Seeking advice on wiring solar system

Need help on wire sizing, grounding and fusing for small 12 volt system I am assembling. Will use this system for my work shop (separate building) light and power tools, and for emergency power for refrigerator in work shop (6.5 amp) when there is a power failure. (Went 3 weeks without grid power after Hurricane Katrina)

Have bought from NAWS the following items so far:
4 ) Kyocera KC130TM panels (7.39 amps X 4 = 29.56 amps) Isc 8.02 Amps each.
Assortment of 10 ga. MC cables for wiring panels to combiner.
1) Tri-Star 60 AMP Charge Controller w/ Meter ( Tri-Star book states I need manual disconnect circuit breaker from controller to batteries. Don't know what type or amperage to use.)
1) Exceltech Pure Sine Wave Inverter XP2000 2000 watts.
1) 4/0 10 ft. long red/black pair inverter cables (batteries are 8 ft. away from inverter)
14) #2/0 (12" long) Battery Interconnect cables ( 7 for main bank and 7 for backup bank in Golf Car)
2) Midnite Baby Boxes. ( 1 for array breaker disconnect in building and the other for ? )

I already have 6) T105 Trojan Batteries bought locally and have placed them in an insulated/ventilated battery box located right outside the building. I have 12 more in 2 Golf cars that are kept charged by another solar array that I hooked up in the past. This will be my backup battery banks for cloudy days. (They are hooked up in series now for 36 Volt golf car motor and could easily be changed to a series/parallel configuration for 12 volts (know how to do this).

Pole mounted array will be 20 feet away from my work shop building where charge controller and inverter will be. Will need 36 feet of wire which includes wire from combiner box on pole, dropping down pole 6 feet, going underground (plastic conduit) for 20 feet and then going up wall 6 ft., and over 4 feet to solar power center in building. (Will also need 10 feet of wire from controller to batteries.)

I have some knowledge but would like advice on type combiner box, sizes of wiring through out system, correct fuse sizes and proper grounding. Want to get this one right.
Any help would be appreciated.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,816 admin
    Re: Seeking advice on wiring solar system
    solarfox wrote: »
    Need help on wire sizing, grounding and fusing for small 12 volt system I am assembling. Will use this system for my work shop (separate building) light and power tools, and for emergency power for refrigerator in work shop (6.5 amp) when there is a power failure. (Went 3 weeks without grid power after Hurricane Katrina)
    Basically, you can wire this using one of two standards... The NEC (National Electric Code) which is very conservative and makes insurance companies/building inspectors happy.

    Or, you can use the standards for wiring boats and cars. Will work fine--but many times those standards allow higher current on smaller gauge wires. And don't use UL/NRTL rated fuses and such.

    Either will work, but since house wiring is readily avialable and speciallized cabling rated for UV (out door) and buried/wet use--it does make sense to follow the NEC recommendations. And for solar RE, you will may times go larger in gauge (wire size) to keep voltage drop to a minimum... A virtual requirement for low voltage DC systems (such as 12 VDC which only have a volt or two before hitting 10.5 volts where most equipment will shutdown).
    Have bought from NAWS the following items so far:
    4 ) Kyocera KC130TM panels (7.39 amps X 4 = 29.56 amps) Isc 8.02 Amps each.
    Assortment of 10 ga. MC cables for wiring panels to combiner.
    1) Tri-Star 60 AMP Charge Controller w/ Meter ( Tri-Star book states I need manual disconnect circuit breaker from controller to batteries. Don't know what type or amperage to use.)
    You can size this either of two ways--one based on the amount of solar panels you have attached, or the maximum output of the controller.

    Using the maximum controller output of 60 amps. Then an NEC derating of 1.25:

    60 amps * 1.25 = 75 amps minimum breaker + wiring

    Round up to 80 amps as next standard breaker and look at NEC for exact wiring size (exposed, or in conduit--in conduit will require even larger wire).

    Note, keep the controller to battery wiring short--voltage drop here is bad because of high currents and voltage errors will slow down the charging of your battery bank (controller will think voltage is higher because of wiring drop and cut back on current).

    For wiring drop calculations, you can find various websites that will do the calculation or download an Excel spreadsheet from this thread:

    voltage drop calculator
    1] Exceltech Pure Sine Wave Inverter XP2000 2000 watts.
    1) 4/0 10 ft. long red/black pair inverter cables (batteries are 8 ft. away from inverter)
    Keep this cable as short as possible--inverters do not work well at the end of long cables with high current draw.
    14) #2/0 (12" long) Battery Interconnect cables ( 7 for main bank and 7 for backup bank in Golf Car)
    Batteries strings need to be "balanced" -- have the same resistance in each string so that they will charge and discharge equally. Otherwise a string with a "higher" resistance will not support the current as well--and the other string(s) will cycle deeper and "wear out" earlier.

    Having two strings where one is in a golf cart can be an issue (golf cart string will probably have longer cables than fixed string).
    2) Midnite Baby Boxes. ( 1 for array breaker disconnect in building and the other for ? )

    I already have 6) T105 Trojan Batteries bought locally and have placed them in an insulated/ventilated battery box located right outside the building. I have 12 more in 2 Golf cars that are kept charged by another solar array that I hooked up in the past. This will be my backup battery banks for cloudy days. (They are hooked up in series now for 36 Volt golf car motor and could easily be changed to a series/parallel configuration for 12 volts (know how to do this).
    I guess you are using a 12 volt 2,000 watt inverter... That is a whole lot of current:

    2,000 watts * 1/10.5 volt min battery * 1/0.85 inverter efficiency = 224 amps minimum
    Pole mounted array will be 20 feet away from my work shop building where charge controller and inverter will be. Will need 36 feet of wire which includes wire from combiner box on pole, dropping down pole 6 feet, going underground (plastic conduit) for 20 feet and then going up wall 6 ft., and over 4 feet to solar power center in building. (Will also need 10 feet of wire from controller to batteries.)
    Use the voltage drop calculator to design minimum wire sizes for each section.

    I would highly recommend that both the charge controller and inverter be way less than 10' from the "main" battery bank. These are the "critical" voltage drop runs. And you will need lots of expensive copper to keep the voltage drops low on a 12 volt battery bank.

    Remember for "safety" and best practices, figure out the maximum current on each wiring section (at operating voltage) then see what the drop would be for each. Normally, a design is 3% maximum voltage drop overall, and probably 1-2% drop is what most people design for for each section of wire drop.

    Some areas, drop hurts less such as on the solar panel to charge controller runs and the inverter AC out to 120 VAC loads--Those drops are at lower current and don't really impact the operation of any equipment.

    However the charge controller to battery run is critical because the battery voltage needs to be accurately measured by the charge controller (even a 0.1-0.2 volt drop at 60 amps is an issue). And the Inverter drawing 224+ amps (2,000 watts at 10.5 volts) can cut out when starting heavy loads (well pump, fridge, etc.) if the battery is low (hot battery, long wire runs, heavy surge current) over too small/long of wire run.
    I have some knowledge but would like advice on type combiner box, sizes of wiring through out system, correct fuse sizes and proper grounding. Want to get this one right.
    Any help would be appreciated.
    You probably did not get many answers here because there are a lot of questions.

    For example, is the Morning Star Controller a MPPT or PWM type charge controller. An MPPT type is much better when you have long wiring runs from the solar panel to the battery shed (yours is on the long side).

    With an MPPT type controller, you can run the panels and Vmp near 100 VDC and reduce the current to amost 1/6th the amount as doing it at Vmp=17volts.

    And what is the battery bank voltage? 12 volts nominal?

    The addition of golf cart batteries into your 12 volt system (???) sounds like an interesting idea--but at 12 volt, the currents are so large (from the 2,000 watt UPS) that it sounds like it may be difficult to get good current sharing between the "fixed" and mobile banks. Also, you should have a fuse/breaker in each battery string to prevent a short (such as in a battery bank/wiring) so that all of the banks don't attempt to provide 1,000's or 10,000+ amps into the short.

    Anyway--sorry I did not answer sooner--sort of a big set of questions to answer.

    Regarding components, take a look at our host's wiring and protection pages... It will give you an idea of what size components are available.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solarfoxsolarfox Registered Users Posts: 5
    Re: Seeking advice on wiring solar system

    Thanks Bill for your answer. It's caused me to rethink a lot of things. I have moved things around in the workshop and my inverter to battery wire will be 6 ft. Instead of 10 ft. This will also shorten the charge controller wire to 12 volt battery bank to 6 ft.. I have also shortened the pole mount/array wire to 25 ft. instead of 36 ft. (Any shorter will give problems with partial shading in the winter). I have used the Blue Sea DC calculator for my wire sizes.

    After reading the TriStar Controller book (PWM 60 AMP) a little closer, I see where your calculations of a 75 amp (rounded to 80 amps) breaker agrees with them. Does this mean, I can't use one of my Midnite baby Boxes between the controller and the battery? They only have a maximum 63 amp breaker. If I can't use it, what kind of breaker box or disconnect do I buy for this?
    Thanks once again,
    Remy
  • BB.BB. Super Moderators, Administrators Posts: 32,816 admin
    Re: Seeking advice on wiring solar system

    You are very welcome Remy.

    Regarding the wiring calculators... Be a bit careful. Some assume 2 way round trip (i.e., 25' run is 50' of wire) and others assume you input the 2 way length yourself (i.e., input 50' for a 25' run).

    Regarding placement of the array--It is very important to get as much full sun as possible (typically 9am-3pm minimum for a south facing array). And any money you spend on better placement (longer/heavier gauge wire) is usually worth the costs.

    Regarding the PWM controller you purchased. Good quality rugged controller. I would highly recommend that you purchase a Remote Battery Temperature Sensor (batteries are very temperature sensitive--cool batteries require higher charging voltages, hot batteries less). AGM's (sealed batteries) are even more sensitive.

    You have purchased the PWM controller--so you are "stuck" with 12 volts for the array to charge controller voltage (really around Vmp=17.x volts).

    If you are really impacted by keeping the wiring runs short (expensive copper wire)--you really have two choices-- 1) Exchange for a more expensive MPPT type charge controller (run higher voltage from array to charge controller) or 2) exchange the inverter to a 24 volt (or possibly 48 volt) inverter. Your TriStar PWM should work at any of those voltages and it will cut your array current requirements to 1/2 or even 1/4 the amperage (and reduce your wire sizing and voltage drop issues by a lot too).

    Anyway--12 volts--so this is what I would look at for the current output breaker/wiring of the PWM charge controller to the battery bank:
    Kyocera KC130TM panels (7.39 amps X 4 = 29.56 amps) Isc 8.02 Amps each.
    • 4 panels * 8.02 amps * 1.25 current derating * 1.25 solar derating = 50 amps design max
    So, your 63 amp box should be fine for your current array (note: the 1.25*1.25 is a very conservative double derating--so there should be no issue there).

    Now, lets look at the voltage drop for 12 volts setup... Vmp=17.6 volts for your panel (if I got the correct ones). And Imp=4*7.39amps=30amps.

    Using a random voltage drop calculator, "normal conditions", 25', and 30 amps we get for:
    • 10 awg = 1.8 volt drop
    • 8 awg = 1.1 volt drop
    • 6 awg = 0.7 volt drop
    • 4 awg = 0.4 volt drop
    • 2 awg = 0.3 volt drop
    For 17.6 volts * 0.03 or less drop = 0.582 volts or less.
    For 17.6 volts * 0.02 or less drop = 0.352 volts or less.

    So, normally, we would recommend less than 3% voltage drop in a wire run--and that would suggest that 4 awg should be the smallest gauge wire for your array to charge controller run.

    A 2% or less drop would be a 2 awg run.

    These are not "small" wires. If you either raise your battery bank voltage to 24 or 48 VDC or used an MPPT type charge controller to allow to run Vmp array at 48 VDC or more--you can really reduce your array wiring size. And/or you can add more panels later with the heavier gauge wire.

    Only you can make the decisions about what trade-offs will work best for you.

    The MPPT controller are much more expensive vs the PWM controllers--but they do have some nice ability/flexibility to allow high voltage solar arrays with lower voltage battery banks to cut down on wiring costs.

    Note too that higher battery bank voltages allow the same single "60 amp" charge controller to manage larger arrays:
    • 60 amps * 15 volts battery charging = 900 watts of solar array
    • 60 amps * 30 volts battery charging = 1,800 watts of solar array
    • 60 amps * 60 volts battery charging = 3,600 watts of solar array
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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