Six years ago, I built my workshop about 150ft from the house with a
100A service. I ran a four-conductor cable to the subpanel in the
shop, and kept the ground and neutral conductors isolated from each
other. I did not bond the sub-panel ground to its own ground rod,
thinking it might cause a ground loop and/or noise in the intercom
circuit bewteen the buildings. More recently, when I put in a spa with
it's own GFCI breaker, I had several people advise me to drive a
separate ground rod for it, and that I should have done the same for my
workshop. So far, I have had no issues, but I want to make sure I did
the right thing.
My searches here have yielded many debates on the need to keep the
ground and neutral bonded only in the main panel and not the subs
(which is how I did it), but nothing about the ground rod question.
Does a remote subpanel need its own ground rod or not?
Thanks in advance,
Unless you have a connection between your garage and main dwelling such as
water pipe, you need to install a ground rod for your sub panel. You don't
need to put in a ground rod to your spa.
You do, however, need to bond any grounded metal within 5' of the spa water
to the bonding lug on your spa panel, using bare solid copper.
Supplemental grounding gets tricky pretty fast.
Excellent choice for the 4 wire service to the garage.
Now if you drive a ground rod at the garage then the grounding conductor
back to the service must be of the same size as your service ground. If not
you COULD have a problem with an fault.... note could..
As for the spa driving another ground rod is just complicating the issue. I
assume that you took a grounding conductor to the spa, if it is connected
correctly you do not need a ground rod there.
Try reading the Soars book on Grounding. Lots of pictures and easy text to
Heck I probably do not understand what your doing cause I can not see it.
If he ran a 100A feeder to the garage panel, the grounding wire is at
least 6 gauge, and maybe 4. IIRC, #6 is the largest wire you are ever
required to use to connect a ground rod to the grounding electrode system.
I would probably drive a ground rod at the garage. I don't know if it
is required or not (but I think it is.) Since there's a seperate
grounding wire already, I don't think it's all that important.
(I wish I could find my code book, even though it's 10 years old.) I
don't think NEC ever requires a conductor that big to connect a ground
rod to the electrode grounding system. In fact, I think there is a
specific exception that says it never has to be larger than #6 copper or
#6 copper-clad aluminum. It probably does require an equipment
grounding conductor (ground wire), or grounded conductor (neutral wire)
that big for 200A or larger feeder circuits, but that's not what we are
Since you ran a grounding conductor with the feed to the subpanel, you do
not need to install a ground rod at that location.
If you feel the need to install a ground rod it should be at the main panel.
What is your current main ground?
Simply not true. Anything beyond a single branch circuit to a remote building
requires a ground electrode system. The only thing a 3 wire vs 4 wire feeder
affects is regrounding the neutral at the remote building.
Greg, I think you misinterpreted the pictures as indicating a ground rod for
each panel. The grounding symbol is used to only illustrate a common
ground. Read the text and you will see that an additional grounding
electrode is not required if a grounding electrode conductor was run with
the service feeder.
The original poster did the job correctly by running four wires and keeping
his grounding conductor separate from his grounded conductor (Neutral).
Sorry John but that is bad information.
Every building that has more than a branch circuit to supply it has to
have a grounding electrode system. That can be as little as a single
ground rod if it has an impedance to ground of twenty five ohms or less.
If the impedance of the single rod is higher you have to add a second
rod. Most electricians do not bother to measure it and just add a
second rod. If the building already has an electrode such as a concrete
encased electrode then you only have to bond the service equipment
enclosure to it and your done. In all cases each separate building gets
a grounding electrode system. There are no exceptions under the US NEC.
The objective is to "limit the voltage imposed by lightning, line
surges, or unintentional contact with higher-voltage lines" and
"stabilize the voltage to earth during normal operation."
250.50 Grounding Electrode System.
If available on the premises at each building or structure served, each
item in 250.52(A)(1) through (A)(6) shall be bonded together to form the
grounding electrode system. Where none of these electrodes are
available, one or more of the electrodes specified in 250.52(A)(4)
through (A)(7) shall be installed and used. Copyright 2002 National
Fire Protection Association.
In short every building
1) needs a grounding electrode system if it is supplied with more than a
2)The building disconnecting means gets bonded to the grounding
Tom, I agree that each building needs to be connected to the grounding
electrode system. However my interpretation is that the grounding electrode
system originates at the main service entrance and can be extended to
secondary buildings along with the power. Therefore a ground rod at each
secondary building is not necessary as long as the building components
(Metal piping, steel, subpanel, etc.) are bonded to the grounding electrode
system that originates from the main service.
I seem to recall an article from a few years ago in one of the trade
magazines that discussed this issue of multiple grounding along a
distribution system. I think one of the problems is that it is possible
that the ground rod in a secondary building could have less resistance than
the primary ground rod (Or water pipe). Since electricity follows the path
of least resistance it is possible that the loss of a neutral could have
current travel away from the main service to a secondary building's ground
rod. Anyway, my memory isn't that great. I could be mistaken.
We can talk what ifs until the cows come home. The requirement in the
US NEC is plain. If the building is supplied by a feeder rather than by
a branch circuit it must have a grounding electrode system and the
building disconnecting means must be bonded to it. It makes no
difference whether there is or is not an Equipment Grounding conductor
run with the feeder a grounding electrode system is still required at
each building. Which conductor of the feeder gets bonded to the
Grounding Electrode System at each building is dependent on whether
there is an EGC in the feeder. Either way you must bond the building
disconnecting means to a grounding electrode at the building that the
building disconnecting means serves. The US NEC does not allow any
other course of action.
Any circuit that starts at one Over Current Protective Device (OCPD) and
supplies other OCPDs is by definition a feeder vis.
ARTICLE 100 Definitions...
Feeder. All circuit conductors between the service equipment, the source
of a separately derived system, or other power supply source and the
final branch-circuit overcurrent device...
Since the "final branch-circuit overcurrent device" are the circuit
breakers that protect the branch circuits which are located in the panel
in the garage the circuit that supplies that panel is a feeder. The
exception in the NEC is meant to exempt small sheds or out buildings
that are literally supplied by a branch circuit. To be a branch circuit
the last OCPD has to be at it's source.
Branch Circuit. The circuit conductors between the final overcurrent
device protecting the circuit and the outlet(s).
That exception that exempts buildings supplied by a branch circuit from
the requirement for a grounding electrode system has a companion
exception that exempts such buildings on residential property from
having a building disconnecting means that is listed as "Suitable for
Use as Service Equipment" The exception says
225.36 Suitable for Service Equipment.
The disconnecting means specified in 225.31 shall be suitable for use as
Exception: For garages and outbuildings on residential property, a snap
switch or a set of 3-way or 4-way snap switches shall be permitted as
the disconnecting means.
When taken together the requirements for buildings that are supplied
from a service that is located in another building come down to all
buildings must have a disconnecting means. All buildings supplied by a
feeder or service must have a grounding electrode system. The
disconnecting means must be bonded to the grounding electrode system by
a properly sized Grounding Electrode Conductor.
I am not being so insistent in order to be argumentative but rather
because I don't want neophyte DIYs to follow unclear or misguided advice
and come to harm.
I think that we have confused everyone who is following this thread enough
so I will end it here. I will be attending a grounding and bonding class in
a few weeks as part of my continuing education requirements. I will
certainly be bringing this issue up in that class.
Out of curiosity a question for everyone: Is the original poster required
by 2002 NEC standards to install a ground rod at his subpanel location?
YES or NO, no more discussion PLEASE! :-)
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