Sub-panel grounding

I'm wanting to place a sub-panel next to my main panel to accommodate future
expansion in my home.
After I have isolated the NEUTRAL, for the GROUND do I need to place a
separate grounding rod or can I just attach to the one already there?
OR do I just use the ground coming from the MAIN box via my conductor?
Thanx.
R. Tarin
Reply to
Randell Tarin
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------------- What is the code in your location? Follow it.
Reply to
Don Kelly
You don't need a separate ground rod. The ground bus in the sub must be bonded to the existing grounding electrode system. This can be done with a short nipple between the panels - but check with the local inspector for the accepted practice in your location.
Ed
Reply to
ehsjr
IIRC, it is also important to *separate* the grounding bus from the neutral bus in the sub-panel. That is, you need *four* electrical connections to the main service panel. Two hots, a neutral and a ground. As you said, the ground *can* be via a short nipple between cases, but it must be separate from the neutral.
daestrom
Reply to
daestrom
Local code has the GROUND bonded to the NEUTRAL at the MAIN. This was accomplished with a built-in bonding strap.
Since the the ground in the MAIN box is bonded to the ground, then a nipple wouldn't work because it would bond the sub ground to the neutral as well.
It seems that I would have to isolate the grounding bar in the sub-panel and run a separate grounding wire to the existing grounding bar.
Reply to
Randell Tarin
That is correct.
The nipple bonds the sub-panel enclosure to the grounding system, which is necessary for safety and code compliance. However, the only place the neutral should be bonded to the grounding conductor is at the service entrance. Therefore, you need two separate bars in the sub-panel. The neutral bar that all of the grounded conductors (white) connect to must be isolated. This means remove or do not install the bonding screw or strap. On the other hand, the ground bar that all of the equipment grounding conductors (green) connect to should be bonded to the metal enclosure, and not isolated.
Ben Miller
Reply to
Ben Miller
True.
Not true. You want the enclosure and the *grounding bus* inside the enclosure to be bonded to the main panel frame and grounding bus in the main panel. The fact that the grounding bus and neutral bus are bonded in the main panel is not an issue.
And, you want the neutral bus in the sub-panel to *not* be bonded to the sub-panel frame or the sub-panel grounding bus. The neutral bus and grounding bus must *only* be connected at the main/service-entrance panel.
So, to power the sub-panel requires: 1) firm electrical contact on the panel (such as the pipe-nipple arrangement, or a bare wire between the main-panel grounding bus and the sub-panel grounding bus). 2) An electrical connection from the neutral bus in the main panel and the neutral bus in the sub-panel with an *insulated* wire. 3) The neutral bus in the sub-panel must *not* be bonded to the sub-panel frame or grounding bus (as Ben said, remove/do-not-install the bonding jumper). 4) One or two 'hot' conductors from the main panel to the 'hot' busbar(s) of the sub-panel.
No, isolate the *neutral* bar in the sub-panel. Running a fourth wire between the grounding bus in the main and the grounding bus in the sub-panel *is* an option. Especially if they are some distance apart.
daestrom
Reply to
daestrom
Is it permissable under NEC to drive another ground rod near the subpanel and bond the ground of the subpanel to it as well as (via 4 conductor) to the Main box? The two panels are on opposite sides of my residence.
Joe
Ben Miller wrote:
Reply to
**THE-RFI-EMI-GUY**
Thanks, I've got it now.
The sub will be right next to the main. I can easily run a fourth wire.
Reply to
Randell Tarin
You can connect as many grounded objects as you want, such as water lines, structural steel, ground rods, etc. to the equipment grounding system anywhere. In fact, this will happen whenever you mount boxes or conduit to buried steel structure, or connect to any buried conduit. If your sub-panel feeder runs underground in steel conduit, then you already have it connected to a grounding electrode!
Just remember that it all must be bonded ultimately to the grounding electrode at the service entrance. It is not acceptable for the earth to provide the sole return path for fault current from the sub-panel to the main panel. As long as you have the fourth wire or a metal raceway between the panels you are fine.
The neutral is only bonded to a grounded object at the service entrance. It is isolated from the grounding system everywhere else in the facility to prevent a parallel return path for current through conduit and equipment enclosures.
Ben Miller
Reply to
Ben Miller
Thanks Ben;
Joe
Ben Miller wrote:
Reply to
**THE-RFI-EMI-GUY**
Make sure you understand *how* to add grounds to your system. Quoting Ben, for emphasis: "It is not acceptable for the earth to provide the sole return path for fault current from the sub-panel to the main panel." Any ground you add must be properly bonded to the grounding electrode system. Ed
Reply to
ehsjr
Now if we could only get the IT industry and some automation vendors to understand that simple concept, and abandon the idea of "isolated" ground rods!
Reply to
Ben Miller
You guys are basically answering my original question:
My MAIN is grounded to an 8' copper electrode via a #4 bare wire. My subpanel will be only inches from this installation. I can bond to the electrode OR to the MAIN grounding bar. As long as the NEUTRAL is isolated in the sub-panel, is there any difference?
Reply to
Randell Tarin
Thus spake Ben Miller:
Thanks, Ben, for an explanation of the technical reason for ground/neutral isolation in subpanels.
What are the dangers and symptoms of having parallel return paths between the the SE panel and a subpanel?
Thanks,
Reply to
DaveC
The danger is the possibility of raising the touch potential on the exposed metal to levels that could be fatal. It might only be a few volts normally, but deterioration of the grounding conductor (which might be accelerated if it carries current continuously) or abnormally high current could raise it. By code, equipment grounding conductors should not carry any current under normal operating conditions.
BEn Miller
Reply to
Ben Miller
Per my original post, I will use 4 wires including an insulated neutral and ground return. The additional ground rod at the subpanel will be bonded back to the main panel via the ground return. The neutral bus bar in the subpanel will "float" and be bonded via insulated neutral at the main panel.
ehsjr wrote:
Reply to
**THE-RFI-EMI-GUY**
Your sub and main are side by side, connected by a nipple. (Don't forget the 4 locknuts) This is what you want your installation to look like:
Ground Ground Rod 1 Rod2 --------- --------- ||===========||======|MainPanel|=|Sub Panel| || || --------- --------- || || || || || ||
Installing a subpanel does not automatically mean you must add a supplementary grounding electrode. In your case, since the sub will be nippled off the main, and want to add a second ground rod, you want it to look like the diagram.
The physical connection of the "grounding electrode conductor" which is the single, continuous unspliced wire from rod 1 to rod 2 to the main panel is important, and must be up to code.
Right or wrong, there are inspectors who will reject your installation if it is physically arranged like this:
Ground Ground Rod 1 --------- --------- Rod 2 ||==============|MainPanel|=|Sub Panel|====|| || --------- --------- || || || || || || ||
Check with the local inspector & do it the way he/she recommends. There's a bunch of rules in the code concerning the grounding electrode system, and they must be followed.
Ed
Reply to
ehsjr
Also bushings
bud--
Reply to
Bud--

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