[RE-wrenches] DC wire sizing

[Darryl Thayer]

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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