[RE-wrenches] Calculating PV Source circuit conductors

[William Miller]

Wrenches:



This is kind of embarrassing, but I have struggled for some time to make
sense of the NEC requirements to calculate source circuit conductors.  I am
tripped up by what I consider to be the pretzel logic of 690.8(B)(2) (2011
NEC).



The section states that circuit conductors shall be sized to carry not less
than the larger of 690.8(B)(2)(a) or (2)(b):



(2)(a) is straightforward.  You take the manufacturer’s 690.8(A) Isc value
and multiply by 1.25 (or the Isc by 1.56).

(2)(b) is the one that trips me up.  It states that we have to use the
“maximum currents calculated in 690.8(A) after conditions of use have been
applied.”  It does not define conditions of use.  What are the conditions
of use?  Are they wire temperature?  Panel temperature?  Conduit fill?



I always assumed it meant to de-rate the ampacity of the proposed wire
caused by conduit fill, ambient temperature and any roof-top temperature
adders.  However this involves applying correction factors to the conductor
ampacity.  These adjustments are not applied to the “currents calculated in
690.8(A)…”  So if I apply correction factors to wire ampacity I am not
doing what the code requires.



I have spent considerable time researching this.  All articles suggest
applying correction factors to the wire ampacity, not to the “maximum
currents calculated in 690.8(A)…”  As an example, this article in Home
Power appears to have it wrong:
http://www.homepower.com/articles/solar-electricity/design-installation/code-corner-pv-circuit-sizing-current-calculations/page/0/1




Anyone have any idea on what the actual intent of the code is?



Thanks in advance,



William Miller





[image: Gradient Cap_mini]
Lic 773985
millersolar.com <http://www.millersolar.com/>
805-438-5600
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20160812/fc23f707/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image002.jpg
Type: image/jpeg
Size: 1460 bytes
Desc: not available
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20160812/fc23f707/attachment-0003.jpg>