High altitude array

In the process of final design of week-end use (future expansion probable) off-grid 24V system to be located at 8'500 ft in CO mountains. Known facts:

- 6 KC130 panels, pre-wired 24v Outback system with MX60 controller, VFX 3524 inverter and 8 -6v 440Ah battery bank wired 4x2, .
- Run from array to controller 10'-12' using 10/2 TC
- Estimated average 1,750 wH daily load (system designed for heavier load - future potential - every one has a plug-in they want to use!!)

Question? - I've received recommendations indicating ability to wire array in series, but reviewing MX60 documentation, I am concerned that at low winter temp of (sub zero) and at altitude, the max VoC would well exceed the 150 limit of the MX60. Several posts have indicated as much as 130% of STC at altitude. The array was oversized a bit to compensate for late afternoon "forest" shading, but I'm concerned with the VoC max of the MX60. As an alternative, or suggestions welcome, I plan to re-design the array into two parallel sets of three panels in series. This should reduce potential VoC issues, but need confirmation that I understand the issue correctly. Also, #10 wire should work, correct? A newbie so hope I have all the terminology correct - Thanks


  • Roderick
    Roderick Solar Expert Posts: 253 ✭✭
    Re: High altitude array

    Yes, don't want to void the warranty of whatever charge controller you choose by designing without sufficient margin. I don't know about the MX60, so will say no more.

    On the #10 wire, if you go with 2 strings, I think that's about 16A of short circuit current. Multiply by 1.25 to get "continuous current" of 20A. Mulitply by 1.25 again to make the NEC happy as far as ampacity of the wire, and that's 25A. At that point, you would have to look at the temperature rating of the wire (is it 75C or 90C? WHEN WET?), and look at the tables in the NEC (National Electrical Code, in the reference section of your library) to see if #10 is enough. If you put the wire into conduit, there might be a derating, too. My gut feel is that #8 would be a sure bet, but you'd have to run the numbers based on local ambient temperatures.

    All that said, in practical terms, with such a short run from panels to equipment, the #10 would work just fine, probably no detectable difference from #8. If you have a strict inspector (assuming your location requires permit/inspection), then there could be some debate.

    Do take a look at this document, it runs through the step-by-step calculations:
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: High altitude array

    “Two parallel sets of three (12 V nominal) panels in series” will work just fine with your 24 V system. The STC Voc of the KC-130 is 21.9 V, so three wired in series will make for 65.7 V. Adding the NEC 690.7 temperature correction factor of 125% for ambient temperature down to -40 F (or C), the design voltage will be 82.125 V – well within the MX60’s self-protection limit of ~140 V.

    FWIW, my system is a “36/24” configuration, and I’d have no reservations at all about installing it at your location.

    Considering your location, your array’s typical mid-day current will typically be ~15 A or so. However, it may occasionally reach 30 A on exceptionally bright winter days (thin air, reflection from snow or ice) or during “edge of cloud” events. I’d suggest #10 wire for wiring each sub-array to the combiner, and then #8 from the combiner to the MX60. But, the NEC rules.

    Jim / crewzer
  • Windsun
    Windsun Solar Expert Posts: 1,164 ✭✭
    Re: High altitude array

    Just for info, on average you gain about 1% output from panels for every 1000 foot in altitude due to less light loss in the atmosphere.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: High altitude array

    wow i gained 1%. do you know how much i lost due to being in an urban area with all of the extra stuff in the air(pollution)? for those that may be wondering pittsburgh is not the smokey city anymore as most of the steel industry has been gone since about the early to mid 80s. 4 steel mills within 3 miles of me were shut down leaving one within that same 3 miles. that one is the site of the first steel mill in the country by andrew carnegie and is called the edgar thomson works. just elaborating somewhat. :-P
  • System2
    System2 Posts: 6,290 admin
    Re: High altitude array

    Thanks to all for the info. Earlier posts indicated as high as 135% of STC at altitude, but doubt my installation will get there. Didn't know about the 1% per 1,000' gain. Thanks. I wasn't looking for the extra power (will take what I can get though), but didn't want to "torch" the MX60. I will be wiring the array 3 x 2 for 32V. The wire capacity looks ok based on the voltage drop calculator (another post) and with the short run to the controller. Will post results when the installation is running. Now if it would quit snowing up there I could start working on the project.

    The forum is a great resource and also "entertaining" at times. crystalmtn