[RE-wrenches] PV GEC to the same point as theutilityservice GEC?

Tue Aug 24 14:40:40 PDT 2010

Ray and Matt,
Thanks for sharing your perspective on the EGC and GEC requirements. I share
Ray's distaste for creating ground loops with redundant GECs, and I'm
thankful for 690.47(C)(3), particularly the last sentence:

"A single conductor shall be permitted to be used to perform the multiple
functions of dc grounding, ac grounding, and bonding between ac and dc
systems."

In the draft of the 2011 NEC, there is completely new wording for 690.47(C).
I find it to be more understandable than the 2008 code, and I recommend
checking it out:

http://www.solarabcs.com/index.php?option=com_docman
<http://www.solarabcs.com/index.php?option=com_docman&task=doc_download&gid=
248&&Itemid=72> &task=doc_download&gid=248&&Itemid=72

As for 690.47(D)... I just can't stop hating this one. I was dismayed to
find that it made it into the 2011 NEC without changes. Is there any purpose
for 690.47(D) other than enhanced protection from direct lightning strikes?
If so, and you're installing in an area where direct lightning strikes are a
problem, shouldn't you be installing a proper lightning protection system
per NFPA 780? As far as I can tell, the main practical consequence from
implementation of 690.47(D) is that you're almost guaranteed to set up a big
ground loop that will suffer induced currents from nearby lightning strikes.
Please, somebody justify the existence of 690.47(D), particularly for those
of us that live in areas where lightning is rare, so I can stop hating it.

Regards,
-Hans

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of R Ray
Walters
Sent: Monday, August 23, 2010 1:55 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] PV GEC to the same point as theutilityservice
GEC?

I think the issue is that some installers are wanting the supplementary
electrode for the array tied to the building grounding system in two places.
I saw this in BIll Brooks seminar too, but then he said it wasn't necessary.
Basically the EGC system already ties both electrodes together as required,
and a 2nd conductor directly between the two electrodes can create a
lightning loop. 
If the electrodes would be close together, you can actually drop the array
electrode, and tie straight to the building electrode. But if it is on the
other side of the building for instance, some have run 2 conductors, 1) the
already required EGC through the AC conduit, and then 2) a separate GEC on
the outside of the building that connects the electrodes a 2nd time. 

This would be the same logic as bonding your electrode of your house to your
neighbor's house: it's not necessary ( they're already bonded through the
services) and it can cause trouble, especially with lightning.
Even my new ground impedance tester only works, if each grounding electrode
is bonded once (and only once) to the rest of the grounding system. 
It's a key point of lighting protection in multiple electrode grounding
systems too: never create loops in your grounding system. 

R. Walters
ray at solarray.com
Solar Engineer

On Aug 23, 2010, at 9:27 AM, Matt Lafferty wrote:

Hi Jason,

I'm a little confused about your intention. I think you might be making this
harder than it needs to be. Everything you need is right there in 2008 NEC
690.47(C).

Where are you tying into the building AC distribution system? Get your GEC
the same place the existing AC distribution system at your point of
interconnection is bonded. If the grounding electrode conductor for the
point of interconnection is building steel (like the delta-wye transformer
scenario described in your original post), source your GEC there.

Your AC & DC grounding system(s) must be bonded together per 690.47(C)(1).
For "normal" interactive, non-backup systems, I use a common ground bus for
both AC & DC (690.47(C)(5). I size it and any bonding jumpers as the smaller
of #8 CU or whatever 690.47(C)(2) & 690.47(C)(7) comes out to. My common GEC
originates at the AC interconnection per 690.47(C)(6) & 690.47(C)(8). I run
the common GEC to and thru the inverter(s) and out to the array thru the
raceway system.

NOTE: Once you are on the array side of the inverter(s), the GEC is a DC
equipment ground. I size it not smaller than #8 and large enough to carry
any unintentional system currents safely to ground based on the DC
configuration. This may result in multiple ground conductors running thru
different conduits. Every pipe (AC & DC) gets a ground wire in my systems.
DC raceways should also be bonded to the GEC (meyers-hub, ground-bushing,
etc.). AC raceways on the load side of an OCP do not require this provision
by code, but it doesn't hurt to go ahead and do it anyway.

690.47(D) is for equipment grounding. This is a separate issue from the
system GECs described in 690.47(C). I consider it to be "enhanced" or
"bonus" equipment grounding. In addition to the requirements of 690.47(C),
you must ALSO bond your rack to the building steel and make sure the steel
is properly bonded to earth, OR run a separate ground conductor from the
rack to a grounding electrode. Either way, this becomes another grounding
electrode system that you need to bond to your 690.47(C) GEC system. If you
are bonding all your equipment correctly, this should happen via the
equipment grounding conductor(s) mentioned in the NOTE above.

I hope this helps clarify it for you.

Matt Lafferty
gilligan06 at gmail.com

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jason Szrom
Sent: Monday, August 23, 2010 6:41 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] PV GEC to the same point as the utilityservice
GEC?

Matt,

Thanks for the reply. I should have clarified, but I am talking about a
standard grid-tied, non-backup system.

I'm still not quite sure I have any solid code sections to back up my
assumption that it is OK to tie the inverter GEC to the nearest steel,
rather than running all the way back to the water main (where the utility
service entrance GEC is tied).

If we have an inverter mounted on a roof, are you saying we can or cannot go
to the closest building steel with the GEC? 690.47(C)(6) sounds like we have
to go back to the water main. While 690.47(D) sounds like we are supposed to
tie to the nearest building steel ("Grounding electrodes shall be installed.
as close as practicable to the location of roof-mounted photovoltaic
arrays").

Thanks for any clarifications,

Jason M. Szrom, PE
Engineer
Solar Energy Systems, LLC
718-389-1545 x13

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Matt
Lafferty
Sent: Thursday, August 19, 2010 2:01 PM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] PV GEC to the same point as the utility service
GEC?
Good questions, Jason.

No. Yes (2008 NEC 690.47(C) & (D)).

Alternative considerations that might result in a "maybe": Does the system
in question have storage (i.e. batteries)? Is your POCC on a Delta system?
(your hi-rise example says Wye... But if it is directly tied to a Delta
system, you might gotta run back to basement.)

If you are talking about a standard, grid-tied, non-backup system, and the
AC distribution between your tie-in and the utility is suitably sized for
the PV, you should hit the GE system where your AC system is getting it's
neutral bond. (Closest steel OK for PV grounding electrode.) Equipment
ground, same thing. Be sure to run an equipment grounding conductor from the
inverter to the array (i.e. don't just bond the rack to building steel).

Regarding your Additional Info items:
1. Yep.
2. Some people on this list and in the AHJ community would argue that the AC
side of the inverter in this case IS a separately derived system in all
cases (I'm not among that bunch). Either way, the output of your entire
system is going thru the building's AC distribution system. ESPECIALLY if
you are on the secondary of a WYE xfrmer, you can and SHOULD source your PV
GEC where the xfrmer neutral is bonded.
3. You are right. The various provisions of 690.47(C) allow/require this. In
particular, (C)5 & 6.

The underlying principle that counts here is, "Get your GEC where your AC
source gets its system ground."

Matt "I'm no building inspector" Lafferty

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jason Szrom
Sent: Thursday, August 19, 2010 7:38 AM
To: re-wrenches at lists.re-wrenches.org
Subject: [RE-wrenches] PV GEC to the same point as the utility service GEC?

Wrenches,
Questions:  Does the PV GEC have to be bonded to the ground electrode system
at the same point as the utility service equipment GEC? Is there a code
section to back this up?
Additional info:
1.       The DC side of the inverter is a separately derived system and
therefore needs a GEC to a ground electrode.
2.       The AC side of the inverter is not a separately derived system
therefore 250.30(A)(7) does not apply. This section states that AC
separately derived systems should be grounded to the nearest ground
electrode (building steel in many cases) and therefore does not need to be
bonded to the same point as the utility service GEC.
3.       My thought process is this:  The PV GEC can be bonded to the
nearest ground electrode as long as this electrode is tied to the same
ground electrode as the utility service electrode, forming one complete
'ground electrode system'. My opinion is that this setup is analogous to
this example which is regularly seen in the field.  High rise building with
structural steel bonded to water main with utility service GEC bonded as
well. Delta-wye step-down transformer on an upper level with the derived
neutral on the secondary bonded to closest building steel. Yet there is
still an equipment grounding connection to both the primary and secondary
sides of the transformer. This implies that it is OK and even preferable to
tie the PV GEC to the nearest ground electrode,  but I can't find any
evidence or section in the code that confirms or denies this.
Thanks for any input,
Jason
Jason M. Szrom, PE
Engineer

Solar Energy Systems, LLC
1205 Manhattan Ave
Suite 1210
Brooklyn, NY 11222

718-389-1545 x13
 <http://www.solaresystems.com/> www.solarEsystems.com

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