690.47 (was Enphase) [RE-wrenches]

[Mark Frye]

Ray,

Thanks for giving me some insight into the content of the handbook which I
have not bought.

I see a triangle here with the three vertex being: 1 - the array, 2 - the
exisiting premise AC system electrode which is functionally also the DC
system electrode, and 3 - the new additional "electrodes for array
grounding".

Now, how do I connect the dots?  1 to 2 with the new combined EGC and GEC as
usual. Add 1 to 3 with a new EGC. OK so far.  The only problem comes if I
also try to connect 2 to 3 directly.

I think it is pretty clear to most folks you don't want to make that last
connection.  What I do know is that when the Code was being drafted, there
was specific language stating that the new additional GE need not be bonded
directly to the exisitng GE system.  That language did not make it into the
Code version.  So if we are not careful, 250.50 "...all electrodes...shall
be bonded together.." could be applied.

Can we take it that the new additional GE is bonded to exisitng GE system,
but this is accomplished by the EGC/GEC connection from 1 to 2?
 
Mark Frye
Berkeley Solar Electric Systems
271 Vistamont Dr
Grass Valley CA 95945
(530) 401-8024
www.berkeleysolar.com 

-----Original Message-----
From: Ray Walters [mailto:walters at taosnet.com] 
Sent: Saturday, June 14, 2008 8:00 AM
To: RE-wrenches at topica.com
Subject: Re: 690.47 (was Enphase) [RE-wrenches]


>
I'm all for better grounding, but if we're not careful with the
interpretation here, we're going to start creating ground loops.
If you bond the array directly to the GE, and tie the array to the premise's
grounding through the EGC; you will create an inductive loop if you make any
other connections to the array's GE (say a bare #6 back to the premise's
ground rod, or even back to the inverter's ground bar) An inductive loop
will be subject to lightning induced surges.
Article 690.47 D) though intended to help with lightning, actually could
cause more trouble than it cures if not carefully applied.
Even the pictures in the 2008 NEC Handbook (Exhibit 690.6) shows the
supplemental array GE but it is labeled "optional".
This picture also clearly shows a ground loop created by bonding the AC & DC
electrodes, while all their EGCs are connected as well.
Considering the AC & DC ground systems are already bonded through the EGCs,
I would NOT further bond them directly.
Exhibit 690.4 shows 2 loops and an interesting AC load center that is
grounded through the Inverter chassis.
  I always thought you had to have a GEC from the AC load center   
(picture seems to contradict 690.47C(6)) and what? Now there's no
supplementary ground off the array  (what happened to 690.47(D)?
I paid extra for the Handbook, so I could understand this better.
The more I study the pictures and read the new text in article 690, the more
inconsistencies IMHO.
I can date a system by what weird requirements for grounding were in place
at the time of installation. (remember 4/0 cable as the GEC?)
  It seems we'll soon have vintage 08 systems to redo later.....
Why do I think we're going to have more ground connections required by AHJs
that will quietly have to be removed after inspection to actually insure
some lighting protection?

Ray






> In any case, you have to run an additional GEC (separate from the DC 
> system GEC/EGC run as a single conductor from the inverter) from the 
> array to this new GE.
>
> Mark Frye


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