Need help with wire gauge/fuse amperage

manamongtheruinsmanamongtheruins Registered Users Posts: 4
So I finally got the inverter I think we'll need. Now I need to get another 100 ah 12v battery and the wiring/fuses and I think we'll finally be able to get this going. I have been doing a lot of research on what we need and it's just really confusing to try to find what's relative to our specific components so I figured I'd just try to find someone that knows this stuff and can maybe explain the why and how so I can better understand it.

To start off, let me say what we're trying to power:

The absolute required loads are two small Criterion 5 cu ft chest freezers from Menard's. The thing is, one is going to be overridden with a temp controller and used as a fridge so it shouldn't eat as much watts as the actual freezer one. Aside from that, we might use the batteries for a strip of lights (30 watts) for a few hours a day or a small fan (5 watts) for a few hours a day as well. We run a gas generator for our usual stuff (a/c, internet, projector, Bluetooth speaker, etc) so as far as trying to run something 24 hours a day like a freezer, solar is going to be the way to go so it's gonna be sort of dedicated to that.

Now on to what components we have:

- 2 x 200 watt Grape Solar panels (however 1 got shattered and I'm in the process still of testing it for its efficacy)
- 40a Grape Solar Comet PWM charge controller
- 1 (only 1 right now but the goal is to have 2 and then probably 4 later on but them being $180 each is a bit hard on the budget) Universal Power Group 100ah 12v SLA AGM (UB121000)
- GoWise Power 1500w PSW inverter (PS1005)
- 15' of 10 ga wire with MC4 connectors
- 2 x 20a inline MC4 fuses

Now for my questions/concerns:

I understand that there are four connections we need to make:
Panels - charge controller
Charge controller - batteries
Battery - battery (in parallel)
Battery bank - inverter

So far I'm about 99% sure that I have the panels to charge controller figured out. The panels and controller came as a kit and included the 15 ft of 10 ga wire as well as the 2 20a inline fuses. It said in the manual to cut the cable in half which would be 7 1/2 ft and to hook it all up with the inline fuses on the positives from the panels so I'm hoping that that is correct. The other connections however, I can't figure out. I don't know what wire gauge to go with as well as what fuse amperage to get. As far as wire distance and what not, I plan to have all the components within 5 ft of each other except for the panels to controller which will be the provided 7 1/2' most likely.

Any help or information to finally get this done would be extremely appreciated. This whole pv stuff has been something I've slowly been trying to research for the past year due to money issues and time issues (as always). The wife needs a freezer already and for the past couple years we've been living out of a little cooler having to buy bag after bag of ice especially in the summer (we live off grid). Really eats up the wallet and we also have our first baby on the way so I'm trying to get this figured out! Thanks guys!


  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Welcome to the forum ManAmongTheRuins,

    Can you tell us (nearest major city) your located? Trying to figure out Hours of Sun per day by season, rough temperature ranges (hot--fridge/freezers run longer)...

    The shattered panel--Don't even bother with it... For the most part, even if it works OK now, by the end of summer or when rains hit, the panel/cells will get wet and corrode.

    Generally, I suggest to hold off buying solar equipment until you figure out your loads...  Solar power is quite expensive (as you have found out). And sometimes mixing/matching equipment (on sale, from Craig's list, etc.) does not work out well. I like to do some paper designs first, then figure out the whole system. Much cheaper to toss paper vs "wrong hardware".

    I will take some guesses here... Actual information/numbers on power/energy usage is needed...

    Say you use 12 AWG wiring which is rated for 20 Amps max continuous branch circuit (keep DC wiring short--Going 10's or more feet with low voltage DC current can cause excessive voltage drops).

    For the panels to charge controller... With 1 or 2 panels, you do not need fuses/breakers. With 3 or more panels/strings of panels in parallel, you need a combiner box with one fuse/breaker per string (larger systems).

    You have MC4 20 amp fuses and holders? These are usually for use in the solar array--And your present array is too small to need them.

    Installing one fuse between solar charge controller and the battery bus--That will protect the controller+wiring (fuses/breaker should be installed "close" to the high current source--Which is usually the battery bank in solar DC systems. Install the other 20 amp fuse from battery bus to your DC loads.

    If you have smaller loads (like your 5 watt fan), you can run thinner wire and 1 amp fuse to protect the wire to the fan from overheating/catching fire if their is a short circuit (fuses/breakers protect your wiring, not the attached devices).

    Here is a nice chart for NEC wiring ampacity (simplified chart):

    i.e., 14 AWG wiring is good for 15 amps maximum (with 15 amp fuse/breaker to protect the wiring from shorts).

    Note: adding refrigerators/freezers/AC inverters takes you from a "small" solar power system to a medium sized one pretty quickly--Which is much larger that you seem to have planned here...

    Here is a very quick/rough sizing of what you seem to be asking for...

    Freezer = 215 kWH per year
    215,000 WH per year / 365 days per year = 589 WH per day
    Freezer as refrigerator = ~250 WH per day
    30 Watts lighting * 5 hours per evening = 150 WH per day
    5 Watt fan * 5 hours per evening = 25 WH per day
    1,014 WH per day

    Size of battery bank... 2 days of storage and 50% max planned discharge:
    • 1,014 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max discharge * 1/12 VDC = 398 AH @ 12 VDC battery bank
    2x 6 volt @ 200 AH golf cart batteries in series for 12 volts @ 200 Amps. Add two more batteries (in series) in parallel for a 12 volt @ 400 AH battery bank.

    Suggest for a full time off grid system 10% to 13%+ (to 20/25%) rate of charge minimum:
    • 400 AH * 14.5 volts charging * 1/.77 panel+controller deratings * 0.10 rate of charge = 753 Watt array "minimum" for full time off grid
    And you did not tell us where you are at... For example, if you are in a reasonably sunny area (US South West), perhaps at least 3.0 Hours of Sun per day:
    • 1,014 WH per day * 1/0.52 off grid AC system eff * 1/3.0 hours of sun per day = 650 Watt array "break even" in winter (3 hours per day)
    The 1,500 Watt GoWise inverter--Some bigger issues there... To run it at full 1,500 Watt power--you need around a 600 AH @ 12 volt FLA battery bank... Even a 400 AH @ 12 volt bank is a bit iffy.

    Also, a 1,500 Watt inverter can draw at full power:
    • 1,500 Watts * 1/0.85 AC inverter eff * 1/10.5 battery cutoff = 168 Amps max continuous...
    You need short/heavy cables to run that level of current (do yo have the manual?). Also the connections need to be bolted--Not battery clips.

    And, it would be nice to know the no-load current/power draw... This guy could very easily draw 15 Watts or more... If you run it 24 hours per day (no loads):
    • 15 Watts (tare losses) * 24 Hours per day = 360 WH per day just from AC inverter being turned "on"
    That is probably more energy than your "refrigerator conversion" uses in 24 hours...

    May need to add the inverter to your daily loads..

    I will stop here--I have made a whole bunch of guesses here to show how the basic calculations work... But I really need your actual loads, nearest major city, etc. to give you better answers.

    I would also suggest you stop buying anything for now... You probably need a larger array, larger charge controller (60-80 amps), and I would suggest, at least, 4-6 golf cart batteries (6 volt @ 200 AH). And probably an 800-1,000+ Watt array.

    Golf Cart batteries do require adding distilled water (check levels every month)--But they are the best bank for the buck for smaller systems.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • manamongtheruinsmanamongtheruins Registered Users Posts: 4
    edited July 10 #3
    Thank you for the reply!!
    We are located near Athens, Ohio.
    As far as the shattered panel, we planned on trying to seal it with some kind of waterproof barrier but I haven't figured out exactly what we want to use. I read that polyurethane and a uv resistant wax polish works okay and is relatively cheap. Of course all the really good fixes are expensive and better to just get a new panel instead but if I can in any way, I'd like to salvage it. We live off grid, no running water, etc and I don't make a lot of money so buying a new panel is gonna be difficult.
    As far as the load, I have already figured it out, I forgot to put it in the op. I'm thinking it will be about 50 watts for the both of them together so 1.2 kwh/day. That will be a day or so of storage with two 100 ah batteries at 50% DoD so I plan to try to get 4 batteries altogether eventually.
    Ya! We definitely learned this the hard way, I originally bought an inverter only to find out I bought a modified sine wave which will ruin our sensitive appliances, finally sold it after a few months of trying and had to settle for $50 less than what we originally paid. So yes, we definitely know how it is on that front.
    Hmmm.. I wonder why they sent those 20 amp in line fuses with it. It came as a kit. So you're saying they're not necessary? If we go with the two panels, the 20 a in line fuses should not be used between the panels and the charge controller with the included 10 ga wire?
    So for the controller to battery bank connection you are saying to use a 20 amp fuse, what about the wiring for that connection?
    Next you say to use a 20 amp fuse between the battery bus and DC loads? We won't have any DC loads, it'll go to an inverter to which we will power the freezers. Is that what you meant by that? Sorry I ask for so many specifics, I'm a very detailed person and especially with this, I need to make sure I get all the right information.
    On this next portion you state that fuses/breakers protect your wiring and not the attached devices. Is there anything that can protect your devices or is that not a thing?
    Thank you for the chart! There are a few things I'm not clear on with it though. I'm not understanding the distance part of it. You say 14 AWG is good for 15 amps but how long of a distance can that travel through it before it starts to have loss? Is that distance point at which loss occurs the same for every gauge size? Also, why are there three different temperatures and amperages listed?
    Ah, it seems you came up with a lower number than I did, like I said above, I calculated 1.2 kwh/day.
    Wow! Your calculations here are lining up with exactly what I had as far as the ah needed for storage. That's great!
    Well! I spoke too soon. The panel wattage I'm off on. I was thinking that 400 watts would work... You're saying I'd need at least 650 watts of paneling to break even?
    On the inverter, I don't plan to run it at full power, I just got that in case of upgrades later on but I appreciate the added info for sure.
    Yes, I do have the manual for the inverter. And yes I'm familiar with the phantom residual no load draw sort of thing. We intend to have the freezers load on it 24/7.
    Thanks for all the info! Hopefully I've answered all the stuff you requested as well and hopefully you can help me figure out what wiring gauge and fuse amperage I need for all four connections!
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Solar panels will not under "normal conditions" output more than Isc (short circuit current) under full sun, pointed at sun, on a sparkling clear day, into a dead short.

    Your panels(?):

    Isc = 10.3 amps
    Series Fuse = 20 amps

    If your have 3 parallel strings of panels. The "failure" would be one panel/wiring shorted, and the two other panels feeding into the shorted panel--

    Isc-2 panels = 2 * 10.3 = 20.6 Amps into "shorted panel"

    So, Isc of 2 panels > 20 amp fuse (just barely)... If you only have two panels in parallel, and one gets shorted, then the shorted panel will "see" 10.3 amps from the good panel... And never pop the 20 amp fuse. So, for two panels in parallel, the 20 amp fuse(s) in the array does "nothing useful".

    Don't spend $20+ on some sort of glass/panel coating... It probably will not really last 1 year. The glass is a "structural element" in the panel (the solar cells are almost tissue thin and very fragile). Broken glass, flexing at cracks/shattered zones, failing cells (let alone water/oxygen damage to cells and connections). But if you want to try....

    You might keep an eye out for a good deal on solar panels and solar charge controller (it does have to "match" the panel/solar array configuration--This needs to be designed before buying/installing--But this is for another day. You can get new panels for under $0.50 per Watt... But you do need a new/more expensive MPPT controller (most likely).

    Ok... Lest use 1,200 WH per day. And a fixed array facing south:

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 51° angle from vertical:
    (For best year-round performance)

    You are down to 2.50 hours per day in dead of winter... With refrigerators, freezers, and such that need to run 24x7 (not like optional stuff, vacuum cleaner, filling a water tank during sunny weather, etc.)... It is really a good idea to only plan on using 50% to 65% of predicted harvest (allow for some days of bad weather, overcast, etc.):
    • 1,200 WH per day * 1/0.65 base load fudge factor * 1/0.52 off grid AC system eff * 1/2.50 Hours of Dec Sun) = 1,420 Watt array w/ 65% fudge factor December
    • 1,200 WH per day * 1/0.50 base load fudge factor * 1/0.52 off grid AC system eff * 1/2.50 Hours of Dec Sun) = 1,846 Watt array w/ 50% fudge factor December
    • 1,200 WH per day * 1/0.52 off grid AC system eff * 1/2.50 hours of sun per day (December) = 923 Watt array w/ no fudge factor for December
    A 923 Watt array... Usually means that you will have to cut back on power usage during bad weather and/or run a backup genset at times during Nov thru February or so...

    Solar power is not cheap--And a good bang for the buck are 6 volt @ 200 AH Flooded Cell Lead Acid Batteries. They do need to be checked/distilled water added something like every 1-3 months or so (check every month at least). And they should last 3-5 years. At $100+/- each (4-6 batteries suggested). AGM and other types of batteries generally cost quite a bit more. And they may last longer than "cheap" Golf Cart Batteries (5-7 years). But to get going and have a stable lower cost system... Not a bad place to start.

    Generally, the fuses/breakers are there to protect wiring from getting overheated (overloads, short circuits, etc.).

    Fuses/breakers (Over Current Protection -- OPD) are not usually going to protect a device (AC inverter, charge controller, etc.) from failing--Electronics fail much faster than an OPD can trip. If you get into the "Code" and Design Documentation--There are other secondary protections... You don't want to feed 1,000 Amps from a large FLA battery bank into a DC fan with very heavy copper cable... Yes, the copper cable will not catch fire, but the fan is not designed to take 1,000 Amps and will be a puddle of melted/arching/flaming plastic (the UL Listed Loads will have a maximum current/voltage/etc. that they can be fed... So there is a "real" reason to not "oversize" wiring/OPD to smaller loads.

    You asked how far a 14 AWG cable can carry current for a load... Copper wire is a "resistor" (or has a specific resistance per foot). And how many volts your load can tolerate--You need to look at the current and operating voltage range...

    For example, say you have a 10 Amp load, and I would suggest a maximum 0.5 volt drop for cable resistance. You can look up the resistance of a 14 AWG cable (remember it is round trip or 2x one way trip) will have--Or you can use a simple voltage drop calculator and plug in some numbers... For example this one is pretty easy (and uses 1x way cable length--As always read the instructions):

    Playing with the numbers... 14 AWG wire, 1.0 PF, 12 VDC, 10 Amps (10a*12v=120watts)--Playing with the feet, I get 8 feet from battery bus to load:


    Voltage drop: 0.50
    Voltage drop percentage: 4.15%
    Voltage at the end: 11.5

    If this was 120 VAC... That would be 14 AWG, 1 PF, 120 VAC, 1 Amp (1a*120v=120 Watts) and 3% drop (3.6 volt drop @ 120 VAC typical):

    The answer is 580 Feet to send 120 Watts of power (same as above @ 12 volts) instead at 120 VAC:


    Voltage drop: 3.61
    Voltage drop percentage: 3.01%
    Voltage at the end: 116.39

    A huge difference in sending the same 120 Watts at 12 VDC vs 120 VAC on 14 AWG cable.

    Why different temperature Wiring... That is different insulation, and possibly different installation locations, wet locations, in direct sun, direct burial, etc.... (for example inside house wall vs mounted on roof top in full sun. Higher temperature rated insulation can run "hotter" without melting (and carry more current).

    There are a lot of details in the NEC Electrical Code Book (national electric code)--More than I can address in one post...

    As I keep saying, the details matter with solar and any electrical system.

    For the Freezers, the pumps may take 65 Watts, or they may take 120 Watts--And could take 5x that for a couple seconds starting.

    For a typical 120 VAC refrigeration compressor--Design for upwards of 600 to 1,000 Watts (starting load)... That is a lot of current at 12 VDC:
    • 1,000 Watts * 1/0.85 AC inverter eff * 1/10.5 AC inverter cutoff voltage = 112 Amp surge
    You need short/heavy wiring to manage that amount of current (with 0.5 to 1.0 volt @ 12 VDC drop)...

    If you assume a max continuous load of 300 Watts (both freezers running, lights running), the 12 VDC current would be:
    • 300 Watts * 1/0.85 AC inverter eff  * 1/10.5 volts battery cutoff = 33.6 Amps
    Round up to (roughly) 40 Amp rated wiring/fuse/breaker for AC inverter minimum...

    I am being somewhat conservative here... Trying to ensure that your system works in full sun/summer, as well as during dark and gloomy winter days with a battery at 50% state of charge.

    These "fudge factors" and worst case conditions really push up the system sizing (larger array, heavier cables, larger battery bank, etc.)... But doing it right the first time--You are not struggling a few winters down the road trying to keep the system running.

    None of the above is written in stone... I show my work so you can see the "fudge factors" and assumptions I am making... You may make different assumptions/design choices than I or others here.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • manamongtheruinsmanamongtheruins Registered Users Posts: 4
    Thank you a ton for all the help and info!! You've definitely been the most thorough and helpful of anyone I've come across in the solar power community. 
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    You are very welcome.

    Let us know how you proceed and everything works out for you and your family.

    And, as always, feel free to post more questions as they come up.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • manamongtheruinsmanamongtheruins Registered Users Posts: 4
    BB. said:
    You are very welcome.

    Let us know how you proceed and everything works out for you and your family.

    And, as always, feel free to post more questions as they come up.

    I do have another question if you have time! I got this ( FIXITOK low voltage disconnect and it comes in a 2 pack. I got it because I'm under the impression that due to the nature of the SLA AGM batteries, if I want to discharge to a specific depth to avoid damage, I need to set that with these low voltage disconnects. Assuming I'm on the right track, am I correct that I'll connect it in between the battery bank and the inverter? And also will I need to use 2 gauge wire as well for it? And also do I need to connect one per battery or just one for the battery bank (it will have two batteries so far in the bank) and if I connect one per battery, how does the wiring work for all that. Sorry! I'm always over thinking everything... 
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Interesting idea, however you cannot use this as is. Yes, you would put one of these modules on the DC input to the inverter. But the module is only designed for ~5 to 20 amps maximum (traces on board--5 amps. Relay 20 amps). And your system would need to surge upwards of 112 amps (~1,000 Watts @ 12 volts).

    Or you can purchase an AC inverter with remote on/off input and see about wiring the Amazon module to control the AC inverter that way (save the costs of a large DC relay).

    Lower cost AC inverters may have 10.5 volts as the low voltage cutoff. And you are correct, taking your FLA batteries to 10.5 volts is "dead" and usually shortens the life by a lot (or even will "kill" an older battery bank). The 10.5 Volt battery cutoff voltage is really there to protect the AC inverter and not the battery bank.

    You can get higher end inverters that have 11.5 volt BCO setting... Or even a "smarter" BCO... For example, let the battery get down to 10.5 volts for a short time to support starting surge. And turn off at 11.5 volts for >5 minutes. That is a bit more useful.

    In any case, lead acid battery voltage is not a great estimate of remaining capacity as Voltage vs Capacity varies with temperature, age, and discharge rate of the bank.

    You can get a "true" battery monitor that measures the Amp*Hours in/out (charging/discharging) and use the programmable output to shut down the bank (or set an alarm for you) with an external relay. A couple examples:

    BMS systems are not cheap, and they can drift (they have a reset to 100% function when the battery is fully recharged). And you have to add a good size relay to turn on/off the AC inverter current (or use the remote on/off for the inverter, if present).

    Adding automation to off grid power systems does add complexity and more possible points of confusion/failure.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • roughpointsroughpoints Registered Users Posts: 1
    Difficult to understand the mathematical calculations. However, I am trying to learn them.
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    edited July 18 #10
    Welcome to the forum Roughpoints,

    I try to write the equations like an English sentence... Read from left to right. And I try to use the same "factor" as either * 0.77 or * 1/0.77 (this is an example of accounting for "hot" solar panels loss in Vmp and some normal losses in a charge controller) as "one number" vs using 0.77 in one place and (1/0.77= ) * 1.30 in another place (same value, just the "reciprocal" and having to "magic fudge factor" numbers to explain/remember.

    Please feel free to start another thread/discussion and copy&paste an equation (or set of equations) in the new discussion and we can talk about the math in detail (how to read, how to plug into a calculator).

    This is normal "algebra" -- But I use actual values instead of A/B/Vmp/Vbatt/etc. as variables. I think it is easier to follow instead of:
    • P * 1/E * 1/LVCO = C
    P = Inverter AC output power
    E = Typical inverter efficiency
    LVCO = Inverter programmed low voltage cutoff
    C = Inverter worst cause DC battery bus current for Inverter @ any Power Level

    Vs (in this case, discussing the "worst case conditions" surge DC current an inverter may take when starting a typical North American Refrigerator/Freezer compressor:
    • 1,000 Watts (typical worst case surge for refrigerator AC load) * 1/0.85 AC inverter eff * 1/10.5 AC inverter cutoff voltage = 112 Amp surge
    I guarantee that not everyone follows "my math" the first time through.

    And in these equations, you are welcome to plug in your own numbers/values for your application (maybe 1/0.90 for a 90% efficient AC inverter; 11.5 VDC for LVCO for your inverter, 600 Watts for an "average" refrigerator compressor starting load, etc..).

    Also, many of these "numbers"/"fudge factors" etc. are based on multiple individual factors. For example I use 52% as the end to end efficiency from solar panels to 120 VAC output power... The details being:
    • 81% drop in Vmp from "ideal specification" sheet number to actual "real world" solar panel at operating temperature in full sun on a hot day (Vmp falls as the cells get "hot"--And the marketing numbers are not accurate for real world operation)
    • 95% efficiency for typical MPPT (maximum power point tracking) charge controller efficiency
    • 80% total efficiency for a flooded cell lead acid battery bank
    • 85% for "average" AC inverter efficiency over entire operating range (for example a lightly loaded AC inverter is less efficient vs an inverter loaded at 50% of rated output)
    • Take all the above: 0.81*0.95*0.80*0.85 = 0.52 = 52% overall harvest efficiency from solar panels to 120 VAC power to your loads
    And you can modify for example using Li Ion batteries which are ~98% efficiency:
    • 0.81*0.95*0.98*0.85 = 0.64 = 64% end to end off grid system efficiency for Li Ion battery based system
    Always feel free to ask if something does not make sense to you... I may have made some assumptions that do not apply in your case, or I have been known to make errors/typos/wrong assumptions/guesses.

    The rules of thumbs and fudge factors we use here are not 100% accurate... But we use them to quickly get to a good paper system design quickly and clearly (I show my work and assumptions). They are "conservative" on purpose so that your system does not only work "when new", but 5 years down the road with aging batteries, deep discharge after a couple cloudy days, etc...

    Off grid solar power systems are designed to be "oversize" for your loads. It is easy to make a system work in the middle of a clear/cool/spring day with >5 hours of sun with a few loads. Starting a well pump after two stormy days (no sun), with a 5 year old 1/2 discharged battery bank--Not so easy (or cheap). Solar array, battery bank, wiring, and such needs to be over-sized so that you harvest some power during cloudy weather, and when the sun comes back out... And as you get farther north, you have less sun in the winter and really cannot get away without running a genset during cloudy weather (sun behind hills, behind trees on neighboring property, etc.).

    Take care,

    PS: And sometimes, I just type too much.  :*

    In power, and solar energy--The details matter.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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