sub panel grounding

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  #1  
Old 08-03-06, 12:08 PM
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why would a subpanel be different? I'm not hooking up a welder, but am re-doing some wiring inside an old home. The main panel is in a detached garage, the subpanel is inside the house. The house currently has two prong outlets, and therefore no ground. I was going to run all new wiring (with ground included) and tie the white and bare wires to the neutral bus and the black wires to the breakers on the subpanel. Do I have to do something different since we are talking about a subpanel? Thanks in advance for any help / replies.
 
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  #2  
Old 08-03-06, 12:17 PM
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Sub panels and grounding can be difficult to understand.

The answer to your question is that it depends, because you are talking about separate buildings.

If the feed from the main panel to the sub panel is four wire (two hots, a neutral and a ground), which it MUST be if there are any other metal paths between the buildings, then you must isolate the the grounds and neutrals at the sub panel, with the ground buss connecting to the ground wire from the main panel and the neutral buss connecting to the neutral wire from the main panel. This is the best approach to the wiring.

If the feed from the main panel is only three wire (two hots and a neutral), which it can be under certain situations, then the ground ans neutrals all get connected together to the same buss in the sub panel.
 
  #3  
Old 08-03-06, 12:19 PM
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calizeus, First off, for clarity your best bet is to start a new thread for your situation. Your setup has nothing to do with a 240v welder circuit.

Ok, that said, your situation ALL depends on how the house is fed from the garage. Meaning with 3-wires or 4-wires.
Both can be legal and both are safe, just different for different reasons.
Start by giving us a description of the feeder.
 
  #4  
Old 08-03-06, 12:19 PM
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Code dictates that after leaving the first disconnecting means the neutral and ground wires remain separated, and the neutral bar is isolated from ground in all subpanels. This keeps the neutral from carrying fault current and, more importantly, keeps the ground from carrying current in case of a neutral break on a feeder.
 
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Old 08-03-06, 12:22 PM
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Jedi, not in all situations. As racraft described, detached structures CAN be fed with a 3-wire feeder, and the neutrals and grounds bonded.
 
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Old 08-03-06, 12:41 PM
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As Speedy and Bob say there are exceptions. As for four wire feeders...........(H-H-N-Grd).......somewhat redundant with the others but maybe will give a little more understanding to the reasoning.

Because objectionable neutral current will flow on both the feeder equipment ground and feeder neutral if you have a neutral and ground bond at the sub-panel. Also if the feeder neutral opens all returning neutral current of the branch circuits being served by the sub-panel will be diverted to the feeder equipment ground wire. due to this bonding of the two in the sub-panel. Remember the equipment ground is not to carry current unless of a fault. Its purpose is to provide a low impedance fault path back to Xo (center tap) of the transformer so breakers will trip out on fault.

Review this article: Panelboard = sub-panel

http://www.ecmweb.com/news/electric_...per/index.html
 

Last edited by Roger; 08-03-06 at 12:52 PM.
  #7  
Old 08-03-06, 02:41 PM
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I will have to look tonight to be 100% sure, but I am almost positive that it is only the three large black wires going from the main panel to the sub panel.

So, if I am understanding correctly and if I do in fact only have three wires. Then I can add all of my new ground wires (along with the white/neutral wires) to the neutral bus (and obviously attach the black wires to the breakers).

If I do have four (which I'm assuming would be the three large black wires and a bare or green ground??), then how can I ground the sub panel? Could I add an additional bus (from Home Depot elec. dept.), make that my ground bus, secure all of my new ground wires (coming from outlets, etc.) to it, and run a large ground wire to a cold water line (or drive a rod 8' into the ground and attach it to that?)

Any advice is much appreciated.

BTW, I hope iamhistory is okay. He wrote, "I'll let you know how everything turns out. If I live to tell about it that is "
 
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Old 08-03-06, 02:46 PM
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If this is an existing panel I would assume it is correct. If it is older as you say I bet it is a three wire feeder and should be treated as any other main panel. If this is the cae the neutrals and grounds MUST be bonded.
Adding a ground rod if there is not one will help but will not give you a ground.
 
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Old 08-04-06, 07:10 AM
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Okay, I checked last night and here is the exact situation. Coming from the main panel to the subpanel are three very thick black wires and one fairly thick (about 1/3 the diameter of the other three - or about the size of a CAT5 cable) black wire.

Notes on the main panel: There appears to be only one neutral bus and no ground bus. I say appears because it does look like it used to be one bus. However, it looks like about the bottom third (let's call this piece A) of it either broke off or was broken off and is positioned just below the other two thirds (piece B). Connecting piece A to piece B is a thick (again about the size of a CAT5 cable) wire. The neutral from weatherhead is connected to piece B and then continues to the subpanel. The 4th black wire (the smallest of the four going to the subpanel) is connected to piece A. Also connected to piece A is a bare ground wire that runs down into the ground (actually into concrete - I'm assuming it goes into the ground under the concrete).

Notes on the subpanel: There is only one neutral bus and no ground bus. The third black wire (the neutral) and the 4th black wire (the ground?) are both connected to this bus. I also noticed that there is one ground wire connected right where the 4th black wire connects. That ground wire then continues up in the wall somewhere. I'm not sure what it is grounding as I haven't found any grounded outlets. One other thing I did notice is that a ground wire is connected to a water line by the water heater, but it goes back in the wall so I'm not sure if it is the same one that is connected in the subpanel.

If I'm guessing right in doing my re-wiring (from H-N lines to H-N-G lines and from two prong outlets to three prong), I can connect my hot black wires to the breakers and both the neutral white and ground bare wires to the neutral bus. Based on the information I've given, does it sound like I need to do anything else?

Thanks again for the responses.
 
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