[RE-wrenches] calculating DC voltage drop

Bill Brooks billbrooks7 at yahoo.com
Thu Jul 22 17:01:28 PDT 2010


All,

 

This is all very interesting, but on the dc side, it is primarily an issue
of energy loss for large grid-connected systems. My rule is 2% voltage drop
for dc side and 1% for ac side.

 

When we talk about dc  % voltage drop, most of what has been discussed in
this thread is instantaneous voltage drop. That is interesting, but
year-round voltage losses are more important. There are two ways to get
this:

 

1.       Use a simulation program like PVSyst to calculate the voltage drop
every hour of the day and give you an overall loss for the year.

2.       Take 80% of Imp for the array and use that current with the nominal
voltage of the array (330 Volts for a 600V array). The reason for 80% of Imp
is that half of the energy is delivered above this number and half below. It
makes an ideal "nominal" current of the array.

 

By using one of these methods, you will calculate the accurate energy loss
on the dc side.

 

Bill.

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Allan
Sindelar
Sent: Thursday, July 22, 2010 3:12 PM
To: mike at solren.com; RE-wrenches
Subject: Re: [RE-wrenches] calculating DC voltage drop

 

Mike,
I would respectfully challenge both reasons ;^)} . The responses already
posted today cover them: you don't need to compensate for intermittent
high-insolation conditions when sizing for voltage loss, as this is not a
Code issue; Code addresses ampacity and OCP issues. And being conservative
means you add unnecessarily to wire cost (sometimes substantially at 25%
greater current) when that money could be better spent on more PV, with an
ultimately greater annualized return of total energy per $$ spent. Ray
Walters' recent posts cover this well.

Allan Sindelar
 <mailto:Allan at positiveenergysolar.com> Allan at positiveenergysolar.com
NABCEP Certified Photovoltaic Installer
EE98J Journeyman Electrician
Positive Energy, Inc.
3201 Calle Marie
Santa Fe, New Mexico 87507
505 424-1112
www.positiveenergysolar.com <http://www.positiveenergysolar.com/> 


On 7/22/2010 3:53 PM, Michael Kelly wrote: 

Allan,

 

I use Imp*1.25 because NEC calls out a 125% adder for unexpected
high-insolation conditions (snowscapes, cloud effect, yellow houses next
door, etc).  And because I am an engineer and like to be conservative :).

 

-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: Allan Sindelar [mailto:allan at positiveenergysolar.com] 
Sent: Thursday, July 22, 2010 5:26 PM
To: mike at solren.com; RE-wrenches
Subject: Re: [RE-wrenches] calculating DC voltage drop

 

Mike,
Why do you make this exception, please?

Allan Sindelar
 <mailto:Allan at positiveenergysolar.com> Allan at positiveenergysolar.com
NABCEP Certified Photovoltaic Installer
EE98J Journeyman Electrician
Positive Energy, Inc.
3201 Calle Marie
Santa Fe, New Mexico 87507
505 424-1112
www.positiveenergysolar.com <http://www.positiveenergysolar.com/> 

 
 
 
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