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

Mon Aug 23 12:27:34 PDT 2010

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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