Hi Peter, For your #2, see 310.15(A)(2) exception. Where you have two different ampacities, you can use the higher one as long as the distance of the lower ampacity is less than 10' or 10% of total conductor length (whichever is less). So for 50' of conductor, as long as the above ground conductor length is <5' you can go with the 30 degree C ambient temperature.
Cheers, Dave *--* *Dave Tedeyan* *Project Engineer* *Taitem Engineering, PC* 109 S. Albany Street, Ithaca, NY 14850 Voice: (607) 277-1118 x121 www.taitem.com On Thu, Jun 23, 2016 at 4:41 PM, Peter Parrish < peter.parr...@calsolareng.com> wrote: > I am working with a customer who is doing a complete remodel, and > upgrading to PV and Smart Energy Storage. The house was broken down to > floor joists, open studded walls and open roof rafters. > > > > The client wanted a 8 kW/16 kWh Smart Energy Storage system and a 10 kW-ac > PV system. The Main panel, the Critical Load sub-panel, the Energy Storage > system and the PV inverters will all be either in a detached garage or hung > on the west facing exterior wall of the detached garage. > > > > We have been given a number of 1-1/2” PVC conduits, buried a minimum of > 18”, that run between the main house and the detached garage, so we have > to convey 2/3rds of the PV source circuits and all of the critical load > branch circuits using this conduit. This raises a number of > questions/confirmations: > > > > (1) We cannot mix PV source circuits and critical load branch circuits > in the same conduit. Pretty obvious. > > (2) What ambient temperature should I use in my ampacity calculations? > I assumed something less than 30°C, such as 20°C. But I read somewhere that > one has to be careful when the conduit exists the ground, in that one has > only 18” of conduit above ground before one has to use the full maximum > ambient temperature which in our case is 45°. The argument says that for > the first 18”, the portion of the copper conductors in the ground will cool > the portion of the conductors above ground via thermal conduction. I can’t > find any citation to confirm this argument. Does anyone have any sources of > information on the subject? Even if there is a sound engineering basis for > the argument, soon after exiting the ground, one would need a vault to > splice in a higher ampacity conductors and continue on to either the > critical load sub-panel or the inverters, you have to calculate the > ampacity at 45°C. These buried conduit runs are approximately 50 feet in > length. > > (3) One end of the conduit run is supposed to come out of the ground > inside the building envelope, so we could derate using 30°C or so for air > conditioned space. We cannot count on the garage being air conditioned > however. > > (4) Can we use THHN as opposed to THWN-2? I am assuming the forces of > nature or human stupidity will eventually cause the PVC to crack and the > extra expense of THWN-2 (or another wet rated 90°C conductor) will be a > better choice. > > > > This job is the same one where the electrician claimed that neutrals on a > 120V branch circuit don’t count as current carry conductors in conduit for > the purposes of de-rating ampacity. He’s gone, but his replacement may not > be any more careful in his ampacity calculations. > > > > Faithfully yours, > > > > Peter T. Parrish, Ph.D. > > NABCEP™ Solar Professional #031806-26 > > President, SolarGnosis > > 1107 Fair Oaks Ave. > > Suite 351 > > South Pasadena, CA 91030 > > (323) 839-6108 > > peter...@pobox.com > > > > _______________________________________________ > List sponsored by Redwood Alliance > > List Address: RE-wrenches@lists.re-wrenches.org > > Change listserver email address & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out or update participant bios: > www.members.re-wrenches.org > > >
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