Subpanel grounding & neutral question

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  #1  
Old 03-31-03, 10:18 AM
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Subpanel grounding & neutral question

I know this has been covered before but my simple mind needs affirmation.

I have a detached garage that I added a subpanel to provide power. The neutral bus bar is insulated from the panel and has a connector inside that appears to be there to be bent in to connect into the neurtal bar. I have a breaker in the main panel that feeds the sub. Currently the case connector is not attached to the neutral bus. The neutral and ground conductors are both attached to the neutral bus in the sub. I don't have a ground rod, yet. I have both ground and neutral conductors connected to the neutral bus. How should I proceed to make this right & safe?

Thanks,

Jester
 
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  #2  
Old 03-31-03, 10:45 AM
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It is not allowed to have neutrals and grounding wires interconnected in a subpanel. You will have to buy and install a separate grounding bar for your subpanel and move your grounding wires to it. Make sure that the neutral bar and the grounding bar are electrically isolated in the subpanel. You may need to remove a bonding screw or strap to accomplish this. If you temporarily disconnect either the neutral or grounding wire running from your main panel, you can use your ohmmeter to verify that the neutral and grounding bars are isolated.

Install at least one grounding rod. Depending on soil conditions in your area, the inspector may require two, separated by at least six feet. You might want to install two anyway just to play it safe.
 
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Old 03-31-03, 10:56 AM
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Just to be clear, there are 4 wires feeding the sub?
 
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Old 03-31-03, 03:06 PM
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I thought I'd read somewhere that you could either separate and use a ground rod or bring separate ground & neutrals to the sub and wire as a regular panel. Sberry27, I have 4 wires coming in. 2 hots, 1 neutral, and a ground. I had enough large NM left over from another job to run this out to my shop. Rather short run.

Thanks for the help,

Jester
 
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Old 03-31-03, 03:13 PM
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You have separate grounding and neutral wires. This is good. You still need grounding rods.
 
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Old 03-31-03, 03:18 PM
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So should I connect my ground conductor from the main panel and wire from rod to the new ground bar? Still keep the grounds and neutrals separate at the sub even though they are common in the main?

Thanks
 
  #7  
Old 03-31-03, 03:29 PM
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Yes. Yes.
 
  #8  
Old 03-31-03, 03:51 PM
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Excellent. Looks like I need to go to Home Depot.

Thanks for the help.
 
  #9  
Old 04-03-03, 12:04 PM
snovosel
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My electrician installed a 60 amp subpanel, within which he attached both the ground and neutral wires to only one bus bar. Did he do did it wrong? i just looked and there is only one bus bar in the box.

Is my entire electrical system in jeapardy?
 
  #10  
Old 04-03-03, 12:20 PM
lestrician
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If you are correct that it is a subpanel he did it wrong. Call him and have him come fix it.
 
  #11  
Old 04-03-03, 12:26 PM
snovosel
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Unbelievable!!! No wonder I haven't been able to get him on the phone for the past several weeks. He must have figured out that I figured it out!

I've already run one 20 amp circuit from it for use in my shop (without problem). Just wondering what the potential hazards are in its present state?
 
  #12  
Old 04-03-03, 12:52 PM
jpc1963
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Snovosel:

my electrician just did the same thing with the neutral and
grounds on the same bus bar in a subpanel. I fixed his mistake thanks to the people here.

Joe
 
  #13  
Old 04-03-03, 02:12 PM
snovosel
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Joe, what exactly did you do? Buy another sub panel?

Could I buy a new bus bar solely and screw it into the existing panel and then simply move over the white neutral wire that originates from the main panel?

I'm also beginning to wonder if there are any other issues this guy left me with!! Anything else I should be looking at?

SteveN
 
  #14  
Old 04-03-03, 02:23 PM
jpc1963
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No, I did not buy another subpanel. I bought a grounding bar and installed myself.

You can read all the gory details here ...
http://forum.doityourself.com/showth...hreadid=125640

Joe
 
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Old 04-03-03, 04:26 PM
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Sub panels in detached buildings can be wired one of two ways.

1) 4- wires - keep the neutral (ground) and equipment ground isolated from each other at the sub panel. The sub panel is bonded to the equipment ground.

2) 3-wires - bond the neutral to the sub panel. This method can be used as long as there are no metallic paths between the service panel building and the detached building. Metallic paths being cable tv line, telephone line, water pipe etc.

Most professional electricians install method #1.

Sub panels in the same building as the service panel always require 4 wires. Keep the ground (neutral) and equipment ground isolated from each other. Sub panel is bonded to the equipment ground.
 
  #16  
Old 04-03-03, 05:12 PM
texsparky
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snovesel,
Purchase and install a ground bar in the sub-panel. Do not move the white (neutral) wires from the bus that they are attached to. Instead, you are to move the ground wires to the new ground bar along with the ground wire that is coming from the main panel. Also make sure that the electrician did not install a bonding screw or a bonding strap from the neutral bus to the panel can.

If this a detached building, install 2 5/8" ground rods spaced a minimum of 6 ' apart and run a #6 wire from the new ground bar to the first rod and from there to the second rod. Use 5/8" acorn clamps to make the connection to the rod. ( Tip-- place the acorn clamps over the rods before driving them in the ground as the top of the rod will mushroom and make it difficult to install the clamp) Drive the rods to be flush with or below ground. If you have metallic piping in the detached building it will also need to be bonded to the ground bar with a # 6.
 
  #17  
Old 04-03-03, 05:31 PM
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It's too early to conclude that this is wired incorrectly until snovosel comes back and tells us whether this subpanel is in a attached or detached structure.
 
  #18  
Old 04-03-03, 06:14 PM
snovosel
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The subpanel is in the basement, the main panel is located in the attached garage one level up.

I went to HD and picked up a bus bar. It is the aluminum bar only and does not have a plastic or rubber mounting bracket as does the existing bar in the sub-p. Could I use a small piece of wood instead?

I'm curious to learn why I should only remove the ground from the existing bus? Why not treat it as the gound bus and use the new one as the neutral bus? Does it really make a difference?

Also, if the main panel is does not have separate bus bars (or if they are connected), does it matter then if the sub also fails to do so?

Thank you

SteveN
 
  #19  
Old 04-03-03, 07:17 PM
texsparky
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The ground bar you purchased will mount directly to the panel can. This bonds (grounds ) the panel can. The neutral bar is mounted on insulators and the neutral is not to be connected to the ground at any other point past the Main Service.
 
  #20  
Old 04-03-03, 07:27 PM
snovosel
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Ok, thank you very much!
 
  #21  
Old 04-03-03, 08:57 PM
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Just make sure that the green screw or bonding strap that connects the neutral to the case is not connected. Also make sure that between the panels you have a separate grounding wire (or metal conduit) and neutral wire. This exercise will accomplish nothing and will screw things up unless that is so.
 
  #22  
Old 04-04-03, 04:39 AM
snovosel
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By "a separate grounding wire (or metal conduit) and neutral wire" between panels, do you mean something other than the 3-wire NM feeder cable that came from the main panel? Also, would it make sense to "further" ground this box by running a separate bare copper wire from the sub-p box to a cold water pipe?

I'm not sure what you mean by making sure the "green screw or bonding strap that connects the neutral to the case is not connected". There is a large green screw in the middle of the neutral bus bar. Should I remove it? It doesn't immediately appear to have any function other than to provide an anchor to any wire that might be thread through it.

Where exactly is the bonding strap that connects the neutral to the case? The only thing I see is the neutral bus bar sitting on top of a black plastic bracket.
 
  #23  
Old 04-04-03, 09:48 AM
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I mean 3 insulated conductors and one bare conductors. If that is what your NM cable has, then you're in good shape. Connect the bare to the new grounding bar you are installing.

Absolutely do not further ground this subpanel in any manner.

Yes, you should remove the green screw in the middle of the neutral bus bar.

If you want to ensure that your neutral and grounding are truly isolated in the subpanel, temporarily disconnect the white feed wire in the subpanel (the white wire coming from the main panel). Then use an ohmmeter to measure resistance from the neutral bar to your grounding bar. If all is good, the resistance will be infinite. Try this before and after removing the green screw and let us know the results.
 
  #24  
Old 04-05-03, 11:41 AM
snovosel
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I just checked the main panel. The cable that goes from the main to the sub-p has both the white neutral and bare ground wires attached to the main panel's neutral bus bar.

Should I separate those as well?

As I look into the main panel, it would seem as though there are several circuits have have both neutral and ground wires on either bus bars. Normal in that regard? Or should each bus bar contain only its respective wire, bare copper or white neutral?
 
  #25  
Old 04-05-03, 12:14 PM
snovosel
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I just read that the NEC requires that the metal main panel frame be connected to the neutral and grounding buses by either a bonding screw or a strap supplied with the box. If that's the case, then perhaps I've answered the previous question. Which is to say that in terms of functionality it doesn't make that much difference?
 
  #26  
Old 04-05-03, 12:30 PM
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The grounding and neutral must be connected together in one and only one place in your house, and that place is where your main disconnect it. For most people, that means your main panel.

Do not try to wire your main panel and subpanel the same. The rules are different, and the rules must be different.
 
  #27  
Old 04-05-03, 04:00 PM
snovosel
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Understood. The two panels have different rules that apply.

BTW, I don't know how I missed it, but I just came across Joe's (jpc1963) prior post regarding the same issue. I must have been blind! Sorry to have to put everyone else through this again.

John, I tested the two buses with an ohmmeter per your suggestion. The white neutral from the main was removed. Here are the results.

Without the green bonding screw, I get no reading, it rests at infinity (on the left of the scale). With the green screw (attached to the can), the needle moves across the arc scale about two thirds of the way to the right. I was at Rx1 on the scale. The meter was zeroed out first.

Is this appropriate?
 
  #28  
Old 04-05-03, 04:11 PM
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Thanks for the results. You have confirmed that that green screw belongs in the trash, not in your panel.
 
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