[RE-wrenches] calculating DC voltage drop

[R Ray Walters]

Hi Dana;

I'd say that's a bit overkill for today's market. That money could be making more power, not just preventing losses. 
Definitely a law of diminishing returns on wire sizing. 
If you allocated 50% of the budget to wire, and 50% to PV, your losses would be very low, (immeasurable but unfortunately still there) but your total system production vs. the money spent would be terrible.
I'm only using this ridiculous example to show, that at some point, we all spend money on more PV, not bigger wire. 
I pick that point based on sound economic analysis, not some over applied rule of thumb. As copper costs rise, and PV gets cheaper, that point moves up. 
If copper was still at 20th century prices, and PV were $10/ watt, your 1% might very well be the right answer.

( this is also the point where Bob-O starts hammering me, so I'd better run. ☹☺)

R. Walters
ray at solarray.com
Solar Engineer




On Jul 22, 2010, at 2:58 PM, Dana wrote:

> For array to power center or Inverter; I use 125% of amps @ max power and run the wire calculations at 1% VD.
>  
> We have set Fluke meters on clamped on @ both ends and talked with radios & compared the readings. We have not seen any noticeable VD down to .00vdc.
>  
> Perhaps overkill?
>  
> Thanks,  Dana Orzel
>  
> Great Solar Works, Inc
> E - dana at solarwork.com
> V - 970.626.5253
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>  
> "Responsible Technologies for Responsible People since 1988"
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> From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Michael Kelly
> Sent: Thursday, July 22, 2010 2:13 PM
> To: 'RE-wrenches'
> Subject: Re: [RE-wrenches] calculating DC voltage drop
>  
> Hi All,
>  
> I agree with Ray with the exception that I use Imp*1.25 in my calculations.  I developed a simple spreadsheet for either fixed conductor size or for fixed voltage drop.  You can feel free to use it by downloading here: http://www.mechanicalmike.com/solar/DC_Voltage_Drop_Calculations_Template_04_20_2010.xltx.
>  
> - Mike
> ---
> Michael Kelly
> Applications Engineer
> NABCEP Certified Solar PV Installer ™
> <image001.jpg>
> Solectria Renewables, LLC
> 360 Merrimack St.
> Building 9, Floor 2
> Lawrence, MA 01843
> Phone: 978-683-9700 ext. 167
> Fax: 978-683-9702
> mike at solren.com
> www.solren.com
>  
> From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of R Ray Walters
> Sent: Thursday, July 22, 2010 3:45 PM
> To: RE-wrenches
> Subject: Re: [RE-wrenches] calculating DC voltage drop
>  
> I use actual operating current, without multipliers. I also use actual temperatures, not the worst case temperatures that many use.
> I created a spread sheet that uses the NEC adjustments for wire temperature, (see NEC chapter 9, table 8, FPN 2)
> It makes quite a difference!
> Volt drop and the accompanying loss of energy production needs to be looked at realistically under normal operating conditions. Adding worst case multipliers is necessary for sizing cables and breakers for safety, but it isn't appropriate when calculating energy losses. We shouldn't do an energy loss analysis for a situation that might never occur, or for only a very small % of the time. 
> A correct analysis will reflect actual losses over the lifetime of the system, so that an informed economic decision can be made. Especially on larger systems with long runs, its worth it to you and the client to use real world numbers, not blind over sizing.
> I got into this over a decade ago, when I noticed that actual measured volt drop was much less than calculated, now my calculations match real measurements. 
>  
> R. Walters
> ray at solarray.com
> Solar Engineer
>  
>  
>  
> 
>  
> On Jul 22, 2010, at 1:22 PM, Marco Mangelsdorf wrote:
>  
> 
> When calculating the voltage drop from a PV array to the inverter, what’s the most common accepted practice?  To use the Isc X 1.25 or 1.56 as the multiplier?
>  
> Thanks,
> marco
>  
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