Wire Size

andy300z
andy300z Solar Expert Posts: 39
Hello,
I am in the process of setting up a total of 360w panels feeding a outback 80amp
mppt charge controller. I am setting up a 24v battery bank the only problem i have is that the battery bank is in the shed which is 60 feet away from my charge controller. I have hundreads of feet of 8awg wire. Can this work. or should i spent the money and purchased 6 gauge.

Thanks

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Wire Size

    If I understand you correctly...

    You need to mount the Charge Controller within a few feet of the battery bank... Roughly, you probably want a maximum of 0.05 to 0.10 volts of drop between the charge controller and the battery bank.

    For a 12 volt battery bank, each 0.1 volt drop represents (very roughly) a 10% loss in charging energy at the battery bank...

    Make the long wire runs between the solar panels and the charge controller.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • conntaxman
    conntaxman Solar Expert Posts: 125 ✭✭✭✭✭
    Re: Wire Size

    This wire size is for the tabbing. Would this copper wire size [ 24 ] be good enought to use when tabbing 36 cells together in series?It says 3.5 amp
    .Thanks John
    .
    AWG gauge / Dia.inch/ Max .amp for chassis wiring / Max.amp for power trans

    24
    0.0201
    3.5
    0.577
  • andy300z
    andy300z Solar Expert Posts: 39
    Re: Wire Size

    unfortunatly my shed had no heat so i made a control panel in office in the house so i can monitor the chargeing of the battery in my shed.
    I have panels on my roof which is 25 feet from my charge controller which is mounted in my office now i need to run 60 feet to my batteries and i even change to 24 system to make the run with a smaller gauge wire . so you are telling me even if i used 5 gauge wire the voltage drop will be to much ? which makes the system inefficient ?
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Wire Size
    andy300z wrote: »
    Hello,
    I am in the process of setting up a total of 360w panels feeding a outback 80amp
    mppt charge controller. I am setting up a 24v battery bank the only problem i have is that the battery bank is in the shed which is 60 feet away from my charge controller. I have hundreads of feet of 8awg wire. Can this work. or should i spent the money and purchased 6 gauge.

    Thanks

    Wire your panels in series, (this is a GUESS, since you did not say what your panels produce) and then run the high voltage ( 70V maybe? ) the long distance to the charge controller, which should be near the batteries. The MPPT controller will down convert the high panel voltage, to the battery charging voltage with little loss.
    You MUST move your charge controller to be closer to your batteries.
    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 ,

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Wire Size

    The current for 360 watts of solar panels, 0.77 typical derating, and 29 volts charging:
    • 360 watts * 0.77 * 1/29 volts charging = 9.6 amps typical maximum current
    Using a voltage drop calculator, 9.6 amps, 60' wire run, start with 6 awg wire:
    • 0.55 volt drop @ 60 feet (one way run) and 9.6 amps
    Typically, a 24 volt flooded cell battery bank will charge at 29 volts and float at 27.4 volts... For a 0.55 volt drop, the charge controller will be charging your battery bank at ~28.45 volts using 6 awg wire... Roughly equivalent to 14.2 volts for a 12 volt battery bank.

    It certainly will charge the bank, but at a slower rate. You have the option of cranking the charging voltage up to 29.55 volts, but as the charge current tapers off, the voltage drop falls, and you are charging the bank closer to 0.55 volts higher than you really want...

    So, you are left with a couple options (60' between battery bank and charge controller)--Leave the charge setting at 29 volts and charge slower--Or up the voltage and charge the batteries at a higher voltage as they approach full charge.

    Neither is a great solution--but they both sort of work.

    If you ever add more solar panels to your existing 360 watt array, the voltage drop is only going to get higher (note, the 5 awg wire will be around 0.44 volt drop for a 360 watt array).

    Also, the charge controller needs to know the temperature of the battery bank. Hot batteries need a lower charging voltage and cold batteries need a higher charging voltage.

    Either the temperature sensor in the charge controller itself needs to be in the same room/temperature conditions as the battery bank, or you need to install a remote battery temperature sensor that bolts/glues to a battery post.

    Installing a charge controller in a temperature controlled room without a RBTS is not a good thing either.

    Also, the charge controller should be fine in the same room as the batteries (if you have wide temperature swings, you may need to insulate the room/battery box anyway).

    Lastly, you asked about efficiency... Remembering:
    • Power=Voltage*Current = V^2 / R = I^2 / R
    If you run the battery voltage at 29 volts @ 9.6 amps the long distance vs running the solar array with Vmp=58 volts @ 4.8 amps (for example)--The difference is:
    • Ratio of power loss = 4.8a^2 / 9.6a^2 = 1/4
    Running the "long" wire run at elevated voltage will give you only 1/4 the power loss (assuming same awg wire) as running at the lower battery voltage.

    So, yes, there is an efficiency factor too involved in moving the charge controller from the office to the shed.

    In reality--much of it is a savings in Copper Wiring costs if you run the solar array at a higher Vmp voltage (~60-100 VDC typically--depending on a bunch of stuff).

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

    i'm not actually sure if bb said this or not, but......

    when the distance between the controller and the batteries is great the voltage drop created by the wire will cause the batteries to not charge properly unless you have a controller with remote battery voltage sensing which i don't believe outback controllers have it. for instance if you have a .7v drop, the controller will think the charge may be complete at say 29.0v, but by the time it gets to the batteries it is 28.3v. that .7v is about a 3% rate of voltage drop. we think it is wise to have about a 1% or less voltage drop between the controller and batteries when no remote battery voltage sensing is available. it will be some serious wire to keep that 1% on the v drop or get them closer together.
  • andy300z
    andy300z Solar Expert Posts: 39
    Re: Wire Size

    Do you know which charge contoller has battery remote voltage sensing ?
  • soleil
    soleil Solar Expert Posts: 45
    Re: Wire Size

    Is the only reason you have the charge controller installed in your office away from the battery bank so you can monitor charging? Couldn't you just get a remote monitor and controller, install that in your office, and put the charge controller itself in the shed near the battery bank?

    Something like this (whichever one your controller uses): http://www.solar-electric.com/mate2.html
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Wire Size

    the tristar mppt controllers have a remote battery voltage sense. do not confuse this with a remote temperature sensor which is available and recommended too.
    http://www.morningstarcorp.com/en/support/library/TS.IOM.Operators_Manual.04.EN.pdf
  • andy300z
    andy300z Solar Expert Posts: 39
    Re: Wire Size

    Hello guys thanks for all the advise.
    I recently bought all my equipment from our host of this website. What great service.
    Here is my list of my equipment.

    1. Tristar mppt 60 amp charge contoller
    2. Tristar remote meter 2
    3. (4) 80 watt panel 320w total
    4. Trimetric 2020 (on order)
    5. All disconnects
    6. 6 gauge wire from panel to charge controller.

    The question i have is that my panels are appx 100 feet from my charge controller
    so i hooked it up in series which make appx 68v @ 5amps due to the long run i did this in series which should cut down in voltage drop.
    I want to know with this type of wattage is it sufficient to charge a 24v or 12v battery system which voltage should i used for my battery bank? I currently have
    (4) 12v deep cycle batteries appx 400 total (AH).
    I have not bought the inverter yet i was just debating which inverter to get. p.s my goal is to take my office off the grid which consumes 300watts with no printers and 1200 watts when the printer initialy starts for 5 seconds.

    Thanks
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Wire Size

    all of your pvs in series will work to charge either a 12v or 24v battery bank with about 1.7% on the voltage drop as per your new 100ft run specification using #8 copper wire. now you won't get all of that wattage to the batteries and some like to cut the wattage in half that is reliable to the batteries after all losses. for a 12v system for example you will have the downconversion also causing a 4 fold increase in current. voltage down by 4 and current up by 4. 5a x 4 = 20a. if using the 50% loss factoring this is now about 10a. we like to have the batteries charge between 5% and 13%. working backwards we get 10a/5%=200ah. for a 13% rate of charge it's 10a/13%=77ah.
    if your losses turn out to be not as great then the 13% rate i cited may not be a good idea to use for you will have more current than 10a resulting in a much higher charge rate. this may be fine if the battery you choose can take the higher charge rates. most will take a bit more, but we err on the safe side noting that some agm batteries can take quite a bit more. for this 12v example it would appear a battery between 100ah and 200ah would be in order. if going to a 24v battery bank then you need 2 12v batteries in series with half of the ah rating i calculated for 12v. if you plan on expanding the number of pvs in the future then go with a higher ah figure within the ah range i cited. that means a 200ah battery bank at 12v will accept more pv current readily without getting into overcharge possibilities.
    whether this will suffice for your loads is a different question as you need to not only replace what you use, but overcome the loss factors too in doing it while staying within good parameters for properly charging your batteries.