Parallel Strings [RE-wrenches]

Ray Walters walters at taosnet.com
Tue Feb 5 20:55:47 PST 2008


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Phil;

I recall this paralleling module issue being stomped to death on this  
list several years ago, now we have MC cables.
I've seen many problems with over use of paralleled circuits here.
But the scenario of batteries and other strings feeding a module  
short without tripping the breaker is something I have never heard or  
seen happen. Have you seen this?
It would take a double fault to create this scenario, as charge  
controllers are designed to prevent back feed of the array.
Besides the faulty charge controller, your argument's scenario would  
require a module short of such precise resistance that  the current  
draw would exceed the series fuse rating,
but NOT the rating of the breaker.
Then I'm confused, the module heats up and explodes? What happens?
I would think eventually the charge controller would have enough  
other problems that it would shut off and quit passing current,
or the short would heat up and burn one of those thin silver cell  
interconnects, and the short would then become an open connection.
  Or the short would draw more current  and eventually trip the breaker.
I've seen people do very stupid things with solar modules, and they  
seem to be one of the safest electrical devices on the market today.
They're substantially less lethal than a hair dryer.
Luckily the code and some manufacturers allow limited paralleling of  
strings.
Lower voltage arrays get ridiculous without limited paralleling,
just remember Ben Franklin - " All things in moderation."

Ray


On Feb 4, 2008, at 8:57 PM, Phil Undercuffler wrote:

>
> Mick,
>
> If batteries are involved, then you really need one series fuse per
> string -- branch plugs cannot be used.  Oh, in theory maybe they  
> could,
> but remember you need to protect the module from ALL sources of  
> current:
> the other string plus the batteries.  For instance, say you had a  
> module
> with a short circuit rating of 3.8 amps.  3.8 x 1.56 = 5.9 amps, and
> let's round up to 6 to make the math easy.  In a batteryless
> application, two strings could be connected with branch connectors
> because neither module will see fault currents higher than it is rated
> to handle.
>
> Add a battery to the mix, however, and things get harder.  If you
> protect the combined strings with a 15 amp fuse, a fault in one module
> will see a potential for 21 amps:  15 flowing through the fuse from  
> the
> batteries, and another 6 from the second string.  If you wanted to  
> choke
> down the series fuse rating to stay below the sum of 15 amps, then the
> series fuse would have to be 9 amps.  However, the 9 amp fuse would  
> not
> be large enough to handle the combined array current in normal  
> operating
> conditions.....
>
> So, in theory it may be possible to use branch connectors in battery
> based applications, it would require modules with LOW short circuit
> ratings (1-2 amps, maybe) plus HIGH series fuse ratings.
>
> Finally, it shouldn't be forgotten that once you solve for the series
> fuse rating, the ampacity of the wire in the conditions of use need to
> be addressed.  Temperatures on the roof can be pretty high, and  
> 2008 NEC
> applies an even more stringent derate factor on conductors & conduit
> exposed to sunlight.  It doesn't take long before the ampacity of that
> #10 and #12 wire runs out of headroom for paralleled modules.
>
>
> ------------------------------------------------------------
> Please note new phone number!
>
> Phil Undercuffler
> Director, Battery-based and Off-grid
> Conergy, Inc.
> Our World is Full of Energy
>
> 1730 Camino Carlos Rey Suite 103
> Santa Fe NM 87507 USA
> Office | 505.216.3841
> Fax | 505.473.3830
> www.conergy.us
>
> ------------------------------------------------------------
>
> -----Original Message-----
> From: Mick Abraham [mailto:mick at abrahamsolar.com]
> Sent: Monday, February 04, 2008 6:20 PM
> To: RE-wrenches at topica.com
> Subject: RE: M-C branch plugs: Legal? Safe? [RE-wrenches]
>
>
> Thanks to all who have replied...very helpful...
>
> John Berdner: Thanks for reminding me about grid-tie situations  
> where PV
>
> string fuses are not needed. My present application is a battery based
> system where I must consider possible backfeed from the battery.
>
> "Ray Walters wrote: "...you can't use those connectors to create more
> than 2 parallel strings, so I'm not sure how you would parallel 3
> strings."
>
> Mick says: On page 13 of the M-C catalogs for SolarLine 1 and  
> SolarLine
> 2, they show trios plugged together in parallel. The M-C drawing looks
> nice but one must wonder if plugged trios are really workable in the
> real world, because that would need a PV module where the series fuse
> rating is almost four times greater than the short circuit amperage.
>
> Ray's mention that the Solar World 165s have a series fuse rating  
> of 15
> amps is most interesting, since their short circuit amperage is a much
> lower 5.1. With the fuse rating so high relative to its amperage, this
> module seems especially well suited to plug-parallel...in pairs at
> least.
>
> Thanks again,
>
> Mick Abraham, Proprietor
> www.abrahamsolar.com
>
>
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R. Walters
Solarray.com
NABCEP # 04170442	




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