[RE-wrenches] Buried Conductors

[Dave Tedeyan]

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.parrish at 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
>
> petertor at pobox.com
>
>
>
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