ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals

ECN Shout Chat
Top Posters(30 Days)
Admin 17
Recent Posts
Anyone hiring inspectors?
by sparky66wv. 03/26/17 12:22 PM
Old decora style outlets
by Admin. 03/25/17 11:40 AM
ESA Arc flash course
by TheShockDoctors. 03/24/17 10:15 AM
fuse rejectors
by HotLine1. 03/24/17 07:53 AM
Another Forum Update
by Admin. 03/22/17 03:04 PM
New in the Gallery:
SE cable question
Popular Topics(Views)
231,509 Are you busy
166,213 Re: Forum
160,660 Need opinion
Who's Online Now
1 registered members (sparky66wv), 62 guests, and 12 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate This Thread
Page 1 of 2 1 2
#88680 - 07/06/04 07:13 AM Center-tapped 240V delta subpanel GEC  
LarryC  Offline
Member
Joined: Jul 2004
Posts: 781
Winchester, NH, US
I am assisting in moving a machine shop into a new to us building. The building service is 1200A 240V Delta with a center-tapped phase.

All subpanels located on perimeter walls have only 4 conductors run to them. The 3 phases and the neutral. I am concerned about a lack of grounded Earth conductor. All of the subpanel feeders appear to be run thru EMT buried in or below the concrete slab floor. The building is corrugated metal on a steel I beam frame. The bases of the I Beams are sitting on the foundation walls. I do not see any bonding jumpers from the steel to earth, and only at the building's main CB do I find some sort of bonding jumper.

ASSUMING the neutral is bonded to the building steel, is it OK to install the listed ground bars for the subpanels and bond them to the EMT coming out of the concrete and also to the building steel?

I have been told that the job will not be inspected. Never the less, I am pushing to do the job right.

Thanks


2017 / 2014 NEC & Related Books and Study Guides

#88681 - 07/06/04 04:04 PM Re: Center-tapped 240V delta subpanel GEC  
Tom  Offline
Member
Joined: Jan 2001
Posts: 1,044
Shinnston, WV USA
The EMT is being used as the required equipment grounding conductor for the feeders to the sub-panels. Installing an equipment ground bus to the cabinet should suffice. You could install a grounding bushing or a ground clamp to the EMT & connect it to the equipment ground bus if you want.

I don't believe that connecting each sub-panels equipment ground bus to the building steel will have much benefit.

Tom


Few things are harder to put up with than the annoyance of a good example.

#88682 - 07/06/04 05:08 PM Re: Center-tapped 240V delta subpanel GEC  
Ryan_J  Offline
Moderator
Joined: Aug 2003
Posts: 1,374
West Jordan, Utah, USA
I agree with Tom.


Ryan Jackson,
Salt Lake City

#88683 - 07/08/04 01:41 AM Re: Center-tapped 240V delta subpanel GEC  
Tesla  Offline
Member
Joined: Jun 2004
Posts: 1,273
Sacramento, CA
Our AHJ would have a fit: EMT in the slab! Be it setscrew or compression, I have yet to see an EMT coupling I would trust to carry fault current having been exposed to wet soil.
The high threshholds of OCPDs in motor circuits, the nature of the usage -- craftsmen stomping around with metal in their hands, in a metal building with metal encased motors sitting on a semi-dielectric pad....
I would want separate bonding conductors.
My AHJ would insist on abandoning the old work entirely. End of story.
( I had the 'good fortune' to step in as foreman on just such a 'red tag'. )


Tesla

#88684 - 07/08/04 10:19 AM Re: Center-tapped 240V delta subpanel GEC  
Ryan_J  Offline
Moderator
Joined: Aug 2003
Posts: 1,374
West Jordan, Utah, USA
Tesla: What code section wouls your AHJ cite?

Quote
358.10 Uses Permitted.
(A) Exposed and Concealed. The use of EMT shall be permitted for both exposed and concealed work.
(B) Corrosion Protection. Ferrous or nonferrous EMT, elbows, couplings, and fittings shall be permitted to be installed in concrete, in direct contact with the earth, or in areas subject to severe corrosive influences where protected by corrosion protection and judged suitable for the condition.


Ryan Jackson,
Salt Lake City

#88685 - 07/08/04 11:14 AM Re: Center-tapped 240V delta subpanel GEC  
earlydean  Offline
Member
Joined: Dec 2003
Posts: 751
Griswold, CT, USA
Whose judgement about the suitability of EMT is used? The AHJ? If the AHJ judges the EMT fault path inadequate, then it shifts the burden of proof to the owner (electician, engineer?). Right?


Earl

#88686 - 07/08/04 06:44 PM Re: Center-tapped 240V delta subpanel GEC  
triple  Offline
Member
Joined: Apr 2002
Posts: 184
Wisconsin
Emt is not allowed underground in Wisconsin. Does NH have a state code?


#88687 - 07/09/04 06:14 AM Re: Center-tapped 240V delta subpanel GEC  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
I would say underground is different from being in the slab and as Ryan has pointed out it is one of the uses allowed.

Larry are you sure it is EMT?

I often use EMT down to the slab and then change over to RMC.

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#88688 - 07/10/04 08:16 PM Re: Center-tapped 240V delta subpanel GEC  
LarryC  Offline
Member
Joined: Jul 2004
Posts: 781
Winchester, NH, US
"Larry are you sure it is EMT?"
The conduit coming out of the concrete appears to be the same diameter as the EMT coming out of the panels, and they did use the setscrew style couplers.

The worksite is actually in Vermont, right across the river.

The subpanels in question are 150A and 200A panels.


#88689 - 05/27/05 04:24 PM Re: Center-tapped 240V delta subpanel GEC  
LarryC  Offline
Member
Joined: Jul 2004
Posts: 781
Winchester, NH, US
Well its been almost a year, and we finally moved the rest of the company into this building.

I still have concerns about the ability of the grounding paths to handle any significant fault currents.

My thought is to test the resistance of the ground paths by using a welder to supply a controlled current of 100-200 Amps for a few seconds between the subpanel ground bar and the building's service bonding location and then measuring the voltage drop between the subpanel ground bar and the building's central ground.

Obviously this would have to happen with the subpanel deenergized, along with suitable Kelvin connections to ensure we are only measuring the voltage drop along the ground path and not the v drop across the welder to ground bar connection. Also we would need enough people to look for unexpected arcs or excessive heating across any high resistance ground paths.

Assuming I can get permission to perform these tests, what would be an acceptable range of voltage drops?

Larry


Page 1 of 2 1 2

Member Spotlight
HappyElectrician
HappyElectrician
Penn USA
Posts: 31
Joined: December 2011
Show All Member Profiles 
Featured:

2017 NEC and Related
2017 NEC
Now Available!

Shout Box
Powered by UBB.threads™ PHP Forum Software 7.6.0
Page Time: 0.016s Queries: 15 (0.003s) Memory: 0.8168 MB (Peak: 0.9894 MB) Zlib enabled. Server Time: 2017-03-26 17:06:37 UTC