[RE-wrenches] DC wire sizing

[robert ellison]

I believe that the breakers from CBI, Midnite and Outback are hydraulic /
magnetic and may be operated at 100% While the derating applies to all
others that are Thermal / magnetic,

Bob

On Fri, Apr 2, 2010 at 11:14 AM, Kent Osterberg <kent at coveoregon.com> wrote:

> Erika,
>
> You should refer to NEC 690.8 this information.  There are two issues that
> contribute to the 1.56 factor.  First is that the maximum current from the
> PV array is considered the short circuit current multiplied by 1.25 to
> account for higher than standard irradiance such as cloud edge effect, high
> altitude, and high operating temperature.  The second multiplier of 1.25 is
> because PV current is considered to be continuous.  All conductors (and
> breakers) operating continuously (over three hours I believe) that are
> subject to NEC rules are limited to 80% of the ampacity in Table 310.16.
> The two 1.25 factors equals 1.56.
>
> For normal operation, wires selected under this criteria are conservatively
> rated.  But it is also the objective of the NEC to design power systems in a
> manner that they will remain safe under abnormal circumstances.
>
> Kent Osterberg
> Blue Mountain Solar, Inc.
>
>
>
>
> Erika M. Weliczko wrote:
>
>  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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