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Thread: Overhead DC conductors

  1. #1
    Join Date
    Sep 2012
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    CA, US
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    114

    Overhead DC conductors

    Hi,

    We have an install where there are two structures, maybe 10-12ft apart, where there will be separate strings at each structure. I was wondering if I can run the DC string conductors overhead between structures? I was thinking to run conduit with a weatherhead at each structure and run free air sun/weather resistant wire from weatherhead to weatherhead. Any thoughts on code compliance with this proposition? No traffic will be driving under the wires and they will be over 12ft in height off grade, probably 14-16ft. It is a commercial install with 1000VDC max. Conductors will be XLPE 1KV.

    Thanks!

  2. #2
    Join Date
    May 2011
    Posts
    4,194
    While I'm not too familiar with the rules for overhead conductors, I don't believe they would be any different from any other 1000V conductors, with respect to wiring methods. See first sentence of 690.31(A).

  3. #3
    Join Date
    Jun 2004
    Location
    Cherry Valley NY, Seattle, WA
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    4,315
    Quote Originally Posted by jaggedben View Post
    While I'm not too familiar with the rules for overhead conductors , I don't believe they would be any different from any other 1000V conductors, with respect to wiring methods. See first sentence of 690.31(A).
    I would have to refresh myself as well. I know you can make "homebrew" messenger supported wiring with certain cable assemblies - SE, UF - but I dont remember if you need specific permission for each cable/conductor type to do that, or you're good as long as its rated sun-res, wet, etc. I have never had to do that, so I have only read over that part of 225 recreationally. I use triplex or Quad, which isn't even NEC approved Too late to dig into it now. Ill try to look it up tomorrow if no one else jumps in.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  4. #4
    Join Date
    Nov 2015
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    CA, USA
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    I've done this before. The only thing I had trouble finding was the acceptable sag over a given span. You don't want it too tight but if it sags too much then the middle might be too low on a hot day. I eventually found some tables that provided the sag/span ratios.

    See NEC art. 225 and 399.
    Last edited by pv_n00b; 02-06-18 at 01:12 PM.

  5. #5
    Join Date
    Sep 2012
    Location
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    Quote Originally Posted by pv_n00b View Post
    I've done this before. The only thing I had trouble finding was the acceptable sag over a given span. You don't want it too tight but if it sags too much then the middle might be too low on a hot day. I eventually found some tables that provided the sag/span ratios.

    See NEC art. 225 and 399.
    Sweet. Could you send over the url for those tables?

    I was looking into it more and was concerned since it will be a 1000V system. It is with Solaredge 480V inverters that operate around 850V. The upside though is that if they were ever hit the there wouldn't be any voltage on the lines, or there would be the 1V per optimizer. I found the IEEE C2 for clearance requirements since it is over the 600V listed in the NEC. They have a table that shows 5 meters (16.4 ft) clearance from where vehicles will be driving. This is going over a parking lot at a gas station. I am going to shoot for the higher 18ft but feel confident if its 16.4 ft. I'm good.

    I am concerned though at this point with the tension and sag. So if you still have the link for those tables it would be greatly appreciated.

    Thanks!

  6. #6
    Join Date
    Jul 2009
    Location
    Mesa Arizona
    Posts
    264
    The utilities have a rather expensive software package for these calculations. When I had this challenge several years ago I asked the serving utility, Arizona Public Service (APS) if I could consult with an engineer who regularly did this and received all the support I needed.

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