PV array wire size

Is the reason for using higher pv voltages only to reduce the wire size? Im trying to piece together a small system with what I have,which so far is two racks of four 75watt panels each. The controller WILL be a mx60. The array will be located about 30 feet from the controller and I have an ample supply of 6 gauge cable for the job. So I guess what im asking is does the wire size dictate the voltage from the array?
Any thoughts on the best way to configure these panels will be sooo much appreciated. thanks

Comments

  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: PV array wire size

    Mark,

    “High” array voltage has its pros and cons. Pro’s include “low” array current, which leads to reduced voltage drop in the array home run wiring. This is particularly beneficial for long wiring runs. The result is less power loss and/or the ability to specify smaller (cheaper) wire. Since array voltage drops as temperature increases, another pro is that high-ambient temperature array voltage can be set high enough to meet battery charging voltage requirements.

    There are also several cons. One is that the controller efficiency drops as the input voltage increases, so a trade study might be required to balance this loss against high-voltage gain in the wiring.

    For example, when operating at 500 W input to charge a 24 V nominal battery bank, the MX60’s efficiency drops by ~2-1/2% when the array’s operational voltage is increased from 40 V to 95 V. (ref: User Manual Rev. 7.0, 1/29/07, SW rev 5.10 and 5.11)

    Another con is that all of the major consumer MPPT controllers (i.e., Apollo, OutBack, Xantrex) have operational/absolute input voltage limits in the 140 VDC to 150 VDC range. Circuit breakers used between the array and the controller have similar voltage limits. Accordingly, one has to be especially careful in configuring a high-voltage array – particularly in cold environments – for systems with 48 V nominal battery banks.

    For example, the NEC temperature correction factor for array Voc in ambient temperatures between 13 F and -4 F is 117%.

    We need to know more about your system before we can offer specific configuration suggestions. Please provide the PV modules' brand and model number, and also provide your intended nominal battery voltage. Ambient temperatures can also affect system configuration and performance, so it would also be handy to have an idea about where the system will be installed. Finally, also let us know if you plan to “grow” your system, and by how much.

    Note that PV modules can operate at very high temperatures, so wire rated for 90 C conditions is warranted. Check the printed specs on the wire insulation. Elevated temperatures and/or running the wire in conduit will reduce wire ampacity, and we'll deal with that when we have the additional info requested. However, based on the info you've provided so far (i.e., 600 W array size, 30 foot home run), and assuming the wire is rated for 90C or better, your stock of 6 AWG wire will likely be more than adequate.

    HTH,
    Jim / crewzer
  • System2
    System2 Posts: 6,290 admin
    Re: PV array wire size

    thanks jim
    the eight panels are siemens sp75,4.4amps,17 rated voltage,4.8 short ckt current and 21.7 open ckt voltage.
    these will be installed on the roof of a metal building in baja.summer temps will be VERY HOT.
    I will be powering a small chest freezer/fridge conversion,a small chest freezer,a few CFL's, and a few small fans.
    im thinking 2000 watt pure sine inverter. DC loads will be limited to just the water pump.
    I dont expect this system to expand.
    intended nominal battery voltage will be 12 volts. batteries I dont have yet.Im thinking 600 amp hour bank. yes/no? input please.
    this place will be used ocassionaly,maybe 10 days a month.
    thank you much. mark
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: PV array wire size

    Mark,

    You have two configuration options. One is to wire up the array “4 x 2”, with four modules per series string (“48 V” nominal), and two strings in parallel. This configuration would allow you to use either 12 V or 24 V batteries. The other option is a “2 X 4” array, but this one wouldn’t work for a 24 V battery bank due to low array voltage in the summer heat. I'd probably go the former and not worry about slightly reduced controller efficiency.

    FWIW, my present system is a "48 V array" and a "24 V" battery bank.

    You’ll need to combine the string wiring and install circuit breakers between the array and the controller. A Midnite Solar MNPV-6 would work for up to six circuits. Midnite also makes a three circuit model, but that might limit reconfiguration and/or expansion options.
    See: http://www.midnitesolar.com/MNPV%20Instructions1.pdf

    An appropriate 100% duty circuit breaker for each string would be rated at 4.8 A Isc x 125% = 6 A. The Midnite Solar MNEPV6 6A DC breaker would be perfect. See: http://www.midnitesolar.com/Price_sheet_Dec_20071.pdf

    The space between the PV array and the metal roof is going to get pretty damn hot in the summer. 90C wire between the array and the combiner box is probably the minimum, and 105 C wire might be warranted. Tilting the south-facing array up at an angle of ~ local latitude plus 15 degrees may help with convection cooling the array as well as with winter energy harvesting.

    An appropriate baseline 12 V battery bank size for your 600 W STC array would be ~600 Ah. You’ll need the optional remote battery temperature sensor for the MX60. How long this bank will sustain your loads will depend upon energy usage and local insolation. That’s a separate calculation, and we’d first need to know a lot more about the various loads’ individual energy needs.

    A true sine wave inverter is a good idea. My gut feel is that 2000 W may be a bit large. A very good 1100W unit is the Exeltech XP1100. See: http://www.exeltech.com/xpspecs.htm and http://store.solar-electric.com/exxp12vol11w.html and http://www.exeltech.com/downloads/xp600-1100manual.pdf

    The Midnite Solar E-Panel “Lite” might be an effective way to integrate the MX60 controller, the inverter, the PV array circuit breakers (omitting the MNPV), other “small” DC breakers, the big DC breaker between the batteries and the inverter, and a couple of AC breakers as well. See: http://www.midnitesolar.com/MidNite-Products.html

    HTH,
    Jim / crewzer
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: PV array wire size
    mark wrote: »
    DC loads will be limited to just the water pump.

    More info about the water system, a 12V camper pump ?? Air Bladder pressure system? Gravity feed roof tank ?

    I'd suuggest the 48V array/24V battery bank, less losses, if you can find a 24V DC pump for your needs. Depending on pump duty cycle, you could have it's control switch activate a 24 - 12V downconverter
    ebay: http://tinyurl.com/yr93up $45 12V out @ 5A
    you want the switching style >85% efficient, not the Reducing style: big heat sink, <50% efficient.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: PV array wire size

    Good catch on the pump, Mike. Here's a link to an example of 24 V pumps available from NAWS, this site's sponsor: http://store.solar-electric.com/2088-474-144.html

    HTH,
    Jim / crewzer
  • System2
    System2 Posts: 6,290 admin
    Re: PV array wire size

    you guys are the best! im gonna have to read and reread all of this a dozen times for it to sink in but my immediate question is about the inverter. if a 24 volt battery bank is used, does that mean a 24 volt inverter must be used or is there a way to use a 12 volt model with a 24volt bank?
    thanks again. mark
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: PV array wire size
    mark wrote: »
    ...my immediate question is about the inverter. if a 24 volt battery bank is used, does that mean a 24 volt inverter must be used or is there a way to use a 12 volt model with a 24volt bank?

    The short answer is NO--there is no REASONABLE way of using a 12 inverter (or any appliance) from a 24 volt battery bank other than using another 24 to 12 DC converter or 24 to 12 charge controller and 12 volt bank.

    It will work--but cost you another ~20-40% in losses to go this way.

    And the "cheap" method of tapping a 12 volt pair of wires from a 24 volt bank will end up killing your batteries because of differential charging between the "low" 12 volt and "high" 12 volt sides of the bank (12 volt battery with load will under charge and the 12 volt battery without the load will overcharge).

    -Bill

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: PV array wire size

    mark,
    that's alot of stuff to run on those 8 pvs. please be sure 8 pvs will suffice for your needs.

    "I will be powering a small chest freezer/fridge conversion,a small chest freezer,a few CFL's, and a few small fans.
    im thinking 2000 watt pure sine inverter. DC loads will be limited to just the water pump.
    I dont expect this system to expand."

    on your last comment, i feel you may need to expand.