[RE-wrenches] calculating DC voltage drop

[R Ray Walters]

Marco and I were just discussing this as well. For long term energy loss purposes, you really would want to use annual average voltage and current for the array. (let's see that on a module spec sheet ☺) 
Basically, it would really depend on the installation and location. I could see for a hot roof in a hot climate that voltage on avg. would be lower than MPP, and current would be higher, although, I think 25% higher on avg. would be unlikely.
Also we discussed wire temperature, and not only would it need to be annual averaged, it would also be averaged for the entire run, (ie. 20 ft are air conditioned, and 20 ft are in direct sun, you would average the 2 temps expected).
The whole idea here is to get the calculations as close as possible to the actual losses, so that you will know accurately how many kWh your wire choice will cost over the next 20 years.
The true test of your calculations will be confirmed with actual voltage drops measured on the proverbial "average" day.

R. Walters
ray at solarray.com
Solar Engineer




On Jul 22, 2010, at 2:12 PM, Michael Kelly wrote:

> 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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