DC wire from panels

funideafunidea Registered Users Posts: 6
1.)Does it make a difference if my 100' buried DC wire is stranded wire or not?

2.)If the wire is PVC exterior coated does it still need to be buried in a plastic conduit OR can i just put the dirt on top of the wire?

3.)Is there any codes on the 10 AWG wire that makes it ok to use for DC solar panels?

4.)Do i need 2 conductor or 3 conductor wire for my 10-Sharp 220W?

5.)Where is a good spot to buy this wire?

Thanks

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,823 admin
    Re: DC wire from panels

    Practically speaking, there is no issue using solid copper wire up to 4 ought (0000) with DC wiring (when powering an AC inverter). The specific limitation is called "skin effect" and only affects AC/modulated DC current (and is wire size/frequency dependent).

    Most likely though (I would guess), any 0000 cable you get will probably be stranded anyway because it is easier to handle (easier to bend around corners).

    There would be no reason to use finely stranded cable (like welding cable) unless you need very flexible wiring (you should check insulation against code requirements for temperature, sun light resistance, water resistance, etc.).

    Regarding wiring size... Typically use the NEC to determine the minimum wire AWG for the run (like 15 amp circuit using 14 awg copper wire) and take into account conduit fill, insulation type, on roof or in home/underground wiring, etc.).

    The maximum awg size is usually governed by getting your voltage drop into the 3% to 1% range (many folks aim for the 2 to 1% voltage drop range). So, that will up your wire gauge requirements (and cost you more money because of more copper in the wire).

    Say you had an Isc=10 amp (short circuit current) wire 100 foot (one way) run and Vmp=35 volts. Using a typical voltage drop calculator, we get:
    • 35 volts * 3% = 1.05 maximum voltage drop
    • 35 volts * 1% = 0.35 typically minimum voltage drop
    • 10 amps * 1.25 * 1.25 (NEC derating for solar panel wiring) = 15.6 amps
    • Round up to 12 awg (20 amp) circuit minimum
    • 10 amps @ 35 volts, 100 foot, and 12 awg wire = 3.8 volt drop
    • 6 awg gauge wire = 1.0 volt drop
    • 2 awg gauge wire = 0.4 volt drop
    So, even though a 10 AWG circuit is "safe" for an Isc=10 amp load--you are going to get almost a 4 volt drop.

    To get an "acceptable" voltage drop, you are looking at 6 to 2 awg wiring.

    There is "direct burial" rated wire (that has to be buried at least 18" deep)... But you should look at code requirements too.

    Also, you may wish to bury a large diameter plastic conduit (ideally rated for electrical service) and then you can pull your wire any time you need to (or even bury several conduit/ABS/PVC pipes--you can run signal wire, CAT5 computer networking, a 120/240 VAC circuit for the yard, etc.).

    Regarding the third "ground wire"--You can run the panel frame grounding wire to a ground stake at the array mount... Run two wires back to the home, and do your DC system grounding to your local ground rod/cold water pipe.

    However, lightning / safety grounding is not a "simple" concept--and there are several different methods of grounding. If you want to talk about grounding in detail, please ask.
    BB. wrote: »
    A couple threads about Lightning:

    Off Grid Grounding Technique?
    Another Question, this time about Lightning

    Note, the above are discussions, not a do A, B, and C--and you will be "safe". There probably is no such thing with lightning. Several different techniques are discussed--and a few of those posters even have experience with lightning. :cool:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • funideafunidea Registered Users Posts: 6
    Re: DC wire from panels

    Thanks!

    I found this...it looks like they only sell down to 8 AWG ....ill keep looking.

    http://store.solar-electric.com/hardware---wire.html
  • BB.BB. Super Moderators, Administrators Posts: 29,823 admin
    Re: DC wire from panels

    Unless you are looking for Battery/Welding Cables or UV resistant panel to panel wiring (from NAWS)--you probably will get the heavier stuff from a local electrical supplier. Refer to the NEC (National Electric Code and its Solar GT section, or what applies in your area) for properly rated wire for your needs.

    You did not give me much information--so I gave an example of what you might need for a mythical installation.

    Depending on what you are trying to do (Grid Tied, Off Grid, etc.), size, how far the array is from the home/battery shed, etc.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: DC wire from panels

    If buried, I would put it in PVC piping. The difference between AC and DC is DC has problems with electromigration when there is any humidity moisture between the positive and negative lines.

    The telephone company has great problems with this as their system uses 48 vdc. Also cable company has problems with the DC fed up center of coax to power remote distribution amplifiers.

    Marine or Plumbers 'Goop' sealer is very good for exposed connections.
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