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Sub-pannel to main pannel grounding via metal conduit, ok ?

Sub-pannel to main pannel grounding via metal conduit, ok ?

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  #1  
Old 03-22-06, 05:31 PM
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Sub-pannel to main pannel grounding via metal conduit, ok ?

Hello,

I have a question regarding my old 100A Cutler Hammer sub-panel from the 50's inside the house that I eventually want to replace. It does not have any ground bus bar. The neutral bus bar is isolated from the metal frame of the pannel which is ok for a subpanel. There is a grounding path to the main pannel/meter in the garage via a 2'' metal conduit (that conduit only carries the neutral and 2 hot wires). According to NEC, is the metal conduit a good enough grounding conductor or should I fish a bare copper wire in it ?

PS : I read some study from Georgia Tech that concludes that metal conduit has a low enough impedance to carry safely any fault current (if max distance is not exceeded).. and allows the flow of higher fault current than an equipment grounding conductor as listed in NEC table 250-95.

Any thoughts ? thanks a lot !
 

Last edited by porzechowski; 03-22-06 at 06:01 PM.
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  #2  
Old 03-24-06, 07:30 AM
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This is an interesting question.

The ground between panels actually serves two purposes: it acts as an equipment ground conductor (EGC) to clear fault currents and cause a breaker to trip; and, it acts as a grounding electrode conductor (GEC) which bonds the panels to the electrode rods driven into the earth. Section 250.118 specifically allows rigid metal conduit, intermediate metal conduit and EMT to be used as an equipment grounding conductor (EGC) given that proper fittings are installed on the conduit and boxes. What I can't find is whether or not metal conduit is allowed as the GEC. Perhaps someone else will chime in?

Is the goal of this project to be able to add new grounded circuits to the subpanel? It can't hurt to pull in a grounding conductor. What gauge are the existing wires? For a 100A subpanel you will need either a #8 green or bare copper grounding conductor if the hot wires are #2 or smaller.
 
  #3  
Old 03-24-06, 08:37 AM
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The GEC is specific to the service disconnect and isn't continued to the rest of the system, ie, subpanel. The code articles for each specific type of metal conduit indicate whether it is allowable to be used for a EGC or not.

A separate wire will be needed for the GEC whether or not you get to the water pipe and/or ground rods through the conduit. If it isn't necesssary to run the GEC through the conduit a bonding bushing will be required on the panel end to tie the conduit to the ground buss. If you do use the conduit to enclose the GEC a bonding bushing is required on both ends.
 
  #4  
Old 03-24-06, 11:10 AM
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Hello Itsunclebill and ibpooks,

Yes this is correct, my intent was to add grounded circuits at the subpanel.
(There is lot of places like the bathroom fans where the chassis ground is connected to the nearest water pipe).

I was thinking to either :
- add a ground bus bar bonded to the subpanel chassis and that's it
- change the subpanel and bring in a #6 or #8 bare copper through the conduit (hots are #2).

I think I am going to call in an electrician to check the main pannel... what I know is that from the main panel the GEC is enclosed in a metal conduit and bonded to an outside rod driven into earth.
Obviously it would be easier for me to use bonding bushings rather than pulling the bare wire...
Thanks for the answers !
 
  #5  
Old 03-24-06, 12:00 PM
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You need to read my post again. The GEC has to be a separate wire. IF IT IS IN METAL CONDUIT THERE HAVE TO BE BONDING BUSHINGS ON BOTH ENDS - this is in addition to the wire. The conduit may only be used as the EGC
 
  #6  
Old 03-24-06, 02:17 PM
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Originally Posted by ibpooks
The ground between panels actually serves two purposes: it acts as an equipment ground conductor (EGC) to clear fault currents and cause a breaker to trip; and, it acts as a grounding electrode conductor (GEC)
Did you read this somewhere?

> What I can't find is whether or not metal conduit is allowed as the GEC.

Where did we get the idea that this subpanel needs a GEC?
 
  #7  
Old 03-25-06, 09:06 AM
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NEC Defintions----

Grounding Electrode Conductor---- " The conductor used to connect the Grounding Electrodes----"

Grounding Electrodes---- " A device that establishes an electrical connection to earth"

Metallic underground water lines, and copper rods driven into the ground are Grounding Electrodes
 
  #8  
Old 03-25-06, 10:45 AM
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What does a GEC have to do with a subpanel in the same building?
 
  #9  
Old 03-26-06, 09:27 AM
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The discussion of the GEC is tangential here. The GEC only exists from the main neutral to ground bond at or before the service disconnect to the grounding electrodes. There is no GEC in a subpanel. The 'ground' conductor from main panel to subpanel is an 'equipment grounding conductor'.

The difference is that a GEC is what connects your electrical system to the ground rods, while an EGC is pretty much every other 'ground' wire in an electrical system.

Now we differentiate between what the NEC requires and what I would do if this were my own house.

The NEC permits the use of EMT as the EGC. There are some detailed requirements with respect to 'concentric knockouts' and making a good connection between the conduit and the enclosures at either end, but presuming that all of these requirements are met, then the EMT is a suitable EGC.

To provide a ground bus in your panelboard, you can simply attach a suitable ground bus kit to the enclosure. You will probably need to grind away some paint to ensure a good connection.

However, I would recommend going beyond what the NEC requires in this case. I would pull a copper EGC through the conduit to act as a backup to the conduit itself. Over time the connections on the conduit can corrode and increase the resistance of the equipment ground. A copper wire inside the conduit is a very nice backup.

If you add this wire, however, you _cannot_ ignore the conduit. You must still make sure that you have a good ground connection at both sides of the conduit, and you need to make sure that the ground bus that you add is well connected to the panel enclosure.

-Jon
 
  #10  
Old 03-27-06, 08:48 AM
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You guys are right -- the GEC discussion would only apply to a main panel NOT a subpanel; I should have read that back after posting.
 
  #11  
Old 03-27-06, 02:47 PM
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Thanks Jon and all for the answers - this forum is great :-)

-Pierre
 
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