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
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