ground rod question [RE-wrenches]

Sun May 21 15:56:04 PDT 2006

David, Darryl, Peter and all,

I concur with the opinions already voiced. I'm including code references as
justification and clarification, some of which is based on past
conversations with Mr. Wiles -I hope I have this right...

>From a code perspective, a pole mounted array qualifies as a "separate
structure" (see Article 100 defn. of "structure"). Section 250.32 addresses
two or more structures supplied from a common "service." (As with other code
rules that are applied to PV systems, by inference a PV source may arguably
fall under this code rule, even though it is not a "service"). In this case,
the two structures are the array pole and the building where the PV system
is. 250.32(B) describes the grounding and bonding requirements for two
cases -with and without an equipment grounding conductor running together
with the "service" (in this case, array) conductors. In either case, a
grounding electrode "system" (rod or rods in David's hypothetical) is
required at the pole, since it is by definition a "structure." And a
grounding electrode conductor is required, connecting the equipment
grounding system of the array to the grounding electrode system. If no
equipment grounding conductor is run back to the PV system along with the
array conductors, you're required to bond the grounded array conductor
(usually negative) to ground at the pole -not a good idea when using a Solar
Boost charge controller -I'm not sure about and MX-60. And definitely not
what you want to do if you do run an equipment grounding conductor back with
the array conductors (multiple bonding points).  IMHO, running an equipment
grounding conductor back from the array, buried in the trench to increase
earth contact, is the best approach. By the way, I believe the 2008 code
proposals will require ground fault protection on all PV arrays, not just
those mounted on dwellings. In that case, an equipment grounding conductor
from the array will be necessary to allow the GFP device to operate
properly, (unless a different type of GFP device becomes available).

David, you mention running the #6 ground wire back to connect to the system
ground rods. As a "separate structure", the array grounding electrode system
is technically separate from the grounding electrode system of the house.
The buried ground wire is, technically, the equipment grounding conductor
for the array home run circuit, not a bonding jumper between grounding
electrodes, though electrically it does serves as a bonding jumper. It's a
definition issue -as an equiptment grounding conductor, the conductor sizing
rules of NEC 250.122 apply, including the requirement to upsize the
equipment grounding conductor in proportion to any upsizing of the array
home run conductors. With a single MX-60, the 60 amp PV input breaker would
require a 10AWG equipment grounding conductor from the array. Technically,
you could skip the #6 and  instead run a #10 array equipment grounding
conductor in the conduit along with the array conductors back to the DC
disconnect (as long as the array home run conductors have not been upsized
for voltage drop concerns). You still need the grounding electrode system
and grounding electrode conductor at the array. Though this saves a bit on
copper, running the equipment grounding conductor back to the DC disconnect,
in the absence of a bonding jumper between grounding electrodes at the pole
and house, seems like an invitation to a lightning surge landing in the
middle of the system electronics. And in most cases, the array home run
conductors will be upsized at least one wire size for voltage drop purposes,
requiring the #10 equipment ground to be upsized anyway. The #6 (or larger)
bare wire buried in the trench addresses both issues. A #6 equipment
grounding conductor is essentially a 253% upsizing of the equipment
grounding conductor required for a 60 A circuit. Depending on how much you
may have upsized your array home run conductors, the #6 may be just fine. If
you've upsized your home run conductors more than 253%, you should
additionally upsize your equipment grounding conductor proportionally, and
that upsized wire should ultimately tie all of the way back to the ground
bus in the DC disconnect.

-unless your inspector tells you to do something else  (:-|

-Which is also what may be the answer to Peter's original question, since
NEC 250.50 speaks of grounding electrodes "present at each building or
structure..." -depends on what the definition of "at" is (your honor). It
would be up to your inspector to "approve" of your ground rod placement.
Ground rod spacing for multiple ground rods is covered in 250-53(B), and the
requirements for multiple rods is covered in 250.56. On ground mounts, I
drive the ground rod just beyond the concrete, and try to keep the ground
clamp and grounding electrode conductor buried in the soil to avoid damage
and to prevent a trip hazard.

-By the way, what is going on with the price of copper?!

Dan Rice
Abundant Sun, LLC.

----- Original Message ----- 
From: "David Palumbo, Independent Power & Light" <ipl at sover.net>
To: <RE-wrenches at topica.com>
Sent: Sunday, May 21, 2006 11:46 AM
Subject: RE: ground rod question [RE-wrenches]

|
| Darryl,
|
| Here's a hypothetical for you.
|
| Off grid system with ground mounted top of pole array located 150'(or
more)
| from the house.
| Do you still run a ground wire back to tie in with the other ground rods?
|
| I have always done it like you state, with two ground rods connected to a
#6
| bare ground wire (in the trench with the conduit for the transmission
wires
| (high voltage to MX60)) which is connected to the system ground rods.
|
| Now with copper becoming so dear I certainly do not want to waste it is
not
| necessary. What say you wrenchers?
|
| David
|
|
| Hello Peter
| Below is my understanding of the codes regrading
| grounding.  I have certainly been wrong in the past
| and have had many discussions with inspectors on
| grounding.
| As I see it the ground rod on a PV array is for two
| purposes, One to divert any lightening induced surges
| to earth protecting your equipment and Two to protect
| grounded people from any errant voltages/currents that
| they might contact during fault conditons.  (part two
| is what the electrical inspector is looking for)
|
| For part one The shorter the ground lead the better.
| and this is not NEC but NFPA 780  78? the lightening
| protection code.  The connection should have only
| radius  bends, and a larger outside diameter and of
| course a good ground connection.  Y0our array will act
| as a capacitor with the sky and as lightening
| discharges occur within the county the sky earth
| changes potential and your array will have differing
| amounts of energy to dissipate.  I always drive two
| ground rods and separate them by there sphere of
| influence 8 to 10 feet.
|
| In case two the grounding is to protect against any
| "generated fault currents" currents from the PV system
| or back feed through the system from the Utility or
| other source that should happen to get on the frame.
| This is the one that the inspector is concerned with.
| You have a separately derived system and it needs a
| grounding electrode system.  You can either bring the
| ground wire back to the service location and ground
| the array with one rod or ground with two rods at the
| array.  I ALWAYS GROUND AT THE ARRAY AND BERING BACK A
| GROUNDING WIRE.  When wiring a garage for example the
| ground rod is always placed
|
| Hope this helps and is correct.
| Darryl
|
| --- Peter Duchon <info at asappower.com> wrote:
|
| >
| > What is the max. distance an 8' ground rod can be
| > from a ground mounted
| > array?   Or where is considered best placement for
| > ground rods on
| > ground-mounted arrays?   Ok, that's plural, two
| > questions.  Pertinent NEC
| > 2005 section would be great if anybody has that info
| > handy otherwise.
| >
| > Thanks,
| > Peter
| > asappower.com

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