Calculating string amperages [RE-wrenches]

[William Miller]

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Ray, Colleagues:

I have been scratching my head ever since Ray sent me the e-mail below.  I 
was very certain that at one point in time I had been convinced by a 
Fronius engineer that 3 strings of Solar World modules are allowed without 
fusing.  When Ray sent this e-mail, rechecked my calculations and found 
that they do not work.

Just now I realized that Solar World changed the specified series fuse 
ratings sometime in the recent past.  In 2006 and before, the SW175 had a 
series fuse rating of 20 amps, which allowed the three strings 
((3-1)*5.3*1.56<20).  I actually found a photo of a module label if anyone 
really needs documentation on this.

The series fuse rating now is 15 amps and three strings are not 
allowed.  Two strings are allowed.

I apologize, Ray, for leaving this hanging.  My memory is not what it used 
to be.

William



At 11:11 AM 2/8/2008, you wrote:


>Well we do have some discrepancies here. William was saying we could
>go to 3 parallel Solar World strings, John Berdner here says only
>one, and I'm still saying 2 is good.
>John I have 2 issues with your excellent analysis.
>1) 690.8 B) (1) exception: "Circuits containing an assembly, together
>with its overcurrent device(s), that is listed for continuous
>operation at 100% of its rating shall be permitted to be utilized at
>100% of its rating."
>This means that with the right breaker, we don't have to use the 1.56
>multiplier for array current, only 1.25 which is for edge of cloud
>effect, etc. I have seen Solar world modules exceed their nameplate
>rating, so this multiplier is appropriate.
>2 x 5.1 A x 1.25 = 12.75 Amps on a 15A breaker that is rated for 100%
>duty is acceptable. (most Outback and Midnite solar offerings are
>100% rated BTW)
>
>
>R. Walters
>Solarray.com
>NABCEP # 04170442



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