Grounding panelboards

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Ivan Young

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I am a helper that needs some info on the right way to ground and bond panelboards. I understand in residential the neutral and equipment grounding bars must be grounded to each other with a special screw. All other panels does not nedd to be bonded like this .
In a commercial or industrial situation where there is lots of panelboards how does one know if the panelboard being installed needs the ground and neutral bonded?
Thanks,
Ivan
 
Ivan Young said:
I am a helper that needs some info on the right way to ground and bond panelboards. I understand in residential the neutral and equipment grounding bars must be grounded to each other with a special screw. All other panels does not nedd to be bonded like this .
In a commercial or industrial situation where there is lots of panelboards how does one know if the panelboard being installed needs the ground and neutral bonded?
Thanks,
Ivan
Click to expand...

In short, the neutral and EGC busses are connected together at the first point of disconnect(usually) for all services and a GEC is installed that goes to the GES.

All sub panels have separated EGC and neutral busses.

See 250.24

Abbreviations
 
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jumper said:
In short, the neutral and EGC busses are connected together at the first point of disconnect(usually) for all services and a GEC is installed that goes to the GES.

All sub panels have separated EGC and neutral busses.

See 250.24

Abbreviations
Click to expand...

I agree with that...:thumbsup::thumbsup:
 
Ivan Young said:
I am a helper that needs some info on the right way to ground and bond panelboards. I understand in residential the neutral and equipment grounding bars must be grounded to each other with a special screw. All other panels does not nedd to be bonded like this .
Click to expand...

The screw, or small strap, in a panel bonds the enclosure/can/box not the two busses together. A large strap usually connects the busses, if two busses are present in a main panel.

In a commercial or industrial situation where there is lots of panelboards how does one know if the panelboard being installed needs the ground and neutral bonded?
Thanks,
Ivan
Click to expand...

All sub panels need the EGC and neutral busses separated, and the can bonded to the EGC buss.
 
There really isn't a difference between residential and commercial. Residential can also have multiple subpanels where there should be no neutral-to-ground bond. Simplest way to put it: if there is a service disconnecting means in the enclosure, there should be a neutral-to-ground bond there. If there isn't, then there shouldn't be one.
 
The secret to understanding this is to forget the terms residential, commercial, etc. as electricity follows the same rules in any of those applications. (current still flows from one potential to another if there is a path to flow on)

The reason we ground and bond things together is to create a low impedance path so if there is a fault from an ungrounded conductor to conductive items it will allow for a high level of current to flow and cause operation of the overcurrent device.

The reason we only bond the grounded conductor to the grounding conductor at the source or first disconnecting means is to limit unintentional neutral current from flowing on all bonded objects in the electrical system. That current may be fairly harmless in a system in good condition, but when something happens to open a current path in something not intended to normally carry current - you will be subject to voltage across the points that became opened.

An example say we have a sub panel fed with metal raceway. Lets assume the neutral to can bonding jumper is installed. Lets assume there load on this neutral. With the bonding jumper installed that neutral curent has both the feeder neutral as well as the metal raceway as a path for current to flow through because they are bonded together not only at the source but at the load end of the feeder also. (and there may be other paths through other grounded objects that happen to be interconnected somehow) Lets just assume the feeder raceway is the only other path for now. What happens if the neutral comes loose for whatever reason on either end? The bonding jumper on the load end of the feeder still has a path to the neutral bus and that neutral current is all flowing in the raceway instead of the neutral conductor. This is not something you really want to happen. Now if the continuity of the raceway is interrupted you have a potential shock hazard.

Another issue is if you are loading your equipment grounding conductors with neutral current you are raising the potential to (earth) ground from your equipment grounding conductors and anything bonded to them because of voltage drop in those conductors. This will leave voltage between grounded metal objects and earth ground that can reach levels that will result in shock hazards, the more load the equipment ground conductors are carrying the higher the voltage drop will be.
 
Industrial and commercial installations will very often have transformers that are Separately Derived Systems (SDS). If the secondary of the transformer has a neutral a neutral to ground bond will be required. This may be at any single point between the transformer and the first disconnect. This point may be in the first panel, but is not required to be there.
 
Thanks for all the help. So from what you are`saying, only at the first disconnect are the neutral and the EGC connected. The long green screw bonds the EGC lug to the can and even in sub-panels that screw should be installed. Is this correct?
One last question .In chapter 9 table 8 you have bare, coated, and uncoated wires listed. Is the grounding the only wire that can be used bare?
 
You were getting the idea then you said "and even in sub-panels". Sub panels are past the first disconnect do not bond the neutral to the equipment grounding conductor beyond the first disconnect. I don't think you quite grasp why just yet. I think we covered the reasons fairly well but maybe we need some graphics or links to some graphics that may help some more. I may post something later that shows some of why we are supposed to do the way we do it.

Ivan Young said:
Thanks for all the help. So from what you are`saying, only at the first disconnect are the neutral and the EGC connected. The long green screw bonds the EGC lug to the can and even in sub-panels that screw should be installed. Is this correct?
One last question .In chapter 9 table 8 you have bare, coated, and uncoated wires listed. Is the grounding the only wire that can be used bare?
Click to expand...
 
Ivan Young said:
Thanks for all the help. So from what you are`saying, only at the first disconnect are the neutral and the EGC connected. The long green screw bonds the EGC lug to the can and even in sub-panels that screw should be installed. Is this correct?
One last question .In chapter 9 table 8 you have bare, coated, and uncoated wires listed. Is the grounding the only wire that can be used bare?
Click to expand...

You are correct that the can needs to be bonded to the EGC, and usually the "long green screw" is what is used. However, if the panel allows it, the bar that connects the neutral bus to the EGC bus must be removed. If you don't, and run the green screw in, you will be bonding the neutral to the can. The neutral must be isolated from the can in a sub panel.

As to your last question, the grounded (neutral) conductor can be bare in some cases.
 
Ok, I think I follow what you are`saying, ( I think). When installing a sub-panel, if there is a strap or clip that connects the ground and neutral it should be removed then that long screw will ground the EGC bar to the can. Yes some graphics would be most welcomed and appreciated.
Thanks again guys
 
Ivan Young said:
Ok, I think I follow what you are`saying, ( I think). When installing a sub-panel, if there is a strap or clip that connects the ground and neutral it should be removed then that long screw will ground the EGC bar to the can. Yes some graphics would be most welcomed and appreciated.
Thanks again guys
Click to expand...

some panels it is a strap, some it is a screw, the purpose is the same - to bond the can to the grounded service conductor. Once past the service equipment we keep the neutral separate from the ground by running separate conductors.
 
Ivan Young said:
Ok, I think I follow what you are`saying, ( I think). When installing a sub-panel, if there is a strap or clip that connects the ground and neutral it should be removed then that long screw will ground the EGC bar to the can. Yes some graphics would be most welcomed and appreciated.
Thanks again guys
Click to expand...

kwired said:
some panels it is a strap, some it is a screw, the purpose is the same - to bond the can to the grounded service conductor. Once past the service equipment we keep the neutral separate from the ground by running separate conductors.
Click to expand...

I think when he said "strap or clip", he was talking about what bonds the neutral bus and EGC bus together.

Ivan: You need to note where the green screw is located. It most likely is in the neutral bus and you shouldn't use the screw then as you would still be bonding the neutral to the can. You would either have to move the neutral (and neutrals from the branch circuits) to the other bus, or leave them where they are and not use the green screw. But you would need to bond the EGC bus to the can in an acceptable way.
 
Thanks for all the help you have given me. I want to put this topic rest. So far I understand that only at the first panel should the neutral and EGC bus ben bounded. A bar is what connects them together. The long green screw in turn bonds both the neutral and EGC to the can. All sub-panels should have the neutral and EGC not bounded together, but the EGC should be grounded to the can.
How am I doing so far?
The final questions I have are : 1) Is the neutral bar to be grounded to the can and the first panel? 2) The neutral bar at the sub-panels should not be grounded to the can?
Sorry to be such a pain, but I want to make sure I understand this because one day I may be in a position to have to make these decision and a bad one could cost someone their life or cause equipment damage.
Thanks for all your help.
 
Ivan Young said:
.
How am I doing so far?
The final questions I have are : 1) Is the neutral bar to be grounded to the can and the first panel? 2) The neutral bar at the sub-panels should not be grounded to the can?
Sorry to be such a pain, but I want to make sure I understand this because one day I may be in a position to have to make these decision and a bad one could cost someone their life or cause equipment damage.
Thanks for all your help.
Click to expand...

#1 Yes In the main panel the neutral bus along with the bround bus are both bonded to the

can. This is accomplished by the use of that green screw.

#2 Yes In sub panels Do Not Install that green screw. After the first disconnect the

neutrals and grounds must be seperate.
 
Thanks John. I have noticed in sub-panels that there is some sort of insulating material under the neutral bar, I guess that is to prevent the neutral bar from being grounded to the can.
But in sub-panels dont you have to ground the EGC bar to the can?
 
Ivan Young said:
Thanks John. I have noticed in sub-panels that there is some sort of insulating material under the neutral bar, I guess that is to prevent the neutral bar from being grounded to the can.
But in sub-panels dont you have to ground the EGC bar to the can?
Click to expand...

In all "service rated" equipment the neutral bar is separated from the can/enclosure by a non-conductive mount, usually plastic in newer panels.
Yes you have to bond the can to the EGC, see my post # 15. In some cases you may be able to use the green screw to accomplish that and in others you may have to use a strap or wire.
 
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