[RE-wrenches] NEC 690.9 and Two String Arrays

[Christopher Warfel]

I'd like to suggest that Appendix J of John Wiles Suggested Practices 
Photovoltaic Power Systems and the NEC be read. It provides good 
instruction on how to calculate when an OCPD is needed. It is a function 
of the module's Isc rating, the inverter's backfeed current capabilty, 
and the module's maximum series sting fuse rating. The rule of thumb is 
fusing is usually required above two paralleled sub array circuits, but 
with some modules, a greater paralleled number of subarrays can be 
installed without fusing. It does make troubleshooting more difficult 
and as said, it is inexpensive to spec subarray fusing.
Chris

On 2/21/2011 3:51 PM, Phil Schneider wrote:
> Dan,
>
> My understanding has been that unless the inverter spec lists a "0 A 
> backfeed current" then it is conceivable that fault current from the 
> grid (up to the inverter OCPD) could feed a fault on the DC side, so 
> DC fusing would be necessary (Fronius IG Plus spec sheet, for example, 
> states this).  SMA specs state the maximum backfeed current to be 
> equal to the maximum OCPD for a given inverter size.  However, I've 
> asked SMA tech support about this directly, and they assured me that 
> it isn't possible for current from the AC side to reach a fault on the 
> DC side.  This is not very clear to me, but the cost of fusing two 
> strings is generally quite low, so when I doubt I spec fusing.  
> Curious to hear if others think this is too conservative.
>
> P.
>
> Phil Schneider
> Creative Energies
> p 307.332.3410
> c 307.438.1195
> f 888.456.6625
> www.CreativeEnergies.biz <http://www.CreativeEnergies.biz>
>
>
>
> On Mon, Feb 21, 2011 at 12:52 PM, <dan at foxfire-energy.com 
> <mailto:dan at foxfire-energy.com>> wrote:
>
>     Hey Everybody.. I got the impression that 690.9 ex. allows up to
>     two parallel strings to be connected (thru a disconnect) to a grid
>     interactive inverter (which does provide Ground Fault
>     Protection).. Do I still need to protect the Array from possible
>     inverter back feed? This particular array consists of two pole
>     mounted arrays out in the boonies in the middle of a field. The
>     Inverter is mounted on one of the poles...  Let the egg toss
>     begin... db
>
>
>     Dan Brown
>     Foxfire Energy Corp.
>     Renewable Energy Systems
>     (802)-483-2564
>     www.Foxfire-Energy.com <http://www.Foxfire-Energy.com>
>     NABCEP #092907-44
>
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-- 

Christopher Warfel, P.E.
ENTECH Engineering, Inc.
Energy Utilization Experts
(401)466-8978

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