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Need advice on buried conduit.

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  • Need advice on buried conduit.

    I have an 2.5kW net-metered roof-mounted PV system whose inverter died and was replaced with a 5kW 3x mpp one. I plan to add two pole mounted 6-panel arrays using Panasonic 335W modules, 492V 2010W max per array. I plan to bury conduits, they will be above grade on the poles ~6ft up to the arrays and ~3ft up the side of the house where it goes thru the rim joist to the basement.

    I'd appreciate advice on the choice of conduit. Five #10 wires (2 DC pairs + ground) to the house. 1-1/2" diameter was suggested for ease of pull. Is Sch 40 allowed for the above ground sections, if it's got to be Sch 80, can it be combined with Sch 40 for just the underground parts? Are the NEC rules for DC different than AC? Is 18" burial depth allowed for this?

    Thanks for any advice on this!

  • #2
    3/4" conduit will work fine for 4 current carrying conductors, but I like to use 1" just to have some good margin. Sch 80 only is required where the run comes out of the ground and is more easily subject to damage. Could also transition to EMT there as (around here anyway) the sun will eat PVC gray in a few years. No difference between AC and DC as long as its less than 600V. 18" is burial depth for residential branch circuits. Can be less if covered by concrete or other circumstances.
    I really try to avoid underground circuits as every time I've worked on a conduit that has been underground for a few years - its got water in it - even here in dry Arizona.
    BSEE, R11, NABCEP, Chevy BoltEV, >2500kW installed

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    • #3
      Bury a #4 copper ground in the trench with the conduit and put some warning tape on top of the conduit. The copper ground should tie the local ground rod at the the arrays to the house ground keeping the ground outside the house.

      It makes it a lot easier to trace a conduit if there is an conductor that can be clipped onto with a tracer. Also take good measurements from fixed points and record them somewhere permanently. Not sure if its legal but I have seen some folks who sketch these dimensions with permanent markers inside the door of the main house supply panel along with dimensions to the septic tank clean out and distribution box.

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      • #4
        Thanks solarix for the great advice! Sounds like I can use Sch 40 buried 18" and switch to Sch 80 or EMT for the above ground sections. Interesting about the water problems you have, I've got ~150ft buried Sch40 pvc for electrical service, others for phone, network, garage, problem free for 25 years, but our perc is really good or it may just be luck.

        Thanks peakbagger, your grounding system sounds like the way to go. I've kept photos of electrical and sewage trenches as we built our house, putting copies in the panels makes sure they can be found.
        Last edited by RichardB; 08-22-2019, 12:48 PM.

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        • #5
          The water can form just from condensation in the air. The ground is at lower temperature then the air and there is some airflow in and out of conduit via the boxes on the ends. Warm humid air gets sucked into the conduit from the outside as most homes are slightly negative pressure and then it hits the cooler walls of the conduit and then condenses. Once the water is in there it slowly collects year after year.

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          • #6
            Originally posted by RichardB View Post
            Thanks solarix for the great advice! Sounds like I can use Sch 40 buried 18" and switch to Sch 80 or EMT for the above ground sections. Interesting about the water problems you have, I've got ~150ft buried Sch40 pvc for electrical service, others for phone, network, garage, problem free for 25 years, but our perc is really good or it may just be luck.
            There's probably water in that 150' of conduit.
            And that's OK.
            Any wires put into conduit outside are required to be rated for wet locations.
            So even though you have wires in conduit that has water in it, everything still works fine.

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