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Thread: Combining AC and DC in one trench

  1. #1
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    Combining AC and DC in one trench

    Hey guys and gals, first time poster so I apologize if this has been answered elsewhere.

    I have 3 parallel feeder runs from 3 AC recombiners that are mounted next to an inverter rack running back to an equipment pad. The drawings call for a trench with both AC and DC conduits to be placed along the same plane in one row. The issue I have is there are upwards of 15-20 DC conduits that need to run in the same trench. This would create a trench that is around the realm of 10+ feet wide to maintain my conduit spacing.

    My question is, from an engineering standpoint, would I be able to have a trench half as wide, run all the AC conduits low, maintain a similar vertical spacing with additional backfill on top of the AC runs, and run my DC conduits above. Still maintaining my horizontal spacing between AC-AC and DC-DC but stacking the conduits between backfill?
    Additional question, could I then run another layer of remaining DC conduits on top of the previous DC pipes so long as I maintain spacing and burial depths?

    -The trench would be 48" wide by 48" deep
    -The AC conduits are 3" pvc with parallel 350kcmil
    -The DC conduits are 2" pvc with 8 #10's + gnd
    What are some factors that come into play in this equation?
    If i left anything pertinent out, I'd be happy to include additional info.
    Thanks for any and all replies

  2. #2
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    I'm not aware of any issues. Nothing wrong with AC and DC in the same trench if they're in separate raceways.

    To do it neatly you'd mass produce some wood spacers to install every 5-10ft.
    Last edited by jaggedben; 06-26-18 at 10:37 PM.

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    Heat

    WARNING! Placing multiple power conduits in the same duct bank or trench can lead to overheating of the conductors unless proper engineered design is followed. The NEC ampacity tables DO NOT adequately reflect the need to derate conductors in duct banks and common trenches. Even with a 125% safety margin, it is easy to overheat conductors. Look at IEEE 835 for details on the impact of multiple circuits in the same duct bank. Also, buried in the ampacity calculations is the assumption that soil has a thermal resistivity of 90 rho. That is fine for wet concrete or wet dirt. Dry sand might be 400 rho, meaning the temperature rise can be 4x that of soil.

    I recommend applying the derating factors of 310.15(B)(3)(a) for all conductors in a duct bank or common trench, not just a raceway.
    e^(i pi) = -1

  4. #4
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    I agree with beanland, in particular when conduits are stacked vertically in a trench it is no longer a simple rule of thumb calculation. Simple trenches with conduit laid out in a single layer with enough spacing and it's fairly simple but you can still run into problems with soil that has particularly high resistance to heat flow.

  5. #5
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    Spacing

    Quote Originally Posted by pv_n00b View Post
    I agree with beanland, in particular when conduits are stacked vertically in a trench it is no longer a simple rule of thumb calculation. Simple trenches with conduit laid out in a single layer with enough spacing and it's fairly simple but you can still run into problems with soil that has particularly high resistance to heat flow.
    When installing medium voltage cables in very large PV projects, we found that with horizontal spacing about 2x depth, the influence of adjacent cables was low enough to have little impact.

    Any vertical stacking has a significant impact on thermal performance.
    e^(i pi) = -1

  6. #6
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    Don’t be concerned with different systems in the same trench.
    Be more concerned with spacing between the systems.

    A trench is not a trench once it’s backfilled.
    There is only spacing between conduits in the ground when backfilled.
    Tim
    Master Electrician
    New England
    Yesterday's Technology at Tomorrow's Prices

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