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

David Brearley david.brearley at solarprofessional.com
Thu Jan 27 07:02:31 PST 2011


John,

Bill has written an article for SolarPro that you may find relevant:

http://solarprofessional.com/article/?file=SP3_6_pg68_Brooks&search=

Here¹s an excerpt:

³...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.²

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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