690.47 (was enphase) [RE-wrenches]

[Dick Ratico]

Ray & Mark,

This may prove controversial, but according to 2005 and 2008 NEC Article
250.134(B) Exception No. 2: 

"For dc circuits, the equipment grounding conductor shall be permitted to be run
separately from the circuit conductors."

Accordingly it would seem acceptable to bond the array GE and any and all other
GEs available, with the appropriate size of direct buried bare copper. This
bonding system would also serve as the dc equipment grounding conductor, if one
is not run with the circuit conductors. Doing this eliminates the inductive
grounding loop (but note that metal raceway, if used, enclosing the dc cicuit
conductors could also contribute to a loop), and puts a lot of CU in the ground
to help dissipate any surges. This also reduces the likelihood of potential
voltage differences between GEs, which cause surges to travel over EGCs run with
circuit conductors.

Regards,

Dick
Solarwind Electric
Bradford, VT


You wrote:

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

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 


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