Multiple DC cables in same conduit

HuckMeat
HuckMeat Registered Users Posts: 24 ✭
in Advanced Solar Electric Technical Forum #1
I'm designing the layout of my upcoming system, and am pretty familiar with most of the requirements I need to follow.... But I have not seen much on limitations/restrictions of my DC cables inside my conduit. I wanted to find out if I had to do anything specific other than observing conductor fill requirements.

Do I need to run a seperate conduit for each string? I am using two outback MX60/36xx's. I am planning on running additional conduit for adding additional capacity later, but wanted to try to plan ahead.

thanks in advance,
John
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  • System2
    System2 Posts: 6,290 admin
    #2
    Re: Multiple DC cables in same conduit

    John, I'm not sure on NEC, but ABYC (marine) has charts for this kind of stuff. I'd bet the farm that they have the same.
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  • newenergy
    newenergy Solar Expert Posts: 291 ✭✭✭✭✭✭
    #3
    Re: Multiple DC cables in same conduit

    There are surprisingly few distinctions for DC in the NEC. I'm about 99% sure there's nothing that would indicate doing conductor fill differently for DC than for AC.

    I don't even know if it says you can't mix AC and DC in the same conduit, but John Wiles says not to.

    690.31 (A) Wiring methods, says

    All raceway and cable wiring methods included in this Code and other wiring systems and fittings specifically intended and identified for use on photovoltaic arrays shall be permitted.

    So, I would take that as meaning you should use the standard conductor fill factors.
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  • rollandelliott
    rollandelliott Solar Expert Posts: 834 ✭✭
    #4
    Re: Multiple DC cables in same conduit

    there is a derating factor when using more than one wire in the conduit.

    if more than 1 string is run in conduit derate for more than 3 wires
    for example you have a 18amp string from a pv array you installed a few years ago.

    You used 12AWG wire then because it is rated up to 20a

    However

    Now you want to double your array by installing twice as many panels and installing a second 18amp string in the same conduit.

    18/0.8 derate factor = 22.5a

    Your new wiring must be 10awg since that can handle up to 30a
    unfortunately you'll need to pull that old 12awg and replace it with 10awg as well.

    or just run a seperate conduit since that is typically easier and cheaper for those that do not think ahead a few years......
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #5
    Re: Multiple DC cables in same conduit
    HuckMeat wrote: »
    I'm designing the layout of my upcoming system, and am pretty familiar with most of the requirements I need to follow.... But I have not seen much on limitations/restrictions of my DC cables inside my conduit. I wanted to find out if I had to do anything specific other than observing conductor fill requirements.

    Do I need to run a seperate conduit for each string? I am using two outback MX60/36xx's. I am planning on running additional conduit for adding additional capacity later, but wanted to try to plan ahead.

    thanks in advance,
    John
    You can route DC conductors for multiple strings through the same conduit. Two strings will give you more than three current carrying conductors if you are feeding two charge controllers (if they were feeding the same CC you could combine negatives), so you will need to look at NEC Table 310.15(B)(2)(a) to figure additional conductor derating. If you are planning for expandability, then you will need to predict what the total number of conductors will ultimately be and size them accordingly unless you are up for possibly pulling new wires for everything some day.
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