What gauge wire for ground to subpanel

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  #1  
Old 09-15-07, 08:05 PM
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What gauge wire for ground to subpanel

Hi - I recently discovered that my subpanel (which was installed 20+ years ago and is mounted right next to the main panel) is incorrectly wired as the neutral and equipment ground wires are all fastened to the neutral bar, which is grounded to the box. There was no ground bus in the subpanel, so I have purchased one and am planning to isolate the neutral bus from the box and separate the neutral wires from the equipment grounds.

The feed from the main panel is via a 50 amp breaker, but the feed only has 3 wires - the 2 hot and 1 neutral. I would like to add an equipment ground wire between the main panel's ground bus and the new ground bus I am installing in the sub-panel. My question is - what gauge ground wire is acceptable between the ground buses? Thanks for your help.
 
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Old 09-15-07, 08:09 PM
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I used 6/3wg NM cable on a 60amp breaker for my sub, so you could use that.

It's ok to have neutrals and grounds on one buss in the main panel, and is very common, but like you said, the neutral and ground busses must be isolated in the sub.

What you currently have was legal at the time so you really don't have to make any changes, but if you want too, go ahead.

I'm sure our electrical gurus will chime in to give you some advice.

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  #3  
Old 09-17-07, 07:02 AM
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Normally you'd use #10 copper for an equipment grounding conductor between panels where the other conductors are protected by a 50-amp breaker. If the other conductors are larger than #6 however, you'll need something larger than #10 for the EGC.
 
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Old 09-17-07, 08:28 AM
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Thanks for your responses. I was not aware that the lack of a ground bus in the subpanel met code back then. I need to decide if it's worth correcting, then. The subpanel sits right next to the main panel, so I wonder how likely it is for neutral current to get redirected onto equipment ground - it seems much less likely than if the subpanel were located a far distance from the main. I'd appreciate anyone's advice on this. Thanks for the info on the wire size - if I do go ahead and add the ground bus, I'll check the size of the other conductors - hopefully they're not larger than #6 and I can use the 10 gauge equip ground wire between the boxes.
 
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Old 09-17-07, 09:39 AM
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If the panels are connected with a rigid metal conduit nipple, there's really no need for a separate EGC as the metal boxes are securely interconnected. Three wire feeders to subpanels used to be legal, so leaving it as-is would be acceptable also.
 
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Old 09-17-07, 07:07 PM
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Thanks, Ben. There is, indeed, a rigid metal conduit nipple between the boxes, so I feel better about leaving it as is and putting it on a very low priority to-do list.
 
  #7  
Old 09-17-07, 07:50 PM
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With respect I think an oversite may have occurred. If the sub-panel is fed with three wires and the neutral bar is bonded to the metal of the sub-panel objectionable neutral current will be on the metal conduit between boxes.

 
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Old 09-18-07, 06:27 AM
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Nice illustration - thanks for pointing that out, Roger. I should note that the CATV guy clamped a ground wire onto the conduit as well, so I imagine some objectionable current may be travelling onto the cable sheathing back to the utility pole where, I believe, the CATV cable is grounded as well. What's interesting is that when they upgraded the electric service in this house, it appears they took the original breaker box and used it for the subpanel as the main box is a modern looking square D unit, while the subpanel appears to be a Cutler Hammer unit circa 1949 based on a label I found on it. I don't see any predrilled holes for the ground bar so I was contemplating drilling and tapping a couple of holes for it.
 

Last edited by vgb14; 09-18-07 at 06:28 AM. Reason: typo
  #9  
Old 09-18-07, 09:14 AM
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First thing I would say is that Ben is a very responsible poster as are the others, he probably just didnt have his morning coffee I've had my share of over sights believe me.

The reason your panel (used to be main panel) does not have provisions for a ground bar is that is was manufactured as a service equipment only panel so there was never any intention for the neutral and ground to be seperated in it as in a sub-panel. Field installations of ground bars are pretty common in these type panels, whether or not they are code legal is a matter of debate. Square d's modern panels allow and include instructions on how to do this. I would tell you how we generally do this in my area that is acceptable by our inspectors but that may not be appropriate for your area. So my advice is to visit with your codes department (hopefully you have one) and take that illustration with you. Google "grounding and bonding online training 7" and print it off.

The catv guy must have been in a hurry you need to move that ground wire to the GEC or an electrode and use the proper clamp.

Roger
 
  #10  
Old 09-18-07, 11:03 AM
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Just to add to my last post one thing will be for certain your going to be removing the neutral to case bond.....

Roger
 
  #11  
Old 09-18-07, 03:43 PM
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Wow - this is a great forum. I'll probably add the ground bar to bring it up to date with code - it seems worthwhile even though it's not legally necessary due to grandfathering. I need to make sure that I can isolate the neutral bar from the box. I believe it is possible by loosening a screw, but I need to confirm this with a continuity tester. Also, I was told I had to drill and tap holes for acceptable machine screws for the ground buss as sheet metal screws are a no-no due to its minimal contact points. The ground bar I bought has the machine screws included. As for the catv ground, it seems to be common for installers to do this incorrectly (see http://www.kramerfirm.com/pictures/t...age=1&sort=na). I inquired with the cable company regarding the ground, and they insist what they did meets code - I'll have to look at this again. Thanks again, everybody.
 
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