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  1. #1
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    Default AC and DC in same trench?

    Planning a solar installation using 600v, the conduit run between solar electronics in house and the panels happens to go right past a building where I need to pull 110v, 40A service. Question is can I drop two conduits in the same trench, pull wire for solar DC run through one, and wire for AC run through the other? Can the AC line put some kind of noise on the DC line that interferes with any of the electronics in the solar charging/inverter system? Or is there any other reason not to do this? If they need to be separated, how much distance between them?
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  2. #2
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    Default Re: AC and DC in same trench?

    The answer to your question will depend on how far the lines will be parallel. The further they run parallel, the further away from each other they will need to be.

    Hope someone here can give you more specific information on what the code requires.

  3. #3
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    Default Re: AC and DC in same trench?

    Normally we are required to separate instrument cables (milliamp carrier ) by 12" from high voltage cables or put in a shielding of carbon steel plate. I have never heard of a requirement to separate high voltage cables but then on heavy construction, we dont have high voltage DC, only the low voltage control cables.
    In any case, you should be ok with them if you keep them at least 12" apart.
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  4. #4
    Super Member Gary Fowler's Avatar
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    Default Re: AC and DC in same trench?

    Quote Originally Posted by greasemonkeyok View Post
    The answer to your question will depend on how far the lines will be parallel. The further they run parallel, the further away from each other they will need to be.
    The amount of EM radiation coming off a cable has nothing to do with length of the run. The higher the voltage, the more EM it produces. A coiled length of HV wire can actually overheat itself enough to melt the insulation due to EM but straight runs produce the same amount per given length regardless of overall length
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  5. #5
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    Default Re: AC and DC in same trench?

    Quote Originally Posted by Gary Fowler View Post
    Normally we are required to separate instrument cables (milliamp carrier ) by 12" from high voltage cables or put in a shielding of carbon steel plate. I have never heard of a requirement to separate high voltage cables but then on heavy construction, we dont have high voltage DC, only the low voltage control cables.
    In any case, you should be ok with them if you keep them at least 12" apart.

    I agree, we trenched and buried cable in pipe and placed DC, CCTV cables, and line voltage all in the same ditch but of course in different pipes. The manufacturer usually specified how far to keep communication cables away from line voltage and I've never seen one specify more than 12" apart. If we had several pipes sometimes we'd lay a layer cover with dirt then lay another layer.

    There maybe something in the code about this but I've never had any problems just doing what the device manufacturer recommends.

    We had camera cables and 110 v in the same ditch at a prison we wired and some on the runs were close to 1000'. The 110 v was for the heaters built in the cameras. The camera manufacturer also recommended 12" separation
    "whenever possible" but at least 3".

    I don't think you'd have any problems.
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  6. #6
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    Default Re: AC and DC in same trench?

    Quote Originally Posted by Gary Fowler View Post
    The amount of EM radiation coming off a cable has nothing to do with length of the run.
    This is true, but the longer the distance over which the cables are parallel, the more they will "couple" and the more the one will induce EMI in the other.

    To the OP: I am not an electrician, but I am a designer of computer networks and installer of computer networking equipment and cabling. We know that if we run data cable (Ethernet) parallel to high-voltage AC lines, the data cable must be protected from induced EMI or data corruption will occur. But we're talking about very low-level signals here. It's hard for me to imagine that the amount of EMI created in your situation would damage the components of the solar system. Surely anything that is handling 600 volt DC is beefy enough to take a little EMI from a parallel AC line.

    For me, the main motivation for separating them would be the potential for an insulation failure allowing the lines to short out to each other. 600 volts DC is nothing to fool around with. I don't know what standard practice or code is in this circumstance, but if I were just making it up, I would probably put them in different conduit as an extra safeguard.

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    Default Re: AC and DC in same trench?

    Thanks for suggestions, I talked to building department, they only know about requirement for phone/signal cable separation, showed no interest in trying to find an answer. I've put out the question to a few electrical engineers I've worked with, I'll see what they come back with.
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  8. #8
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    Default Re: AC and DC in same trench?

    Quote Originally Posted by BeezFun View Post
    Thanks for suggestions, I talked to building department, they only know about requirement for phone/signal cable separation, showed no interest in trying to find an answer. I've put out the question to a few electrical engineers I've worked with, I'll see what they come back with.
    Can I suggest that you contact an electrician? An EE may have a theoretical understanding, but this doesn't exactly sound like the domain of most EEs I know.

  9. #9
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    Default Re: AC and DC in same trench?

    I would think that you would be ok at that voltage, as stated by others it is mainly a problem with low voltage wiring. Are you by chance using shielded cable? Probably not I would assume, but if so than I would definitely say it would be fine. If you are running shielded cable than be sure to only ground one end of the shield.

  10. #10
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    Default Re: AC and DC in same trench?

    Quote Originally Posted by mikehaugen View Post
    I would think that you would be ok at that voltage, as stated by others it is mainly a problem with low voltage wiring. Are you by chance using shielded cable? Probably not I would assume, but if so than I would definitely say it would be fine. If you are running shielded cable than be sure to only ground one end of the shield.
    With Ethernet cable, you can buy it with foil shield in the insulation. If you can't get a hold of that, you can accomplish the same thing by putting the cable in metal conduit and grounding the conduit. Which leads me to the question... if you're putting metal conduit in the ground, do you really need a separate earth ground? What are you going to do? Put a ground rod in and attach a wire to the conduit? The whole conduit is a ground rod!

    Maybe the "metal conduit" trick only works above ground.

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