Positive or negative ground?

I'm specing a DIY system for my home and am going to use a Xantrex GT3.3 (240) with 16 Evergreen 190's in a single string.

The Xantrex 3.3's come in both postiive and negative ground. Is there a standard or an advantage to having either positive or negative ground?

The 3 line drawing that SRP (AZ utility) sent me to check out has a Sunpower with a positive ground. Their rules didn't specifiy either.

Can you help?

Thanks!

Mikey

Comments

  • System2
    System2 Posts: 6,290 admin
    Re: Positive or negative ground?

    The "standard" here in North America, is the same as our cars, Negative Ground.
    As far as I know anyway.
    Cheers
    Wayne
  • System2
    System2 Posts: 6,290 admin
    Re: Positive or negative ground?

    Thanks Wayne!

    That's kind of what I figured.

    Mikey
  • Roderick
    Roderick Solar Expert Posts: 253 ✭✭
    Re: Positive or negative ground?

    Excellent! Single string, no fuss. Some tips:

    1) Panels are the lion's share of the cost, so shop carefully.

    2) Buying a "kit" is not necessarily cheaper, and may lull you into not doing the calculations for yourself to see if the components are compatible.

    3) If you go with conduit for your wiring, might as well use THWN-2 wire, which generally only comes in #8 and larger. Not too expensive, given the lengths involved. And you won't have to justify your calculations to the inspector. You could use USE-2 or other high-class wire, but it would cost more.

    4) If using mounting racks, I suggest one or two long rows. Don't make my mistake and try to do a lot of little rows, unless you really need to.
  • System2
    System2 Posts: 6,290 admin
    Re: Positive or negative ground?

    Thanks Rod!

    I got an out the door price for the 16 panels and 3.3 inverter for 17.4K from AZ wind and sun. I'm a few hours from their home base & figured I'd pick em up and save the shipping.

    As for shopping around, it's pretty hard to find em at any price!

    I'm making my own mounting system with 3 rows of 5 & one of one to distribute the weight across my roof.

    Thanks again and don't hesistate to let me know if you have a better source for panels!

    Mikey
  • System2
    System2 Posts: 6,290 admin
    Re: Positive or negative ground?

    And just to educate myself better...

    The only difference between THHN and THWN-2 wire is the THWN-2 can be exposed to oil.  Do you think that is a necessary feature if I'm using it in conduit ont eh roof and through the attic?

    Thanks again!

    Mikey
  • Roderick
    Roderick Solar Expert Posts: 253 ✭✭
    Re: Positive or negative ground?

    Hi, Mike.

    Your system has slightly more output than mine, and the price you got is excellent. I think you could take your wife or significant other out for dinner with the money you saved. At a really nice restaurant. Ten to fifteen times.

    If you look in the library, they usually have a copy of the National Electrical Code, and that's part of where I learned about wire. The THWN-2 is rated for 90 degrees C when wet. Often, the -2 adds extra temperature. THWN is only rated for 75 C when wet. Conduit is considered a wet location by code, because of the possibility that the conduit could fill with water (and it does in real life). In your case, with just 7 amps running through the wire, you would probably be fine with #10 or even #12 THWN. By my calculations, with the code-required deratings for solar, and ambient temperature (can get very hot on a roof in my area), I found that #10 could handle a panel with a short circuit current of 11.7 amps. If you live in the low desert, you might need to derate even more for temperature than me.

    Also, for me, the inspector's shortcut was to reject THWN wire in conduit on the roof. If he had seen THWN-2, or USE-2, he wouldn't have given it a second thought. Plain THWN required showing my calculations, flipping through the NEC, and it didn't hurt to have the John Wiles article on hand. I could imagine there might be some inspectors out there unwilling to entertain such a discussion.

    If you get 100 feet of #8 THWN-2 at (say) Home Depot, that's still only about $65, compared with maybe $40 for #10 THWN. And if you even expand in the future, the #8 will have headroom for more current. For example, if you someday wanted to go to 18 panels, that would probably exceed the operating voltage of your inverter in a single string, so you would have to rewire for 2 strings of 9. But you wouldn't have to change wire.
  • System2
    System2 Posts: 6,290 admin
    Re: Positive or negative ground?

    The Phelps Dodge website listed THHN/THWN either damp at 90 degrees or wet/oil at 75 degrees.

    http://www.pdic.com/NR/rdonlyres/C6DE5DA7-9D33-42D2-ADC3-CB91C6FB8A32/0/pdic01073.pdf

    You would think it would be more specific...

    I was planning on using AWG #10 but was thinking about #8 to minimize V drop losses.

    You are correct, 16 panels is 589V which is really really close to max V. But I did run the Xantrex sizing tool at a min temp of 23 degrees. We never see 23 degrees here, certainly not in the day time, but I wanted to make sure I was very conserative with my figures.

    Thanks again!