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.
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.
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.
On 10/17/05 6:54 PM, in article NQW4f.72191$ email@example.com,
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
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
Ben Miller wrote:
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
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
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?
On 10/23/05 9:54 AM, in article firstname.lastname@example.org, "Ben
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
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:
Rod 1 Rod2 --------- ---------
|| || --------- ---------
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:
Rod 1 --------- --------- Rod 2
|| --------- --------- ||
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.
I am not the original poster. My main and sub panels are about 70 feet
apart, thus I am going to ADD a supplementing ground electrode outside
the Sub panel as well as provide continuous bond of the ground to the
main panel and its existing electrode.
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?
Please, no "Go Google this" replies. I wouldn't
ask a question here if I hadn't done that already.
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.
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.
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