Separate conduit - NEC interpretation

[ggunn]

I know who John is, he has even posted here before.

But what you describe is not what the 2008 NEC says, it is 125% twice for a total of just over 156%.

Take a look at 690.8 I don't see anything limiting it to just the OCPD.

Yes, the NEC says that conductors must carry no less than (1.25)(1.25)(Isc), but if you derate for temperature, distance from roof, and conduit fill, those deratings often amount to more than the 1.25 factor for continuous use. OCPD sizing is determined by (1.25)(1.25)(Isc) with no alternative, which is why I said that it was "just for OCP". That was a simplification, of course, and not precisely accurate.

John Wiles writes in his published article "Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices" (updated March 2007 to include NEC2008 changes) that in sizing conductors for PV source circuits, either the 1.25 factor for continuous use or the lumped deratings for conditions of use are to be used, but not both.

This just in from John Wiles:
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Gordon:

Yes. [In response to Question about grouping PV source circuits to different inverters in common conduit: can I do it? - G]

BUT... Who established those voltage drop limits? Not code requirements, but the 1% on ac side is OK and recommended. Both DC and AC represent not only voltage drops but percent energy losses over the life of the system.

From recent analysis of fires on large PV systems, I no longer recommend putting multiple dc source circuits or multiple PV dc output circuits in one conduit. A single fault in one circuit (either line to line or line to ground ) can spread to other circuits/systems in the same conduit. And with the sun feeding the current, things get bad quickly.

Be sure to MEGGER all conductors after installation.
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[Smart $]

...but if you derate for temperature, distance from roof, and conduit fill, those deratings often amount to more than the 1.25 factor for continuous use. ...

Thing is... 2008 NEC 690.8(B)(1) does not say derating can offset this second 125%. I know it comes from 210.19(A)(1) for branch circuit conductors and 215.2(A)(1) for feeders. But as noted earlier, PV system conductors are not branch or feeder circuit conductors. The 2011 NEC has revised this subsection to allow it.

 

[ggunn]

Thing is... 2008 NEC 690.8(B)(1) does not say derating can offset this second 125%. I know it comes from 210.19(A)(1) for branch circuit conductors and 215.2(A)(1) for feeders. But as noted earlier, PV system conductors are not branch or feeder circuit conductors. The 2011 NEC has revised this subsection to allow it.

I was confused by this as well. I used to determine conductor sizing by starting with (Isc)(1.25)(1.25) and then applying conditions of use derating, which was more conservative than was needed but in the small systems I was dealing with it didn't make much if any difference to the system cost. In the larger systems I am working with now, though, it makes a difference, and I defer to John Wiles on the methodology described in Appendix I in this publication:"Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices" http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/PVnecSugPract.html