[RE-wrenches] calculating low string voltage

Ray Walters ray at solarray.com
Thu Dec 1 17:22:30 PST 2011


Again Thanks Bill and David;

Continued excellent information.
I just jumped on Solectria'a string sizing online tool, and what are 
they using for the Vmpp coefficent? just a tiny bit higher than the Voc 
coefficient.  SMAs new design tool just lists the Voc coeff.   I think 
some manus might need to be following this thread as well, and tweak 
their sizing tools appropriately.

Ray

On 12/1/2011 5:54 PM, Bill Brooks wrote:
>
> Ray,
>
> There is an explanation on the www.solarabcs.org/permitting 
> <http://www.solarabcs.org/permitting> webpage for the interactive 
> solar map. The 2% ASHRAE data represents 14 hours in a month---meaning 
> temperature will be at or above that value for 14 hours in a typical 
> month. This is likely to correspond to at least one 3-hour timeframe 
> which is considered "continuous" in the NEC. The Copper Development 
> Association has chosen this number for ambient temperature 
> calculations in the NEC. The NEC Handbook (not the NEC) uses this 
> number in their example problems.
>
> The 0.4% ASHRAE date represents 3 hours in a month. It is much more 
> conservative number. I put it in the data to provide a context for the 
> 2% number and it allows engineers to consider a more conservative number.
>
> Of course, the mean extreme minimum drybulb temperature is also on 
> that website for 690.7 max system voltage calculations. Enjoy---its 
> free thanks to SolarABCs.
>
> Bill.
>
> *From:*re-wrenches-bounces at lists.re-wrenches.org 
> [mailto:re-wrenches-bounces at lists.re-wrenches.org] *On Behalf Of *Ray 
> Walters
> *Sent:* Thursday, December 01, 2011 3:58 PM
> *To:* RE-wrenches
> *Subject:* Re: [RE-wrenches] calculating low string voltage
>
> Fantastic Info everyone, I think I've got a full grasp on this topic 
> now, from using the correct coefficient to the correct weather data, 
> to the adders for installation type, and a nice link to the applicable 
> data without spending $180 for the ASHRAE book.
>
> I'm guessing the 2% number is 2%  of the time year round 24/7, not 2% 
> of the time the sun is shining, so losses could be higher than 2% of 
> production?  Sorry to keep asking questions, but I know my class will 
> probably ask the same thing.
>
> Ray
>
> On 12/1/2011 2:57 PM, Gary Willett wrote:
>
> RAY:
>
> This "Solar Reference Guide Map" web site is also linked to on the 
> SolarABCs site and provides the ASHRAE 0.4% & 2% high temperatures, as 
> well as the PV module max temp based on distance above the roof (0.5, 
> 3.5, and 12").
>
> http://www.solarabcs.org/about/publications/reports/expedited-permit/map/index.html
>
> Regards,
>
> Gary Willett, PE
>
>
> On 12/1/2011 14:56, Gary Willett wrote:
>
> Ray:
>
> Look at Appendix F in the Expedited Permit Process for PV Systems - A 
> Standardized Process for the Review of Small-Scale PV Systems Prepared 
> by Bill Brooks, P.E. Brooks Engineering
>
> This document provides the ASHRE temperatures for a lot of locations 
> around the USA.
>
> http://www.solarabcs.org/about/publications/reports/expedited-permit/pdfs/Expermitprocess.pdf
>
> Regards,
>
>
> Gary Willett, PE
>
>   
>
>
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