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Thread: Direct burial of conductors of different systems in same trench

  1. #1
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    Direct burial of conductors of different systems in same trench

    Hello guys,

    I'm a young engineer and new poster so you'll have to forgive my inexperience. I'm dealing with a project that requires distributing 5kV service lines across a campus as well as stepped down voltage (120V/208Y & 277V/480Y) distribution to certain facilities. In order to save space and lower cost, I was thinking about running the 5kV and the lower voltage lines in the same trench. From my understanding of the NEC 300.3(C) there is no recommendation against doing this. Furthermore, combing the values from table 300.5 & 300.50 (Minimum cover requirements for 0-600V & above 600V respectively), the lower voltage line requires 18in. minimum while the 5kV line requires a 30in. minimum cover spacing for direct burial, resulting in 12in. seperation between the conductors of different systems. Am I correct to assume that the code allows for this practice?

  2. #2
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    A few more thoughts

    I'm still trying to figure out the correct approach to resolving this issue. I'm unconvinced by my own logic based on tables 300.5 & 300.50, since under certain conditions the resulting distance is not 12in. like the case I initially presented. It seems like comparing the values between the two tables (assuming identical wiring method for both systems) always results in the minimum 6in. seperation, which does conform to spacing between runs of the same system voltage, but again no rules or recommendations are explicitly stated. NEC 300.4(C)(2)(e) also seems a bit relevent where it allows for conductors of different voltage ratings in manholes "if the conductors of each system are permanetly and effectively seperated from the conductors of the other systems...", but again this may be referring to seperate trenches with different system voltage ratings entering the same manhole for tapping, allowing access for maintenance or other uses. Any help would be appreciated guys.

  3. #3
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    Pragmatic solution

    Since I couldn't find a solid answer in the NEC, I did the next logical thing and checked out some of the older jobs that are archived on our company server. In a job from about 10 years ago, the designer is using the same trenching to run MV(5kV) and LV but has them laid is seperate duct banks with 12" of seperation between the duct banks. I think I'm going to go with this option for the 50% design submittal (hopefully I don't get beat up too hard at the design review meeting if this is a mistake). In any case, my father's a really knowledgable and experienced engineer but he's overseas right now working his coordination studies/arc flash magic & making the big bux and I'm having difficulty reaching him, but as soon as I do and know the right approach I'll post it FYI. In the meantime I'd still appreciate any thoughts on this issue.

  4. #4
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    Quote Originally Posted by skeshesh View Post
    Furthermore, combing the values from table 300.5 & 300.50 (Minimum cover requirements for 0-600V & above 600V respectively), the lower voltage line requires 18in. minimum while the 5kV line requires a 30in. minimum cover spacing for direct burial, resulting in 12in. seperation between the conductors of different systems. Am I correct to assume that the code allows for this practice?
    You are correct if these are customer owned systems. If any part is POCO (power company) owned, you would have to check their specs.

  5. #5
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    Separation is an issue the NEC does not address. However, please make sure you allow adequate room to repair the conduit if damaged.

    Also, I like to see the duct bank or conduit backfilled using sand or 1 sack concrete, dyed red. When you dig and see the sand or concrete, you are there.

    Forget the ribbon. An excavator will tear thru that and the conduit at the same time.

    Welcome to the forum, we look forward to your participation.
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    Ancora Imparo

  6. #6
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    I think Tom is on the money thinking of the future repairs. Much nicer if you have room to lay one next to, rather than under, the other.

    The colored fill is great too. But don't forget the marking ribbon entirely. If they are not in concrete it will be required on the "service" laterals. If they are all feeders, then no 300.5(D)(3), but still a great idea. Not all diging is mechanical.

    Don't overlook the 24 in. burial under the parking lots and streets either in your layout.

    Welcome.

  7. #7
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    Interesting thoughts, I have a question does the LV wire under 600 enter the same vault as the high voltage? Our code generally only Spec's what happens to 600V wire (day in and day out) not as to mixing these sizes of services as you've decribed (and not that I'm the untimate NEC Nerd).

    But one thing does come to mind, I can't place it per article though, If you send the LV Service lower there is a deduction valve needs to be applied, IE your going deeper than whats listed in Chapeter Nine or in the 300's with Wire methods all in respect to the LV service, but I can't recall where I JUST read it!!! ...
    = : )

  8. #8
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    Also, I like to see the duct bank or conduit backfilled using sand or 1 sack concrete, dyed red. When you dig and see the sand or concrete, you are there.
    I'm definitely show backfilling using sand in my detail drawings, but I've never specified a color before, but it does sounds like a good idea. For future reference, I have good theoretical understanding and research skills to find what I need, but nothing replaces experience so please don't hesitate to educate me on the practical stuff that you'll only learn on site not looking at a piece of paper. Thanks for the greeting and I'm looking forward to learning a lot from you guys as well as hopefully contribute something.

    I think Tom is on the money thinking of the future repairs. Much nicer if you have room to lay one next to, rather than under, the other.
    Thanks Volta - in fact I do lay them horizontally next to each other, since I'm keeping ground electrode conductor continuity in both systems and I've always seen the ground run above the phase conductors.

    Interesting thoughts, I have a question does the LV wire under 600 enter the same vault as the high voltage? Our code generally only Spec's what happens to 600V wire (day in and day out) not as to mixing these sizes of services as you've decribed (and not that I'm the untimate NEC Nerd).
    Cadpoint - take a look below, I'm pretty sure that this part of the code (NEC 300.4(C)(2)(e) clearly allows for the different systems to to enter the same manhole. I'm a bit unsure though as to what "permanetly and effectively" means. I would think that it's talking about fastening the cables on the rack on opposite sides of the manhole, but I'm not sure.

    NEC 300.4(C)(2)(e) also seems a bit relevent where it allows for conductors of different voltage ratings in manholes "if the conductors of each system are permanetly and effectively seperated from the conductors of the other systems...", but again this may be referring to seperate trenches with different system voltage ratings entering the same manhole for tapping, allowing access for maintenance or other uses. Any help would be appreciated guys.

  9. #9
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    Installation of duct bank, and installing the duct bank 12 inches horizontally apart sounds like a good idea. I think that sand is universally understood to alert excavators that there is something below the sand, coloring it may not make much differenc, but could be a lot of extra work.

    Fastening the cables in the manhole to the racks is good.
    Take a look at Article 110, Part V, for manholes.
    Instructor, Industry Advocate

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