Wiring Help!! Off grid 24v no inverter just all DC

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lakelots
lakelots Registered Users Posts: 5 ✭
in Solar Beginners Corner #1
I have a 2 panel (135 watt keocera) 4 battery system with a steca1515 charge controller and midgnight solar coombiner with 2- 15amp breakers.

4- batteries are 6v each so I'm wiring batteries in series to get to 24v but I'm new to wiring dc so any help with how to wire together panels,batteries,combiner,charge controller would be big help.

I'm thinking run each of the panel's straight to the two midnight 15amp breakers in the midnight combiner parallel? plus and minus then to charge controller -fuses, batteries straight to controller-fuses, what do I do for dc consumer? straight to light string from controller? or I've got a 24v marine panel with breakers I could use to switch my dc lights and dc fan.

Just limited knowledge on dc wiring and want to make sure I don't screw it up.
Thanks in advance guys
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Wiring Help!! Off grid 24v no inverter just all DC

    I have to ask: why 24 VDC if you're not running an inverter? There's much more 12 V stuff readily available.

    As to how to wire it, you should have instructions with every piece you bought. They're usually very good at explaining where the wires go, what size to use, and what breakers/fuses. The basic wiring is simple enough: (-) tends to be common throughout. Panels fused before the downlead (although 270 Watts isn't much, fuses are always a good idea) with a disconnect at the charge controller. Output of the CC is fused/breakered before the batteries. Then you need fuses/breakers to protect each circuit off the batteries. You have to use the right size wire/circuit protection at each stage.

    For instance your Kyoceras are "12 V" panels rated at 7.63 Amps max. So to get "24 VDC" you wire them in series and include a 10 Amp fuse for circuit protection. The size of the wire from the array to the charge controller is determined by the distance run: http://forum.solar-electric.com/showthread.php?t=29
    The Steca 1515 is a 15 Amp controller, so it should run through one of your 15 Amp breakers with no trouble.

    Are you planning on expansion? That should be considered too. easier to put in larger wires to begin with than change it all out later.

    Also what are your batteries? It's easy to get into trouble with mis-matched charging/battery capacity.

    And standby for more answers; you will get lots of help from this forum! :D
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #3
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    Re: Wiring Help!! Off grid 24v no inverter just all DC

    For your system, you do not need any breakers/fuses between the solar array and your charge controller. You normally only need over current protection on the arrays when you have more than 2 strings in parallel--then you need to the specified short circuit protection fuse in each parallel connected string (typically, some sort of combiner box).

    From the charge controller to the battery you will need the appropriat wire and fuse/breaker connected directly to the battery (or battery bus bars). Keep the wiring between the Charge Controller and the Battery Bank short and heavy gauge wire. Long runs of small diameter wire will tend to reduce the efficiency of the charge controller (charge controller will not charge batteries as fast because of high voltage drop by long runs). If you have a choice, put the "long runs" between the solar array and the charge controller (you will still have wiring losses--but charge controller will work better with short CC to Battery run).

    Generally following the recommendations in the User Manual for the Charge Controller will be just fine.

    Connect your DC wiring to loads directly to the battery bank/bus bar common point (not at the base of the charge controller).

    Wire from Battery bank to "first" fuse should be short and well mounted/protected (fuses/breakers are to protect wiring runs from short circuits--so fuses should be as close to the battery as practical).

    To run your loads, typically you would run a heavy gauge wire with large fuse from the positive post/bus of the battery bank (say 10 AWG with 25 amp breaker) to another bus point. From there you can run 14 awg wire with 15 amp fuse/breaker to each load (and/or smaller wire and fuses to loads).

    Run all return wires back to the negative terminal/battery bus (no fuses/breakers in return leads).

    You can follow NEC rules for wire gauge vs fusing/breakers (usually conservative assuming wire in conduit/insulated areas). Or use a boating (or equivalent) wire vs protection for exposed wiring (typically less conservative).

    Did I make sense?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
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    Re: Wiring Help!! Off grid 24v no inverter just all DC
    BB. wrote: »
    Did I make sense?

    -Bill

    Moreso than me, as usual. I had "he's going to expand" stuck in my head and started with fusing the array. Sort of addressed the non-need, then slipped up again.

    Can't help but wonder what the intent is here; the end usage, as it were.
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  • lakelots
    lakelots Registered Users Posts: 5 ✭
    #5
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    Re: Wiring Help!! Off grid 24v no inverter just all DC

    Thanks for the fast help!
    I'm still a bit uncertain on a couple things, I have that midnight combiner with the 2-15 amp breakers inside, can I bring both sets of solar panel wires into that then wire the panels in series (+-) inside that and then go to Charge Controller, also I'd bring in panel ground wire that is connected to both panels into this combiner box,

    Or should I just take panel wires directly to CC in series without going through the combiner, I guess if I did that I could use combiner for the loads?

    any good DC wiring book recommendations? I definitely need more knowledge on the matter. Thanks again for your help I do appreciate it.
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  • lakelots
    lakelots Registered Users Posts: 5 ✭
    #6
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    Re: Wiring Help!! Off grid 24v no inverter just all DC

    I'm just wanting to use this setup to power some CFL lights 24v dc which I already have probably 5-6 of those and then a 24v ventilation fan for summer months. I have this in a old family barn my children love to play in, so just basic lighting, vent fan for hot summers for now, in the future I guess we could add inverter to hook up radio or other small appliance.

    Thanks again
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #7
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    Re: Wiring Help!! Off grid 24v no inverter just all DC

    Solar panels pretty much output near maximum current in normal operation... Shorting out a panel only outputs a little bit more current. So placing a fuse/breaker in series with one string does not really provide any extra protection (unlike fusing/breaker on a battery which can output huge amounts of current into a dead short--even a single 12v "car sized" lead acid battery can output 1-4,000 amps into a dead short.).

    You only need series protection fuses (one in each parallel string) when you have more than two strings in parallel (use the recommended fuse size by the panel mfg.).

    Probably a good place to start would be reading about Electricity for Boaters - BoatSafe.com and how to wire a boat... Much of the DC side is very similar (and even a bit better than you need if you account for vibration and water/weatherproofing).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #8
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    Re: Wiring Help!! Off grid 24v no inverter just all DC

    i'm confused why you have a combiner unless in the future you wish to expand on your pvs. the kcs will be in series and with the proper fusing and wire gauge used for your wire run the combiner features are not needed.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
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    Re: Wiring Help!! Off grid 24v no inverter just all DC
    lakelots wrote: »
    Thanks for the fast help!
    I'm still a bit uncertain on a couple things, I have that midnight combiner with the 2-15 amp breakers inside, can I bring both sets of solar panel wires into that then wire the panels in series (+-) inside that and then go to Charge Controller, also I'd bring in panel ground wire that is connected to both panels into this combiner box,

    Or should I just take panel wires directly to CC in series without going through the combiner, I guess if I did that I could use combiner for the loads?

    any good DC wiring book recommendations? I definitely need more knowledge on the matter. Thanks again for your help I do appreciate it.

    Connect the panels in series on the roof; no need for two sets of down lead and less Voltage loss with the higher array Voltage (possibly not an issue in your case, but technically correct). As Bill says, for two panels in series there's really no need to put a fuse on the panels - except for the future planning aspect.

    Remember you've got just about enough solar panel to keep 225 Amp/hrs of battery "happy". (A bit shy by my usual recommendations.) What those batteries are will make a difference in how well this project works too.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #10
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    Re: Wiring Help!! Off grid 24v no inverter just all DC
    Cariboocoot wrote: »
    Remember you've got just about enough solar panel to keep 225 Amp/hrs of battery "happy". (A bit shy by my usual recommendations.)

    I'd hate to try to equalize a 24 volt 225 AH battery bank with 270 watts of solar, While I had this same situation, I had planned on adding panels when I purchaed the battery bank.
    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.
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  • lakelots
    lakelots Registered Users Posts: 5 ✭
    #11
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    Re: Wiring Help!! Off grid 24v no inverter just all DC

    Thanks again everyone, I'm going to do some more reading to add to my limited dc knowledge and I'll get back with you guys on this. Thanks for all your help and time.
    OH ya
    My 4 batteries are
    UB-GC2
    200aH 6VDC
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  • lakelots
    lakelots Registered Users Posts: 5 ✭
    #12
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    Re: Wiring Help!! Off grid 24v no inverter just all DC

    For some reason I was thinking this had to be a 24v system from the start? I guess because of the 4 6v batteries, but it does seem to make better sense to just wire the 4 batteries and whole system as 12v.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
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    Re: Wiring Help!! Off grid 24v no inverter just all DC
    lakelots wrote: »
    For some reason I was thinking this had to be a 24v system from the start? I guess because of the 4 6v batteries, but it does seem to make better sense to just wire the 4 batteries and whole system as 12v.

    There are advantages to 12 VDC systems. Readily available 12 VDC equipment for one thing. There are disadvantages too: higher DC current for the same Watts. You'd wired your panels in parallel on the roof, unless you have an MPPT type controller (not really necessary with this small of a system). I think the Steca is a 12 or 24 V PWM type? In 12 VDC configuration you'd need two sets of MC4 connectors so you can plug in to each panel, then combine the (+) & (-) leads in a weather-tight box. There's slightly more line loss with the lower Voltage, but if the run is short and the wire big enough this won't be noticeable in practical terms.

    The biggest downside is in charging: the Steca 1515 is 15 Amps at either Voltage: 15 @ 12 VDC = 180 Watts, 15 @ 24 VDC = 360 Watts (roughly speaking). So with 170 Watts of panels @ 12 V you're about at the controller's limit. Also, you'd have 400 Amp/hrs of battery (4- 6V @ 200 Amp/hr; two in series to make 12 V, twos sets paralleled = 400 Amp/hrs) which isn't going to be "happy" with only 15 Amps of charge current (Amp/hr capacity * 5% minimum = 20 Amps approximately). These calculations are based on nominal values and "rules of thumb"; they are not the definitive engineering answer. You could divide in to two separate banks, and use them alternately.

    Just some things to think about.

    But hey; people keep telling me I don't know what I'm talking about so by all means: wait for further answers. :p
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