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Replacing old subpanel with conduit as ground: should I add bonding bushings?

Replacing old subpanel with conduit as ground: should I add bonding bushings?

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  #1  
Old 11-13-12, 11:56 PM
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Replacing old subpanel with conduit as ground: should I add bonding bushings?

I have a 1950s Stab-Lok subpanel for the oven and cooktop I want to replace because those Stab-Lok breakers are known to catch on fire. It has 3 wires and the flexible metal conduit is the EGC. There is a junction box in the attic in the FMC run (j-box used to serve another circuit, but not anymore).

I understand that according to NEC, FMC can be used as the EGC.

For extra safety I am thinking of adding two bonding bushings inside the j-box and another one inside the subpanel. Then I'd run a ground wire only between these bonding bushings, terminating on a ground bar that I would add to the subpanel. The purpose would be to ensure continuity in case corrosion ever develops between the locknuts.

The rest of the conduit goes all the way to the main panel 100 feet away (it's a long house) and I am sure this conduit is carrying ground nicely all the way to the main panel without further junctions. It would be very difficult to run the extra ground wire all the way to the main panel.

Would this idea be ok?
 
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  #2  
Old 11-14-12, 01:19 AM
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Flexible metallic conduit can only be used as the equipment grounding conductor under some pretty strict conditions. I would strongly urge you to get a real grounding conductor through the entire conduit.
 
  #3  
Old 11-15-12, 11:52 AM
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I am thinking of adding two bonding bushings inside the j-box and another one inside the subpanel. Then I'd run a ground wire only between these bonding bushings, terminating on a ground bar that I would add to the subpanel... The rest of the conduit goes all the way to the main panel 100 feet away (it's a long house) and I am sure this conduit is carrying ground nicely all the way to the main panel without further junctions.
If it is all flex, pull the new green wire from or to the panel. I wouldn't bother with the bonding bushings - bit of a PITA and impossible to install around existing wiring - but I would bond to a ground screw in the J-box.
 
  #4  
Old 11-15-12, 12:04 PM
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I wonder if it is somehow possible to pull a new green wire through 120 ft of metal flex with existing wires. The fact that the ground can be a smaller gauge helps. Maybe I need a long fiberglass pull rod, but I don't know if it would be possible to push the fiberglass through, next to existing wires?
 
  #5  
Old 11-15-12, 03:33 PM
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I wonder if it is somehow possible to pull a new green wire through 120 ft of metal flex with existing wires. The fact that the ground can be a smaller gauge helps.
Sure. Just be sure to pull in the easier pull direction.

I would tie a pull string onto one of the existing wires and pull that in, then use the pull string to pull the wire you just pulled out, the new EGC and a second length of pull string into the pipe. A bit of wire pulling lube, a helper to feed the pipe, securely hitched and taped pulling heads and 2-way communication will help a lot.

Use the clear lube, not the yellow stuff.
 
  #6  
Old 11-15-12, 04:31 PM
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What size circuit is running through the FMC and what size is the FMC?
 
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