What gauge wire should I be using with my new 24V system.

Hello all,

I have a small system (2-230watt 24v panels)

What size wire should I run from the panels to the controller?

Is 6 gauge correct for connecting my batteries to each other.

Thanks so much

Comments

  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: What gauge wire should I be using with my new 24V system.
    darrylgood wrote: »
    ... (2-230watt 24v panels)

    What size wire should I run from the panels to the controller?

    This depends on how far the panels are from the controller and if they are in series or parallel?
    darrylgood wrote: »
    ... Is 6 gauge correct for connecting my batteries to each other.

    This depends on the distance between the batteries and the inverter draw (or total draw if your using DC voltage)
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • darrylgood
    darrylgood Solar Expert Posts: 25
    Re: What gauge wire should I be using with my new 24V system.

    The panels are around 35' - 40' from the controller, 2 24v panels wired in series

    The inverter is a 1200 but the draw will usually be quite low - all use will be AC and the batteries will be right beside each other.

    Thanks Photowhit!
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: What gauge wire should I be using with my new 24V system.
    darrylgood wrote: »
    The panels are around 35' - 40' from the controller, 2 24v panels wired in series

    The inverter is a 1200 but the draw will usually be quite low - all use will be AC and the batteries will be right beside each other.

    Still not enough info. I will ASSUME that your panels have Vmp of 30 volts, in which case 10 gauge cable will give you a 1% voltage drop between the panels and the controller.

    As far as cable size from the battery to inverter, that depends on the current through the cable. That will depend on the battery voltage (which you have not told us) and the peak power that the inverter draws from the batteries (if your inverter puts out xxx peak watts, it will draw more than xxx watts from the battery). Your inverter manual should provide info on fusing and cable requirements.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • darrylgood
    darrylgood Solar Expert Posts: 25
    Re: What gauge wire should I be using with my new 24V system.

    Yes Vmp of 30 volts

    As for the batteries, there are 4 12v's - wired as 2 24v

    Sorry if I have left anything out. Please let me know if you need any more info.

    Thanks so much for the help guys
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: What gauge wire should I be using with my new 24V system.

    As VtMaps said at 10 gauge your in great shape, and likely OK with 12 gauge from panels to MPPT charge controller(?)

    6 gauge wire is fine for up to 50 amps at high temps, at 24v that's about the limit for a 1200 watt inverter and you might exceed that momentarily at peak rates, always nice to use a size larger cable. Keep the battery wiring short and use the same length wire between the batteries and attach your leads to the positive of one set and the negative of the other, this should keep your resistance the same across the 2 sets of batteries.

    If you have a long run from the batteries to the inverter you might consider a heavier cable, but a 1200 watt inverter running on 24 volt battery bank should be fine. I have assumed that your using @100 amp 12V batteries? 2x2 bank, for a 24volt 200amp battery bank? or close to this size a small sized battery could create low voltage problems with a heavy load.

    A minimal sized 24v battery bank for a 460 watt array would be around, 120Amps based on NOCT(Normal Operating Cell Temperatures) values (used 80%) and a C/8 (charging at 1/8th of the battery capacity)
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • darrylgood
    darrylgood Solar Expert Posts: 25
    Re: What gauge wire should I be using with my new 24V system.

    Great, 10 gauge it is, on a side note my controller is 30 amps, is this large enough?

    The inverter is quite close to the batteries, so we should be fine there

    Yes exactly, 2x2 @100 amps - Great 6 gauge and a short run to each.

    Also for my 10 gauge wire run from the panels to the controller, I have a Y splitter for my mc4 connections for the 2 positive and 2 negative connections into the 2 single wires that run to the controller. Is this ok to wire them like this?

    Thanks so much
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: What gauge wire should I be using with my new 24V system.
    darrylgood wrote: »
    Also for my 10 gauge wire run from the panels to the controller, I have a Y splitter for my mc4 connections for the 2 positive and 2 negative connections into the 2 single wires that run to the controller. Is this ok to wire them like this?

    No now you have a new problem, you didn't happen to buy these for a guy down on the blvd that claims they are 24 volt panels did you?

    These panels with a vmp of 30 are NOT high enough voltage to charge a 24 volt system with out a MPPT type battery charger. Just posted on a shopping site that was directing people to a cheap source of "24 volt" panels that are NOT designed to charge a 24 volt system with a cheap charge controller.

    If these happen to be perlight 230 watt panels here is what I posted there;


    While this is a good price and a good panel, "Bad Company" has a very bad habit of calling panels in the 26-32volt VMP range "24Volt" panels. This is NOT true!!! To properly charge 24 volt systems, with an inexpensive Pulse Width Modified Charge Controller (like the cheapy linked in OP) It requires a panel with a VMP of at least 34 volts.

    These panels are made for use with a more expensive MPPT (Multi Power Point Tracking) Charge Controller. These start around $240 for a model that will handle 2 panels in a 24V system and up. A single panel will not properly charge a 24 volt system.

    Perlight 60 series Spec Sheet

    Let me walk you through the NOCT (Normal Operating Cell Temperature) values. The panel is rated at 25 degrees C. but the NOCT value when the sun is hitting it during a normal summer day will be 47 degrees. The VMP is reduced .35% for each degree above 25 degrees so the VMP of this in normal operating conditions is 30 x 92.3% or 27.69 Volts. This is before voltage loss in your charge controller and in the wires into and out of the battery and is already below the 28.8 volts you normally would charge a FLA(Flooded Lead Acid) battery and barely above the 27.2 volts you would float a 24 volt battery.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: What gauge wire should I be using with my new 24V system.
    darrylgood wrote: »
    The panels are around 35' - 40' from the controller, 2 24v panels wired in series

    Those two panels in series should work well with an MPPT controller and a 24 volt system
    darrylgood wrote:
    I have a Y splitter for my mc4 connections for the 2 positive and 2 negative connections into the 2 single wires that run to the controller.

    But now you indicate that the panels are in parallel. If they are in parallel, Photowhit is correct:
    Photowhit wrote:
    These panels with a vmp of 30 are NOT high enough voltage to charge a 24 volt system

    I am now too confused to offer any more advice.:confused:

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • darrylgood
    darrylgood Solar Expert Posts: 25
    Re: What gauge wire should I be using with my new 24V system.

    Absolutely my mistake. Sorry yes the 2 24v panels are wired in Parallel. (beginner mistake :) )

    I have attached a picture of the specs off the back off my panels. I am stuck now, should I not be running it as a 24v system?

    Any advice would be greatly appreciated as I am just setting this up now and can change anything up now.

    Thanks :)

    Attachment not found.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: What gauge wire should I be using with my new 24V system.
    darrylgood wrote: »
    yes the 2 24v panels are wired in Parallel. (beginner mistake :) )

    I have attached a picture of the specs off the back off my panels. I am stuck now, should I not be running it as a 24v system?

    Those panels in parallel cannot charge a 24 volt bank. period. If you put them in series they will be able to charge a 24 volt bank, but you will need an MPPT controller (actually you can use a PWM controller, but it will utilize less than half the power that your panels can produce).

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • darrylgood
    darrylgood Solar Expert Posts: 25
    Re: What gauge wire should I be using with my new 24V system.

    Thanks vtmaps good to find this out noew :)

    Can the current panels be set up to power a 12v battery bank?
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: What gauge wire should I be using with my new 24V system.
    darrylgood wrote: »
    Can the current panels be set up to power a 12v battery bank?

    Yes, but. If you put them in parallel with a PWM controller you will realize less than half their potential. If you put them in series with a PWM controller you will realize less than a quarter of their potential.

    No way around it: you need an MPPT controller with those panels.

    Its getting harder to find large panels that can charge a 12 or 24 volt battery bank with a PWM controller. When you do find them they cost more per watt than the type of panels you have. The industry is moving towards panels that are designed for grid-tie inverters. These "grid-tie" panels work perfectly well with battery systems, but require an MPPT controller to realize their potential.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: What gauge wire should I be using with my new 24V system.

    It's hard to find them, but they are out there. Northern Arizona Wind and Sun recently had some Astropower(?) panels at around $1 a watt that were true 24V nominal panels, and there is another supplier that has them at about the lowest price available. Email me if you are looking for true 24V panels. At this point your best bet would be looking for a MPPT charge controller.

    There are also few MPPT charge controllers in the medium size. There is a rogue that should be released this month and Midnite is promising one sometime this year.

    Morning Star makes a 15 amp, but I don't think It will handle the VOC of these combined with out risk of destroying it on a cold winter day.

    Right now I'd say the morning star 45 amp CC would be your best bet and allow for some expansion.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • darrylgood
    darrylgood Solar Expert Posts: 25
    Re: What gauge wire should I be using with my new 24V system.

    So what would be the ideal system for these panels? I do own the panels and the 4 12v batteries now, should I go 12 or 24v system. And what would be the best way to wire the sytem up for the best productivity. ( I have no problem returning the 30 amp controller if I need to)

    Again, sorry I still learning..... and need th step by step :)
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: What gauge wire should I be using with my new 24V system.
    darrylgood wrote: »
    So what would be the ideal system for these panels? I do own the panels and the 4 12v batteries now, should I go 12 or 24v system. And what would be the best way to wire the sytem up for the best productivity. ( I have no problem returning the 30 amp controller if I need to)

    My advice: return the controller.

    Now, as far as designing a system is concerned, let's start from the beginning.

    Step 1: define your loads. AC or DC loads? What will be the peak load (watts). What will be the total energy use per day (kilowattHours).

    Step 2: choose a battery voltage and capacity

    Step 3: buy enough panels and charge controller to keep the battery happy.
    It is possible to design a system backwards (start with panels or battery and try to figure out what sort of loads they can handle), but why would you want to do it backwards? The whole purpose of a system is to supply your loads. If you don't know your loads, what is the purpose of your system?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • darrylgood
    darrylgood Solar Expert Posts: 25
    Re: What gauge wire should I be using with my new 24V system.

    I will give you a quick run down on the use: It's for a small cabin that will be used only on weekends. The purpose of this solar set up is to run a small bar fridge while we are gone during the week and a laptop and light or two while we are there. I did the math before I bought the panels and with the 460w panels and the 4 batteries it will meet our requirements for sure. If we do happen to run out, we can use the generator :)

    The load will be ac, not sure if I should go 24v or 12v that's what i am hoping you guys can help with. So I think we have decided that I 100% need a mppt charger, can I go with a 20 amp?

    Thanks again

    I would also like to keep the 1200 w inverter I have but am open to suggestions.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: What gauge wire should I be using with my new 24V system.

    Have you measured the fridge power with a Kill-a-Watt type AC power meter? Generally, adding an electric fridge to any off grid power system takes it from being a small system to a medium sized system.

    Also, would like to know the brand/model/specifications of AC inverter--The "idle losses" can be significant.

    But since I don't have any of that information yet--I will take a stab at it. A typical bar fridge is probably around 320 kWH per year. The typical 1,200 watt inverter is probably ~85% efficient and idles at 12 watts. The 1,200 watt inverter probably will start/run the fridge (right on the edge--good batteries, short/heavy copper wiring from battery to inverter).

    The math might look something like this (note that some models of inverters have low power modes, or may even draw more power--just a guess at "nominal" numbers). Use two day of battery storage and 50% maximum discharge:
    • 320,000 WH / 365 days per year = 877 WH per day (0.887 kWH per day) for fridge
    • 877 WH per day * 1/0.85 inv eff + 12 watts * 24 hours per day = 1,320 WH DC power per day from battery bank
    • 1,320 WH per day * 1/24 volt batter bank * 2 days storage * 1/0.50 max discharge = 220 AH @ 12 volt battery bank

    To charge a 24 volt @ 220 AH battery bank (4x 6 volt 220 AH golf cart batteries in series) at 5% to 13% rate of charge:
    • 24 volts * 220 AH * 1/0.77 panel+controller losses * 0.05 rate of charge = 343 watt array minimum
    • 24 volts * 220 AH * 1/0.77 panel+controller losses * 0.10 rate of charge = 686 watt array nominal
    • 24 volts * 220 AH * 1/0.77 panel+controller losses * 0.13 rate of charge = 891 watt array "cost effective maximum"

    And, Assuming three seasons in North Bay ONT (again, just guessing, showing process) with PV Watts, fixed array tilted to latitude:
    Month      Solar Radiation kWh/m2/day)
    1      3.46     
    2      4.94     
    3      5.88     
    4      5.27     
    5      5.50     
    6      4.95     
    7      5.50     
    8      5.03     
    9      4.23     
    10      3.63     
    11      2.04     
    12      2.31     
    Year      4.39      
    

    Toss bottom three months, leaves October with 3.63 hours as the "break even" month (may need genset):
    • 1,320 WH DC power per day * 1/0.77 panel+controller derating * 1/0.80 battery eff * 1/3.63 hours of sun = 590 Watts of solar array minimum

    So, the range of solar panel for a 320 kWH per year fridge + a 12 volt tare loss AC inverter running 24 hours per day would require a solar array from 590 Watts to 891 Watts (minimum to cost effective maximum) for a "rules of thumbs" system design.

    Numbers should be pretty close--Please feel free to adjust with real numbers, actual location, and season(s) of use.

    Refrigerators use more power during warm weather and when opened/closed (adding new food to chill).

    Also note, the battery cutoff voltage for most AC inverters is around 21 volts--That is pretty much a "dead battery" bank and probably needs to be replaced. On the other hand, if you can program the inverter for around 23 volts (and more than a few minutes that low), you could save the battery bank, but come back to a "warm fridge" full of science experiments--Not pretty either.

    I would be hesitant to leave an off grid system running a refrigerator for week+ at a time without somebody dropping by once in a while to check up on it. Would probably be better if you can shut the system/fridge down while you are gone for more that a few days. Hardware failure, a run of clouds, etc. can all give you heart burn.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • darrylgood
    darrylgood Solar Expert Posts: 25
    Re: What gauge wire should I be using with my new 24V system.

    Hi BB this is quite a small fridge and only uses 240, so your numbers all seem quite close. I calculate that with 460w we should just make it. At this point it is just a hobby, so I have no problem adding to it in the future. Also, the location is Parry Sound Ontario and it will only be used in the summer months.

    So after all this great help from everyone here I think I finally have a clear view.

    Please let me know if this system set up is correct for my 30 vmp 24v panels:

    2 -24v panels in series.

    Than to a mppt controller (what is the smallest size mppt controller I can get? is 20amps to small)

    4 12v batteries with 100 am hours each, wired to have 2 24v sets.

    1200w (peak 2400w) 24v inverter

    On a side not is there any plus to setting these panels up as a 12v system or should I stick to the 24v system outlined above.

    Thanks!!!!!
  • darrylgood
    darrylgood Solar Expert Posts: 25
    Re: What gauge wire should I be using with my new 24V system.
    vtmaps wrote: »
    Yes, but. If you put them in parallel with a PWM controller you will realize less than half their potential. If you put them in series with a PWM controller you will realize less than a quarter of their potential.

    --vtMaps

    So in parallel is better better with the PWM controller? Sorry i thought it was said that with the low vmp it should be in series?

    Thanks vtMaps
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: What gauge wire should I be using with my new 24V system.
    darrylgood wrote: »
    So in parallel is better better with the PWM controller? Sorry i thought it was said that with the low vmp it should be in series?

    Thanks vtMaps

    A PWM type controller has no ability to "down convert" higher Voltage into current. Basically any power from the panels that is due to Vmp above "system normal" (17-18 for 12 Volt, 34-36 for 24) will be lost.

    So to maximize panels with a Vmp of 30 on a 12 Volt system using a PWM controller the panels should be in parallel. That way they will effectively be Imp * charge Voltage "Watts". If you put two in series on this, all the power that would come from the second panel will be wasted instead of just some.

    With a 24 Volt system the 30 Vmp runs the risk of being too low under normal operating conditions to achieve charging Voltage for the batteries. Put two in series and you will have enough Voltage, but at the expense of a large portion of the power of the second panel (basically Imp * Voltage difference between string Vmp and "ideal" Vmp).

    Yes; clear as mud isn't it?
  • YehoshuaAgapao
    YehoshuaAgapao Solar Expert Posts: 280 ✭✭
    Re: What gauge wire should I be using with my new 24V system.
    A PWM type controller has no ability to "down convert" higher Voltage into current. Basically any power from the panels that is due to Vmp above "system normal" (17-18 for 12 Volt, 34-36 for 24) will be lost.

    So to maximize panels with a Vmp of 30 on a 12 Volt system using a PWM controller the panels should be in parallel. That way they will effectively be Imp * charge Voltage "Watts". If you put two in series on this, all the power that would come from the second panel will be wasted instead of just some.

    With a 24 Volt system the 30 Vmp runs the risk of being too low under normal operating conditions to achieve charging Voltage for the batteries. Put two in series and you will have enough Voltage, but at the expense of a large portion of the power of the second panel (basically Imp * Voltage difference between string Vmp and "ideal" Vmp).

    Yes; clear as mud isn't it?

    For 24 volt PWM, You can always use 72 or 80 cell panels. They will usually be 280+ watt panels (80-cell 300+ watt). 72 cell runs 35-36 Vmp, 80 cell runs 39-40 Vmp. If in hot AZ weather you should use 80-cell. 72-cell will work in cooler weather.