wire gauge for panel to controller

Hi. Wonderful forum. 
Installing 2 - 210 W MONO panels tomorrow on my 2019 Keystone 5th.  I have an existing Renogy 40AMP Rover charge controller and two existing 100 AH Lithium batteries.  Have successfully used a Renogy 100 W suitcase for a year.    Inverter / charger Multiplus will be stage two in a month or two.  Three questions:

1)  Renogy tech guy says to run panels in parallel.  I had planned on series.  

2) Current solar plug for the suitcase is 8 gauge wire running 10 feet to battery bank.  Going to connect with a bar, 10 guage from the new panels that will have a run of about 10 feet from panels to where it will junction to the solar plug and then onto the controller.  Total run to charge controller from the panels is about 18 feet then.  Question:  Will 10m guage be safe or do I need to go 8 guage?

3) Should the batteries be in series or parallel?  Currently they are in series.

Thanks for weighing in.


  • atownmike55atownmike55 Registered Users Posts: 4
    Sorry  Current solar plug for the suitcase is 10 gauge wire running 8 feet to battery bank
  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Welcome to the forum Mike,

    In solar/battery power, details matter.
    • MPPT Controller: 15-100 VDC input
    • 520 Watt solar max input for 12 volt bank (vendor spec)
    • You have battery temperature compensation is turned off (no temp compensation for Lithium Ion batteries)
    And need to know exactly what solar panel(s) you will be installing...

    Do you have existing solar panels that you wish to continue to use? If yes, need Vmp and Imp rating for those panels

    What are the Vmp/Imp ratings for your 210 Watt panels?

    For MPPT solar charge controllers, the recommended Vmp-array (from very cold to very hot climates) runs around:
    • 15 volt max battery charging * 1.3 minimum MPPT controller voltage requirement = Vmp-array-std~19.5 volts minimum suggested for MPPT
    • 100 VDC max MPPT controller input * 0.67 arctic freezing weather = Vmp-array-std ~ 67 volts max (i.e., -40F/C condtions)
    Such is why site temperatures are important... If you are on a nice Caribbean island where it is 75F all year long--Then you may have other options for series/parallel array options.

    Where is the system located (nearest major city)? And/or what are the min/max temperatures for your location (Voc/Vmp change with temperature; aka panel Voc voltage rise as temperature falls, and Vmp falls as temperature rises).

    Unless there is something very different about your panels (high Vmp/Voc voltages)--Yes, you can probably put them in series without any problems, and you can now run smaller AWG wiring and/or longer wiring as needed (such as park RV/Camp in shade, put panels in sun).

    200 Watt panels are fairly large format... And can be difficult to move with one person. Is this a fixed installation (rack mount, RV mount) or is this a portable installation (stake to ground--prevent wind from blowing them over and making expensive modern art)?

    For current vs AWG, this simplified NEC table is pretty conservative:


    And you can use a voltage drop calculator for AWG vs Wire Length (one way run for this website) vs Current:


    Typically for solar panel and general wiring, you design for around 1% to 3% max voltage drop. For battery to charge controller wiring, design for ~0.05 to 0.10 volt drop max (short/heavy cables from controller to battery bank). And for 12 VDC wiring suggest a max of 0.5 VDC drop from battery to loads.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 9,363 ✭✭✭✭✭

    3) Should the batteries be in series or parallel?  Currently they are in series.

    What voltage is your system  ?  12V   24v ?   That needs to be addressed first ?
    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 ,

  • atownmike55atownmike55 Registered Users Posts: 4
    Hi guys.
    Thanks for the quick reply.  

    Yes, the two 210 V Newpowa Mono s are now fixed on the roof.  I am south of you 3 hours in San Luis Obispo area.  Will be traveling mainly in the west and southern states.  I have a 2000w generator for backup but would like to use it seldom.  I have the option of connecting them in series or parallel and that is now my biggest question.  I just went with 8 guage wire.

    So this is a 12 volt system. The panels generate:

    Voltage at Pmax (Vmp)


    Current at Pmax (Imp)


    Thanks for the help on choosing series or parallel for the panels.

  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Connecting the two panels in series is the correct choice for you.

    10 AWG would have been fine... 8 AWG is nice too.

    The size of the array (and the battery bank) really depends on your electrical loads... If it is for lightning and other smaller loads, a smaller array can work fine. If you use an Induction cook top, 12 VDC refrigerator, Instapot, etc.. (i.e,. electrical vs propane), then the more panels you stick on the roof, the happier you will be.

    To show you a bit more about the math... Say you want two days of energy usage (no sun/cloudy weather) and run the Lithium bank from 90% to 20% state of charge (70% of capacity) for longer battery life:
    • 200 AH * 12 volt battery bank * 0.70 of capacity * 1/2 days usage * 0.85 AC inverter eff = 714 WH of 120 VAC power per day
    Just to pick a different location, say your array is mounted flat to roof and you are in Fresno Ca:

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a horizontal surface:


    And you camp from February through October--3.48 hours of sun per day (Feb):
    • 714 WH per day * 1/0.61 end to end AC off grid system eff * 1/3.48 hours of sun per day = 336 Watt array "break even" for February
    714 WH per day should be enough to run LED lights, cell phone charger, a small laptop computer... But if you are running a DC refrigerator, induction cook top, etc... Then you probably want more power than that.

    Not saying the above is "right or wrong" for you and your needs... Many RV folks are OK with just one day of battery storage (not much room in RV, driving a lot using engine charging, etc.). And that is fine too. But you really need to look at everything--For example, getting 12 volts from the tow vehicle to the RV trailer to charge the battery bank needs some heavy wiring or using an Inverter in the truck to make 120 VAC--Ship to the Trailer--And an 120 VAC to 12 VDC battery charger (sending high current 12 VDC any distance needs very heavy wiring).

    For example, say you want to charge your bank in 2 hours of driving:
    • 200 AH * 14.8 volts charging * 1/2 hours of charging * 0.70 bank capacity = 1,036 Watt load (on 12 volt RV bus)
    • 1,03614.8 Watts of charging load / 14.8 volts charging = 70 Amps @ "12 VDC"
    • 1,036 Watts * 1/120 VAC * 1/0.85 AC inverter/charger eff = 10.2 Amps @ "120 VAC"
    Much easier to send 1,036 Watts @ 120 VAC vs @ 12 VDC... Of course, now you need an AC inverter on the truck. And a 70 Amp AC battery charger, etc... (use the AC charger with your AC genset for backup)...

    Just an example of how the math works... You may run your genset for 4 hours to recharge--And then use a smaller AC charger, etc...


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • atownmike55atownmike55 Registered Users Posts: 4
    Wow !  Thank you for such a thorough explanation.  I will hook up today.
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