[RE-wrenches] DC wire sizing

[Chris Worcester]

I have seen 1360 w/m² when cloud edge effect took over. We usually measure 1100 to 1150 w/m² on a cold clear day here in Truckee.

 

Sincerely,

Chris Worcester

Solar Wind Works
NABCEP Certified PV Installer
Phone: 530-582-4503
Fax: 530-582-4603
 <http://www.solarwindworks.com/> www.solarwindworks.com
 <mailto:chris at solarwindworks.com> chris at solarwindworks.com
"Proven Energy Solutions"

 

From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Darryl Thayer
Sent: Friday, April 02, 2010 10:35 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] DC wire sizing

 


Erika
you are correct, the solar radiance can exceed STC by about 125%, any one who has spent any time with a pyrometer has observed this.  From experience's  continuously operated  circuits fail  fasted than  intermittent circuits.  for the temperature and conduit fill, they are valid of course, but solar are not overcurrent protected, it is possible to have a solar circuit be shorted and stay sorted for months or even a year.  If this happens it is still hoped that the system will not cause a fire.

Darryl 

--- On Fri, 4/2/10, Bill Brooks <billbrooks7 at yahoo.com> wrote:


From: Bill Brooks <billbrooks7 at yahoo.com>
Subject: Re: [RE-wrenches] DC wire sizing
To: erika at repowersolutions.com, "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>
Date: Friday, April 2, 2010, 11:14 AM

Erika,

 

The most correct answer (which is a really funny thing to say) is to use John Wiles’ “5-step program”—that’s what I call it. It is published in one of the appendices of his latest “Suggested Practices” document. If you can actually follow what he leads you through (that’s why I call it the 5-Step program), it will lead you to the correct answer.

 

My short answer, that can result in a larger than necessary conductor, is to determine the required overcurrent protection device (OCPD) rating and then size the conductor accordingly. The reason it may be conservative is that the NEC allows you to round to the next larger standard OCPD in 240.4(B). This is the way all conductors are sized in the electrical industry. I’m all about simplicity and being a little conservative is always better than being overly liberal (I’m not making a political statement here, but if the shoe fits…).

 

Bill.

 

From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Erika M. Weliczko
Sent: Friday, April 02, 2010 6:27 AM
To: 'RE-wrenches'
Subject: [RE-wrenches] DC wire sizing

 

To my understanding the 156% on PV source and output circuits is related to the ability of PV to deliver more than rated and be continuous.

Therefore, the wire has to be able to carry this current, so now the temperature and fill corrections are applied to find the wire capable of the 156%.

 

I am in a debate where the question is why correct for temp and fill on 156% of ISC and spend all that extra money when the normal operating is at Imp. Or why correct the 156% but why not correct the Isc or Imp.

 

I am going to stick to the fact that the circuit has to carry the 156% under all conditions…

Thoughts? 

 

Erika


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