Bonding screw on Square D service panel

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  #1  
Old 12-01-06, 06:40 AM
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Bonding screw on Square D service panel

When I moved into my house 3 1/2 years ago, we had a new Square D (QO)100 amp service panel installed. I have noticed that the electrician did not install the bonding screw that bonds the neutral/ground bar to the panel, and assumed that he knew what he was doing.

Can anyone tell me under what circumstances the bonding screw should be installed?

The reason I'm asking this now is that I've run out of room on the neutral/ground bus (even with doubling up on the ground wires of same #), so I bought the appropriate expansion ground bar kit for this panel. The instructions only say to screw the ground bar to the panel, so I'm now wondering if I need to install the bonding screw - otherwise how will this expansion bar be connected to the existing ground/neutral bar?

Thanks for any help!
 
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  #2  
Old 12-01-06, 06:47 AM
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The bonding screw is used to make the neutral bar a ground bar.

The ground bar is always bonded to the panel, to ensure that the metal box is properly grounded.

The neutral bar is only bonded to the panel when it is a dual purpose ground bar and neutral bar.

In a sub panel the neutral bar must be floating (NOT connected to the panel).

If this is your main panel then the bonding screw should be installed. It is not absolutely necessary if there is a second bar which is bonded, since the panel will be grounded by the second bar.
 
  #3  
Old 12-01-06, 07:58 AM
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Just a few things to check to cover all your bases. The fact that the neutrals and grounds are landed on the same bar indicates that this is a main breaker panel, if so then this panel needs the bonding screw installed just as Bob has described. Provided this is the first panel after the meter that also includes the main disconnect. Three wires will enter the panel from the meter H-H-N.

If you have a main disconnect out by the meter or before the panel that would probably indicate the panel is main lug only (no main breaker). If this is the case then the panel should be wired as a sub-panel with neutral floating (insulated from the metal can)...ie..grounds and neutrals on different bars with the ground bar bonded to the can. You would have four wires entering the panel H-H-N-Grd from the disconnect.

If your ground bar kit is one for your make of panel then there should be a set of raised swaged holes that are on the backside of the panel can where the new ground bar should be fastened. If the panel is the main disconnect panel then install the green bonding screw this will bond the neutral bar to the new ground bar via the metal of the enclosure. You can then land both neutrals and grounds to this newly installed bar.

Roger
 
  #4  
Old 12-01-06, 10:55 AM
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Thanks for your quick responses!

In response to your questions/clarifications/answers:
1. Yes - This is the first (and only) panel after the meter and does include a main disconnect breaker. And Yes, both neutrals and grounds are landed on the same bar. Based on this, it looks like I need to install the bonding screw (and it seems like it should have been installed from the start).

2. Yes - the ground bar kit is the model referenced for my panel model on the Square D website, and there are sets of wholes to fasten it to on both sides of the back of the panel.

So, to clarify: I will install the bonding screw, and the ground bar kit. The bonding screw will connect the main neutral/ground bar to the expansion ground bar via the panel itself. This will then allow me to terminate both grounds and neutrals on any of the above bars.
Correct?!?

(Seems to my sense of logic (which of course is based on a non-professional point of view) that bonding the neutral/ground to the panel could create a situation where the panel itself could at some point be "hot." - But then I probably don't have a complete grasp of the overall neutral/ground concept!)
 
  #5  
Old 12-01-06, 12:24 PM
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Yes you can land both neutral and ground to this bar. It is common when possible to move some existing grounds to the new ground bar and use the opened holes to land your new neutrals. This puts only the grounds on the bar you added. But this is just cosmetic.

You are in no danger of shock or an energized panel. Return current on the neutrals will follow the lowest impedance path to the center tap of the serving transformer via the service neutral. Reason for the bonding of neutral and ground at the main panel and only at the main panel. Ground wires of the branch circuits, more appropriately called equipment grounds, are for safety and do not carry current. They only carry current in the event of a short to ground (fault current). This facilitates or allows massive current to flow to trip out the circuit breaker. Many people error in thinking that the ground rods and grounding to the water pipes does this, however this is not the purpose of the grounding electrode system. This is that big fat solid copper wire you see in the main panel.
Neutrals on the other hand do carry the return current and will shock you or worse but not likely in the main panel given that bonding is correct to the grounded service (neutral) conductor. Touch a neutral on a branch circuit that has a load operating on its circuit anywhere but the main panel and it will bite you. If you were to take a neutral loose from the neutral bar in the main panel and the branch breaker is still on and a load is operating on the circuit watch out because you just removed the bonding to the grounded service neutral conductor and you have just become the only impedance to ground. It is best just to consider a neutral as a hot wire and dont touch it period unless the circuit is deenergized by the circuit breaker being turned off. If your working in the main panel turn the branch breakers off to remove load from the main breaker and return current to the neutral bar then turn the main off to take power off the two hot busses of the panel. Make sure you are standing on a non conductive surface.

This bonding and grounded and grounding subject is one of the most difficult subjects to grasp and I'm sure others here may have some things to add to what I have said and may even disagree. You seemed curious to understand so gave a short and hopefully accurate answer.

If you consider the fact that your bonding screw wasnt installed and something would happen that would cause a single hot wire to contact or pass voltage to the metal can of the enclosure then that metal of the enclosure would come to120 volts. Since the bonding screw is missing there is no way for the fault current to travel a path of low impedance to the service neutral then to the center tap of the transformer. No breaker would trip. So you can see the danger in not having the bonding screw in place when its required.


Roger
 

Last edited by Roger; 12-01-06 at 12:47 PM.
  #6  
Old 12-01-06, 12:47 PM
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Roger:

Thanks very much for your explanation. I will certainly agree that this is a complex/confusing subject, and your explanation helps a lot!

Thanks!

(I assume that it would be best to at least turn off the main breaker, and even better to flip the service disconnect on the meter outside, before installing the expansion ground bar kit.)
 
  #7  
Old 12-01-06, 12:58 PM
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Whoa!!

You have a main disconnect at the meter?

Please describe what you have this sheads a whole new light on things if this disconnect kills the power to your main breaker panel. Is it a circuit breaker rated in amps equal to your service?

Roger
 

Last edited by Roger; 12-01-06 at 01:16 PM.
  #8  
Old 12-01-06, 08:55 PM
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Sorry, my mistake. I wrote this morning's posts at work and was operating from memory. For some reason I was thinking there was a disconnect on the meter itself. I looked when I got home tonight and that is NOT the case. The main breaker on the panel is the first of any kind of disconnect on the main line.

Sorry to confuse the issue.

Does this mean that the only way to shut off the power coming into the panel is to call the electric company?
 
  #9  
Old 12-01-06, 11:31 PM
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Ok no problem....everything stays the same.

By turning the main breaker off in the panel you are only energized at the lugs of the main breaker. There will be no neutral current or hot busses that the breakers connect to. This is normal when workinging in this type panel. Just take care not to contact the main breaker lugs. Some people will make a cardboard guard to keep from contacting the lugs with a wire or person. Square d on some panels makes a guard that you can install. But to answer your question..... yes....if you want all power removed from the panel the meter will need to be pulled. If that is your desire then you will need an authorized person to do that.

What exactly are you wanting to add to the panel and what equipment/appliances are you going to serve?

Roger
 
  #10  
Old 12-03-06, 08:19 AM
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When you install the ground bar, doesn't the mounting screws themselves "bond" the bar to the cabinet? Why is a separate bonding screw necessary?

Sorry to hijack your post, I was just reading along and the question came to mind.
 
  #11  
Old 12-03-06, 09:10 AM
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When you install a ground bar the screws provide the bonding.

The bonding screw being discussed is for the factory installed neutral bar. You screw in the bonding screw if this is to be a main panel, you remove it and add a ground bar if this is to be a sub panel fed with four wires..
 
  #12  
Old 12-08-06, 11:19 AM
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Originally Posted by Roger View Post

What exactly are you wanting to add to the panel and what equipment/appliances are you going to serve?

Roger

I'm just adding the expansion ground bar because I've run out of room on the factory installed neutral/ground bar (that's even after doubling up on the grounds of the same gauge). I need more room.
 
  #13  
Old 12-11-06, 05:17 AM
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After reading this post I realized I needed to install this screw in my panel. I assumed that there would be a place for me to screw this supplied screw through the bar & into the cabinet in one fell swoop, but I don't see such a convenience. There is a hole which passes through the bar & the cabinet, but it is not threaded, nor is the bonding screw long enough. How do I go about installing this screw without tapping & threading the cabinet & installing a grounding lead from the bar?

EDIT: I should have mentioned it's a brand new 40 slot, QO 200 Amp panel.

Thanks in advance,
Dan
 
  #14  
Old 12-11-06, 07:26 AM
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If the bonding screw is not long enough then you are not installing it in the correct location or it is not the screw that came with the load center or the incorrect screw came with the panel. I suspect you are not finding the right location. If this panel is one that has neutral bars on both sides of the main breaker then there is a metal bonding strap that runs horizontally between them. In most of the QO's the green bonding screw installs through this bonding strap. You will jsut have to look and see. There may be a location designated on the spec. sheet on the inside cover. The panel must be the service equipment disconnecting means before this screw is needed. There is one exception for a detached building with a 3 wire feed its sub-panel.

Roger
 
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