[RE-wrenches] calculating DC voltage drop

Dana dana at solarwork.com
Thu Jul 22 13:58:38 PDT 2010


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

F - 970.626.4140

C - 970.209.4076

web - www.solarwork.com

 

"Responsible Technologies for Responsible People since 1988"

 

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_Calculat
ions_Template_04_20_2010.xltx.

 

- Mike

---

Michael Kelly
Applications Engineer

NABCEP Certified Solar PV Installer T

toplogo
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 <http://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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