external bonding of liquid tight conduit (LFMC)

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Apps EE

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senario
13.8KV underground feed to a motor with PVC conduit in a duct bank. 90degree Elbows and stubups are HDGS, and vertical rise is HDGS. Last 4 to 6 feet is LFMC to the motor terminal box. LFMC is 6" liquid tight. A separate ground conductor is run with the power feeds inside the conduit. Installation is in an electrical Non-Hazardous area.

Question;
Is it required to install an external bonding jumper across the liquid tight flexible metal conduit? Or, is the internal separate ground wire satisfactory?

I believe there are 2 separate issues.
1. Provide ground fault return path. The internal ground wire provides this path.
2. Make sure the conduit is electrically continuous and grounded. Can the LFMC internal ground make the lower HDGS conduit electrically continous? Note, motor terminal box has a ground conductor down to the ground grid. Switchgear is also connected to same ground grid as well as upstream transformer neutral.
 
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Apps EE

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Yes it is an NEC installation but

Yes it is an NEC installation but

Many times on high voltage larger horsepower motors, i.e. 20,000HP and larger. We can not use 4" or smaller liquid tight. As a result we use 5 and 6 inch versions. On some of the smaller, less than 10,000HP we can use 4", depends on the ampacity and cable size chosen.

The LFMC absorbs some of the vibration transmitted from the motor to the terminal box to the conduit. In addition when the 5" or 6" HDGS stubs up, it is never in the exact location you want it to terminate at the terminal box. We can get a 1" to 2" offset with the LFMC.

Above 4" it is not U.L listed. Above 1-1/4" it is not U.L. listed as a ground return path.
 
don_resqcapt19

don_resqcapt19

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retired electrician
I don't see an need for an external bonding conductor, although that is a standard industrial practice in some areas. The internal EGC will satisfy the code requirements.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
don_resqcapt19 said:
I don't see an need for an external bonding conductor, although that is a standard industrial practice in some areas. The internal EGC will satisfy the code requirements.
Click to expand...
I believe Apps EE is asking about bonding of the rigid, since the 6" LFMC is not UL listed as suitable for carrying fault current.

IMO, the GRC, especially the riser, needs to be bonded to the EGC per 250.86.
 
don_resqcapt19

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
Smart $ said:
I believe Apps EE is asking about bonding of the rigid, since the 6" LFMC is not UL listed as suitable for carrying fault current.

IMO, the GRC, especially the riser, needs to be bonded to the EGC per 250.86.
Click to expand...
Smart,
You are correct, I didn't read the original post closely. Some type of bonding is required for the rigid between the PVC and the LFMC. In this application, the only method available is an external jumper around the LFMC.
 
Mike01

Mike01

Senior Member
Location
MidWest
Wow

Wow

:Dholy sh^% in what type of facility do you find a 10,000+ HP motor not that it's bad or anything i just have never even herd of such a beast must be one hell of a job. (would be lying if i said I was not jelous would love the oppurinity to be working on a project like that)...
 
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Apps EE

Member
Thank you

Thank you

I also believe it is a bonding issue, to make the conduit system electrically continous.

Even though the conduit has an internal ground, the LFMC >1-1/4" is not U.L. listed to carry fault current. From that I assume it is also not listed to make the conduit electrically continuous from the transistion of the PVC to rigid conduit upto and thru the LFMC to the grounded motor terminal box.

"Exposed non?current-carrying metal parts of fixed equipment likely to
become energized shall be connected to the equipment grounding conductor"
..... as well as all conduit needs to be electrically continuous.

I see no other way except to install a bonding jumper over the LFMC.
 
D

danger30

Member
So, technically you need to need to bond around any LFMC over 1 1/4" to make your raceway "electrically continuous"?
 
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