[RE-wrenches] Cable Sizing - revisited, Ambient Temp

[John Wadley]

David B.,
     Thanks for the reference to this excellent article by Mr. Brooks.  I'm a subscriber and fan of SolarPro and have adopted use of this article on several projects already.  In fact I'm butting heads with a customer now over this min string size check.  I've shown them the calculations, referenced this article directly and I'm sure they are aware of the weight behind Mr. Brooks expertise, but they are still willing to implement the shorter string size.  I plan to make sure they sign something in writing stating they have been informed of the potential consequences of their decision.
 
     If I see a situation where I think the contractor might install conduit that does not meet standard practices or may experience abnormally high ambient temps, I'll use the more conservative number (ASHRAE Extreme Annual Mean Maximum Design Dry Bulb Temperature ) to protect myself and the end customer.  Otherwise, I'll have to force them to show conduit runs on a site layout drawing and say any change from that has to be approved through me.

Best Regards,
John Wadley, PE
NABCEP Certified Solar PV Installer (TM)
Wadley Engineering

> Date: Thu, 27 Jan 2011 09:02:31 -0600
> From: David Brearley <david.brearley at solarprofessional.com>
> To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
> Subject: Re: [RE-wrenches] Cable Sizing - revisited, Ambient Temp
> Message-ID: <C966E527.B6E2%david.brearley at solarprofessional.com>
> Content-Type: text/plain; charset="iso-8859-1"
> 
> John,
> 
> Bill has written an article for SolarPro that you may find relevant:
> 
> http://solarprofessional.com/article/?file=SP3_6_pg68_Brooks&search=
> 
> Here?s an excerpt:
> 
> ?...it is important for system designers to perform detailed low dc voltage
> calculations for specific array configurations. Designers should use the
> highest expected continuous ambient temperature for calculation purposes.
> According to the Copper Development Association, the highest ASHRAE
> temperature data that is likely to create a 3-hour continuous condition, per
> the definition of continuous found in NEC Article 100, is the 2% Annual
> Design Dry Bulb Temperature, which is also found in Appendix E of the
> Expedited Permit Process for PV Systems. For designers who feel that the
> ASHRAE 2% temperature is not high enough, the same table also includes
> ASHRAE Extreme Annual Mean Maximum Design Dry Bulb Temperature data, which
> can be used for even more conservative voltage or ampacity calculations.?
> 
> I realize you are talking about a different set of calculations, but the
> rationale for which data to use may still apply.
> 
> David Brearley, Senior Technical Editor
> SolarPro magazine 
> NABCEP Certified PV Installer ?
> david.brearley at solarprofessional.com
> Direct: 541.261.6545
> 
> 
> 
> On 1/27/11 1:20 AM, "John Wadley" <wadleyjk at hotmail.com> wrote:
> 
> > Dave,
> > Thanks for responding in Mr. Brooks place. Since ASHREA 2% is not the
> > very worst case, it seems like it might be possible for the ampacity of the
> > wire chosen to dip below the rating of the OCPD protecting it, if there is not
> > much margin. I've been trying to rationalize whether this would become a
> > safety issue. I don't think it would since the OCPD protects the wire from a
> > current source increasing beyond the expected design output. I don't think
> > there is much chance of that for a PV module (unless there was a short between
> > two strings). I think the increased heating would more likely increase wire
> > resistance/voltage drop and lower production. With enough voltage drop, the
> > inverter might shut off.
> > I guess my new concern is in the most severe case where there is solar
> > concentration on a short section of conduit. Here, the heating effect of both
> > the elevated ambient temp and reduced wire resistance might lead to premature
> > failure of the wire insulation. If the combined heating effects exceed the
> > 90C rating of the wire, does the insulation embrittle or melt? In either
> > case, I foresee a grounding fault, and if the GFCI failed, it could spark a
> > fire.
> > I know the best solution is to keep conduit shaded and avoid these worst
> > case solar concentrating conditions. Sometimes. when I design a system for a
> > new contractor, I don't always know exactly where they plan to run conduit on
> > a roof (nor can I control it) and I start "what-if'ing" whether my design
> > numbers will be conservative enough to prevent a system failure or a fire.
> > 
> > Thanks and regards,
> > John Wadley, PE
> > NABCEP Certified Solar PV Installer (TM)
> > Wadley Engineering
> > 
 		 	   		  
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