Wiring sizes for 1920 watt off grid system

System
System Posts: 2,511 admin
in Off Grid Solar & Battery Systems #1
I am after some pointers to calculate the sizes and methods for wiring using 'best practice'

the system comprises of:

12x160 watt solar panels. 6 arrays of 2 panels

Panel specs: Vmp 35.5, Imp 4.51

controller 48v 40amp
inverter 48v 2000w pure sine wave. to 220vac @ 50Hz

Battery bank:

4 banks of 4 VRLA (PV type) @ 200aH (total 16 batteries)



So:

I want to know the best method of wiring these batteries and the size of wire that should be used.

Also;
the wires from the controller to the inverter
the wires from the controller to the batteries.

I have guestimated the distances to be around 2 metres from controller to inverter etc.

Note that each array of 2 PV wires will come direct to the controller, so this is not an issue.

An additional question for this is the placement and type of safety switching/ fusing.

The physical battery size is:525x240x245mm of course some allowance between them is needed.


Looking forward to some answers and hope that this thread will help others.

Simon
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Wiring sizes for 1920 watt off grid system

    You may want to try and get that invert closer to the batteries: those 'heavy wires' carry the most current of any part of a system, and should be kept as short as possible to minimize loss.

    Calculator thread:
    http://forum.solar-electric.com/showthread.php?t=29

    Also, unless I'm doing the math wrong (it happens!) your array is only good for about 400 Amp/hrs of battery - 800 maximum (basically PV watts divided by system voltage should be 5-10% total Amp/hrs of batteries). So if your total battery package is 800 Amp/hrs (4 banks @ 200 each) you're on the edge. The engineers on this forum can get you right on with specifics.

    But let's ask the "$64 question": have you calculated your usage yet? The first thing to know is how many watt/hrs of power you need (really need - no electric toasters!) between re-charge times (usually daily).

    If you've already bought the equipment, do give brands & models of charge controller and inverter.
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  • System2
    System2 Posts: 6,290 admin
    #3
    Re: Wiring sizes for 1920 watt off grid system

    calculator thread is a good start, and the spreadsheet opens fine in openoffice even when converted into ODS format.

    My assumption with the battery interconnects after talking to several people is that they should be based on the loads.

    the inverter is 2kW @ 48v, therefore battery interconnects should be rated at about 42 amps.

    will post my system design calculations as I am now unsure about the battery bank size!!

    Simon
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  • System2
    System2 Posts: 6,290 admin
    #4
    Re: Wiring sizes for 1920 watt off grid system

    I revisited my calculations, and yes I made some wrong assumptions for the battery bank. below are new calcs


    The 5-10% of the PV wattage and aH size of battery bank does not take into account the Insolation figure used, I guess you may be using something i around the 3.0 mark. (which would make the PV size much larger)

    This system is to be installed at my Sisters house in a northern area of Australia, so the figure of 4.5 Insolation is correct.

    Insolation figure from here:
    http://eosweb.larc.nasa.gov/cgi-bin/sse/grid.cgi?uid=3030
    (damn good resource, bookmark it!)

    Read on for my calculations.



    Battery bank sizing

    Watt hours per day 6900
    Days of autonomy 2

    13800

    DOD 0.5

    27600

    Battery temperature derating
    1.11

    Battery bank capacity -min watt hours
    30636

    Battery bank voltage 48


    Min Ah capacity of battery bank
    638.25

    Battery capacity in Ah 160

    Battery voltage 12
    no of batteries per sting
    4

    no of banks of strings 4

    total batteries required
    16
    Ah capacity of battery bank
    640


    Solar panels sizing

    Insolation 4.5
    derating 0.8
    Min PV watt 1920
    Min PV size 160
    Min PV units 12
    end PV size 1920
    PV arrays 6
    of PV panels 2
    controller size -amps 40
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Wiring sizes for 1920 watt off grid system
    aquatooth wrote: »
    calculator thread is a good start, and the spreadsheet opens fine in openoffice even when converted into ODS format.

    My assumption with the battery interconnects after talking to several people is that they should be based on the loads.

    the inverter is 2kW @ 48v, therefore battery interconnects should be rated at about 42 amps.

    will post my system design calculations as I am now unsure about the battery bank size!!

    Simon

    Whereas 2kW / 48V = about 42 amps, you've left no 'margin for error'. Connections and conductors are never perfect, so in terms of wiring for batteries to inverter always go with the largest size wire you can practically apply. Especially true the longer the 'run'. Welding cable - 2/0 gauge - is usually the default recommendation. Remember; in terms of current tis is going to be the heaviest draw.

    Another potential battery problem you've got: it isn't recommended to have more than two banks in parallel. This is due to difficulties in getting even current flow through all banks. With two banks, you 'cross-wire' the invert & charge controller so the positive from one goes to one bank, the negative to the other. This helps keep current flow even through all batteries for both charging and discharging.

    If you batteries are typical 12V 160 A/hr Marine/RV Deep Cycle you may be disappointed with the performance. I've got those and they don't come up to snuff. The best deal in A/hrs is the deep cycle electric forklift/golf cart type, such as Trojan or Deka. You'd need 8 to make 48V, but they can be 200 to 400 A/hr.

    Also, 50% DOD is generally not recommended by the experts here as it significantly shortens battery life.

    What is your PV array voltage (nominal)?

    The first thing you should look into is reducing your consumption. 6.9 kW/hrs is more than double what I use, including running a full-size water pump and digester pump. What are your loads?
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  • System2
    System2 Posts: 6,290 admin
    #6
    Re: Wiring sizes for 1920 watt off grid system

    6900 watt hours covers my sisters high usage. big family....


    Looking at options and a battery catalog it seems going for a single bank of 2v batteries (24pcs) might be better.

    50% DOD. noted. the specs for 2v give 1200 cycles at 50%, so I think this is ok

    600 or 800 aH sizes are available, so I may have to re-do design figures again.


    Will be thinking about this and post updated design in a few days...

    Panel specs: Vmp 35.5, Imp 4.51

    So nominal voltage will be 71

    Simon
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