Question about off grid system wiring.

12Voltian12Voltian Registered Users Posts: 5
Hi, I'm putting together a 12 volt system to run a DC fridge and freezer. I have 5 grape solar 100 watt panels, a Xantrex C35 controller, a Xantrex Pro Watt SW 3000 watt inverter, a Power Max 75 Amp charger and 6 Trojan T105 batteries.
Even though this is a small system I would like to put in some fuses/protection. Can someone please recommend the size needed and an appropriate source? The schematics I've seen seem to place one both before and after the charge controller. Also, are switches really needed on a system this small? Thoughts? Thanks...

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,235 admin
    Ok... Lets slow down a minute here... Do you have a good idea of how much energy usage from your fridge/freezer? Some are very energy efficient, other types are very energy inefficient.

    Knowing your loads, then we can size the battery bank, Then size the solar array (based on size of battery bank AND hours of sun per day).

    Since you have the battery bank--Lets start there. 6x 225 AH @ 6 volt batteries will give you a 675 AH @ 12 volt battery bank. We recommend charging a battery bank at 5% to 13% rate of charge from solar array. 5% can be fine for weekend/seasonal use... 10% or more is good for full time off grid use:
    • 675 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 636 Watt array minimum
    • 675 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 1,271 Watt array nominal
    • 675 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 1,652 Watt array "cost effective" maximum
    Since we don't have any loads--Lets assume you use 25% of battery power per day (2 days of storage, 50% maximum discharge):
    • 675 AH * 12.0 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge = 1,721 Watt*Hours per day (120 VAC power)
    Using PV Watts V2 for SHREVEPORT, LOUISIANA, fixed array tilted to latitude of 32 degrees from horizontal:
    Month    Solar Radiation ( kWh / m2 / day )
    January        3.68
    February    4.52
    March        4.96
    April        5.48
    May        5.59
    June        5.83
    July        5.86
    August        6.04
    September    5.56
    October        5.49
    November    3.99
    December    3.76
    Annual        5.06
    


    To supply 1,721 Watts of 120 VAC power (AC inverter) per day, toss out bottom three months (assume generator use), we get 4.52 hours of sun for February (break even month):
    • 1,721 Watt*Hours per day * 1/0.52 end to end AC system eff * 1/4.52 hours of sun per day (Feb) = 732 Watt array minimum (based on load+hours of sun)
    Lastly--A 675 AH @ 12 volt flooded cell lead acid battery bank will support a maximum inverter size of roughly (based on 400 AH @ 12 volts per 1 kWatt of AC inverter):
    • 675 AH * 1/400 AH per 1,000 Watt of AC inverter = 1,688 Watt or ~1,500 to 2,000 Watt AC inverter recommended maximum
    So, your 3 kWatt AC inverter is a bit large for this size battery bank... Also, is is very difficult to wire up the DC battery bus wiring to support much more than 1,200 to 2,000 Watts worth of current:
    • 3,000 Watt Inverter * 1/0.85 inverter eff * 1/10.5 volt cutoff * 1.25 NEC derating for wiring+fuse/breaker = 420 Amp 12 VDC branch circuit rating to support a 3kWatt inverter
    So, as you can see, I would like to understand your system needs a bit more before I suggest ABC fusing/breakers and wiring for your setup (of course, the inverter installation manual would hopefully give some guidance too).

    Based on the math above (and my guesswork), what would/should your system look like?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • 12Voltian12Voltian Registered Users Posts: 5
    Hi Bill, thanks for the speedy reply. Not only am I new to solar but I'm new to posting on forums as well. I should have given more information.

    First, the inverter. It just happened to come with the kit. Even though it is called a Pro Watt SW 3000, it is only rated at 2000 watts continuous. The 3000 is surge. I will not be using it much anyway. Maybe for small things if the batteries charge permits. I need to get some type of meter for monitoring the system. There is a faceplate display monitor for the C-35 that I am considering.

    Next, the load. I will be for sure powering a Sun Danzer DCF 225 freezer. At 86 degrees ambient (worst case scenario) it consumes 702 watt hours a day. Sun Danzer says this includes 4 days of no sun. Supposedly, my location's (Gulfport MS) average annual sun is 4.8 hours. I would like to be able to power a Sun Danzer DCR 225 fridge as well but I don't know if the system will do it. The fridge uses about half the energy of the freezer. I know some folks feel AC powered freezers/fridges are best but I am committed to the 12 volt DC approach.

    I do not want to run the batteries down more than 30 percent. I have had Trojan batteries last over 5 years (nearly six) in a golf cart with meticulous charging regimen and maintenance.

    The battery box , charge controller, inverter, and charger will be in the garage. The panels are outside the garage, about 10-12 feet from the battery box. I am thinking of using 8 gauge wire from the panel to controller. Even though it might be overkill, 10-12 feet is cheap enough. Won't it decrease the voltage drop and allow a little bit charging? Would 2 gauge wire be good enough for the battery interconnects or should
    it be heavier?

    I built the panel rack so the tilt could easily be adjusted each month. It does not track, however.

    So I'm getting ready to wire this thing up.

    Anything else you can think of to help me put this together correctly is much appreciated. I'm trying my best not to take the ready, fire, aim approach to this. I did do some calculations prior and think I'm close to having a properly sized system. Your calculations and advice are very helpful. Thanks again for taking the time to respond.

  • BB.BB. Super Moderators, Administrators Posts: 32,235 admin
    If you are not going to power heavy loads with the AC inverter--I would highly suggest the MorningStar 300 Watt 12 volt TSW inverter (with remote on/off, and "search mode"). Very nice for smaller 12 volt system.

    For fusing/breakers--I showed you how you can calculate the branch circuit rating (x 1.25 NEC derating for wiring/fuses/breakers). You can use the NEC (US National Electric Code) to conservatively rate your current vs wire gauge:

    https://lugsdirect.com/WireCurrentAm...ble-301-16.htm

    You can see that the NEC would rate 2 awg at somewhere around 95 to 130 Amps.

    Or you can use a marine chart:

    http://www.boatus.com/boattech/artic...-terminals.asp

    And put something like 178 to 210 Amps through a 2 awg cable (much less conservative).

    Also, you need to calculate the voltage drop... For a 12 volt AC inverter, I would suggest a 0.5 volt maximum drop at maximum rated continuous power for the inverter.

    And I would suggest around 0.05 to 0.10 volt maximum drop from charge controller to battery bank (for fast/accurate battery charging).

    http://www.calculator.net/voltage-drop-calculator.html

    More or less, you need a fuse/breaker for every + wire that leaves the battery bank. And remember fuses/breakers are there to protect the wiring from overheating/catching fire.

    A "special case" for fusing / breakers is when you parallel strings of solar panels--Most of the time, when you put 3 or more panels in parallel, each string should have its own fuse or breaker (protects a shorted string from being feed too much current from the other panels).

    And the fuse vs breaker question--Breakers are very nice because you can also use them as on/off switches...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 32,235 admin
    The Xantrex C35 is getting a bit long in the tooth--But certainly a meter can be helpful.

    Or you can go with a true Battery Monitor like the Tri-Metric or Victron models.

    A good quality Hydrometer is gold standard for flooded cell lead acid batteries (remember to rinse well with distilled water before putting away).

    A very nice debugging tool is an AC/DC current clamp DMM. This one from Sears is good enough for our needs and not very expensive.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • 12Voltian12Voltian Registered Users Posts: 5
    The info charts you provided are very informative but that is assuming you know the current draw. This is where I'm having a problem trying to figure out what fuses and switch ratings I need. I'm more ignorant than you think! I will have 5 100 watt panels wired in parallel so I need a fuse for each? The specs for the panels say the voltage at maximum power point is 18 volts and the current at maximum power point is 5.56 amps. I haven't seen any fuses this small. Why are they all rated at 300 volts, 30 amps etc.? Wouldn't I just need 5 fuses that were greater than 18 volts, 5.56 amps? Do people use car fuses for this? Do they work with 18 volts? Should the fuses be mounted in a box at the panels or could they be mounted on the panel support? I realize they would have to be waterproof. Seems like I will need a box at the panels anyway. Concerning the gauge of the wire from the panels to the controller, it will be carrying 5.56 amps x 5 current, or 27.8 amps, correct?

    How about the fuse between the controller and the battery? I don't understand why you need one.

    Would the fuse between the battery and inverter be based on the inverter? Divide the inverter wattage by 12 volts to get the amps? Do you use the continuous wattage or surge wattage when figuring the amps?

    That Sears meter looks very useful. I have a cheapo hydrometer I use on the golf cart batteries- I will probably get a better one.

    Thanks again. The information links look very useful once I figure out what I'm dealing with.



    P.S. Does the system I am proposing seem balance now that I've stated the intended use and components?
    P. P. S. Is the reason you advise a smaller inverter is because it uses less power from the system? I surely don't need the one that came with the kit. It is huge and heavy.
  • BB.BB. Super Moderators, Administrators Posts: 32,235 admin
    12Voltian wrote: »
    The info charts you provided are very informative but that is assuming you know the current draw.

    Yep--To do this right, you have to know your power needs/current flow. Or you way over design and spend a lot of money doing that. (or under design, and have all sorts of maintenance issues/popped fuses/tripping breakers/inverters that do not supply more than a few hundred watts to the loads, etc.).
    This is where I'm having a problem trying to figure out what fuses and switch ratings I need. I'm more ignorant than you think! I will have 5 100 watt panels wired in parallel so I need a fuse for each? The specs for the panels say the voltage at maximum power point is 18 volts and the current at maximum power point is 5.56 amps. I haven't seen any fuses this small.

    From what I have seen, many of the smaller panels do not have a "series fuse" listed in their documentation. Many times these are panels not really intended to be installed under NEC/Building codes but for small portable power systems and such.

    More or less, you can start with:
    • 100 Watts Pmp / 17.5 volts Vmp = 5.71 Amps Imp
    • 5.71 amps Imp * 1.25 NEC Solar derating = 7.14 amps (estimated) Isc (short circuit current)
    • 7.14 amps * 1.25 NEC derating for wiring/fuses/breakers = 8.9 Amp minimum fuse/breaker/wiring rating
    So, for that panel, I would estimate around 10 Amp design for wiring/fuse rating.

    You can use any fuse/breaker rated for DC voltage > ~21 volts (Voc STD--Voltage open circuit standard test conditions).

    Yes, people do use glass car fuses (many are rated for 32 VDC--Check the specs.). And some fuses/holders have been less than reliable. For example, one poster here (Wayne from NS Canada) had these types of automotive holders "melt down" with just normal current flow:

    Attachment not found.

    For solar--We tend to have many hours of high current flow (solar charging, running pumps, AC inverter with heavy loads) compared to what is done in a car (run a radio, cigarette lighter for 1 minute, toaster for 5 minutes, (suggest running genset at 80% load when running battery charger for multiple hours) etc...

    I highly suggest that you get good components and be conservative in your design.
    Why are they all rated at 300 volts, 30 amps etc.? Wouldn't I just need 5 fuses that were greater than 18 volts, 5.56 amps? Do people use car fuses for this? Do they work with 18 volts? Should the fuses be mounted in a box at the panels or could they be mounted on the panel support? I realize they would have to be waterproof. Seems like I will need a box at the panels anyway. Concerning the gauge of the wire from the panels to the controller, it will be carrying 5.56 amps x 5 current, or 27.8 amps, correct?

    You can get combiner panels that hold fuses or breakers (breakers are nice for debugging, and you don't have to keep spare fuses around--But in general, a well designed system properly installed should not blow fuses).

    http://www.solar-electric.com/installation-parts-and-equipment/midnite/pvarco.html

    And you start to see why we tend to avoid a bunch of 100 Watt panels when 2x 250 Watt panels would work fine (no combiner box needed, higher voltage means smaller awg wiring, use a MPPT charge controller--More expensive controller and less expensive "GT" type panels).

    I suggest that you do several paper designs first and cost them out/figure out their +/- for you first. Otherwise, you may end up with a cardboard box full of parts you cannot really use.

    27.8 amps for votage drop calculation.

    27.8 amps * 1.25 NEC solar derating * 1.25 NEC wiring derating = 43.4 for amps for "NEC" based design

    Yes, derating * deratings gets a bit overkill at times--But that is how the NEC roles.
    How about the fuse between the controller and the battery? I don't understand why you need one.

    If there is a short circuit on the wire leaving the battery bank (connector comes loose, sharp piece of metal grounds the leads, etc.)--The fuse/breaker is to stop the hundreds to 1,000's of Amperes that a large lead acid battery can feed into a dead short.

    Also, if there is a wire failure, FET/Power Transistor failure in the MPPT controller, it can (in theory) short out the battery output too--Again, the fuse/breaker is there to prevent fires (and not really there to "protect" the electronics).
    Would the fuse between the battery and inverter be based on the inverter? Divide the inverter wattage by 12 volts to get the amps? Do you use the continuous wattage or surge wattage when figuring the amps?

    Use the maximum expected wattage (continuous)... A 2,000 Watt inverter and you expect a 1,200 Watt maximum load:
    • 1,200 Watts * 1/0.85 inverter eff * 1/10.5 AC inverter low voltage cutoff * 1.25 NEC wiring+Breaker deratings = 168 Amp minimum branch circuit rating. Round up to 175 amps (standard fuse/breaker size).
    Note we are looking at "worst case" design... A 1/2 discharged battery (11.5 volts) and 0.5 volt drop (1,200 Watts) gives you ~11.0 volts at the inverter. The extra 0.5 volt drop is for surge current/prevent false trips at the inverter due to low input voltage when under load/surge conditions).
    P.S. Does the system I am proposing seem balance now that I've stated the intended use and components?
    P. P. S. Is the reason you advise a smaller inverter is because it uses less power from the system? I surely don't need the one that came with the kit. It is huge and heavy.

    The inverter you have may draw 20-40 Watts just "turned on"...If you only have a few hundred watts of loads (LED lighting, computer/power tools/cell phone charging, etc.)... A 100 Watt load and 20-40 Watts of wasted power is a big deal. The inverter I pointed at uses ~6 watts Tare (turned on), and much less in standby or remote power off.

    Conservation is key to a happy solar power system... You have to watch for "vampire" loads that can significantly waste your precious power (both from solar panels, and from battery bank).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • 12Voltian12Voltian Registered Users Posts: 5
    Bill,
    Thanks so much for your help. I think I'm starting to get it (well, a little more anyway) :-). You are pointing out a lot of the things I did not know (deratings, efficiencies etc.) . Sometimes, you can be so ignorant that you don't even know what questions to ask. I can see there is a lot more to this than just sticking parts together using the manufacturers specs. My system will be strictly stand alone, not connected to anything in the house. I will probably try to follow the NEC guidance as much as practical for longevity and cool running if nothing else. My wire runs are so short that the cost of bigger wire is insignificant. You are correct, Grape Solar did design the small system to run without fuses, breakers etc. And I guess I'll find out if it is balanced as far as panels to batteries to load. I am definitely going to order that hydrometer- it is much better than mine. Thanks for pointing it out. I will hold off ordering the small inverter for now. If I keep the big one turned completely off it won't draw power, right? What is your opinion on the water saver caps? They seem like a good idea, especially in the south. I've found that if I charge the golf cart at night the batteries don't use as much water. The batteries in a solar system will be charging during the day. Maybe the caps will help?
  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    Fusing is very important in any electrical system. Your system is not really 'small' in the sense that theres enough energy stored in 6 golf carts to blow your house up quite adequately, therefore you MUST have fusing of everything that goes into and out of the battery. My advice is forget fuses, get yourself a midnite big baby box (of if you need more room a MNDC), some 150V breakers (these are only 11 bucks each and make things really easy and very safe.).

    As bill said you also need fusing on each pv string, to prevent an array fire, should a single panel short. Midnite combiners are nice there. Youll find as most of us did that these breakers also help service the array, homerun and disconnect. You also need a breaker in and out of the charge controller, and one for the inverter. The writeup in my sig shows how i did it.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • 12Voltian12Voltian Registered Users Posts: 5
    That was perfect Zone Blue. Just what I needed to help finish the wiring. Thanks
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