CODE, 9E. This easy-to-understand...

books.google.com

 

[Bill Stanis]

Thanks for all the input.
Lincoln is supplying all the welding equipment and is supposed to be out next week. Hopefully they can shed light on this topic. Our EE just basically stated to install whats on the prints........ YES BOSS........
I still want to understand and will add to this thread later for what its worth.
Thanks again.

 

[Bill Stanis]

Maybe the engineer wants the welding tables tied to an earth ground to reduce electromagnetic interference because arc welders typically generate lots of broadband noise. If so I'm not sure how effective that will be.

Tying the metal tables to EGCs of the electrical system could lead to welding currents getting into the EGCs if it's not done right (for example, if multiple return paths are present). See 630.15 and its informational note, and the commentary on pages 402 and 403 of this book:

Interpreting the National Electrical Code

Demystify and accurately interpret the National Electrical Code! Help your students master all sections of the 2011 National Electrical Code (NEC) with the accurate, thorough coverage found only in Surbrook/Althouse's INTERPRETING THE NATIONAL ELECTRICAL CODE, 9E. This easy-to-understand...

books.google.com

I will check out this book.
Thanks

 

[sameguy]

PVC pipe

 

[LarryFine]

I think it would be better to connect each table to the nearest building steel.