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 [email protected]
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