Fuse size for small solar system

Options
bratan
bratan Solar Expert Posts: 32 ✭
in Solar Beginners Corner #1
Hi,

I'm looking for advise on what size fuse I will need for this little solar system I'm building. It consists of 100 Watt solar panel, 30A PWM charge controller, 125Ah AGM battery and 600 Watt pure sine wave inverter. It will be located in my shed, and will power backyard LED lights (50Watt to start, but I might expand it). I know 600 Watt inverter is probably overkill, but I might expand system and also use it to power additional devices (i.e. laptop, networking equipment) during power outage.
I want to add an inline fuse between inverter and battery (wire is 4 AWG) but not sure what size fuse I need. 60 Amps?
Also i'll probably need a fuse(s) between inverter and light(s) in case of short circuit, or it's not necessary?
· Share on Twitter

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Options
    Re: Fuse size for small solar system

    Welcome to the forum.

    You don't need a fuse on the solar panel at all. You would if there were more than two in parallel, then each one should be protected according to its maximum series fuse rating.

    You should have a fuse on the charge controller output to protect against a short in it which could cause the battery to dump current through those wires. The output of a single 100 Watt panel is <10 Amps, but if you are planning to expand I suggest you ramp the circuit up to handle the 30 Amp controller's full output to start with. That would mean 8 AWG wire and a 50 Amp fuse.

    You definitely need one between the battery and inverter. A 600 Watt inverter can draw 60 Amps from 12 Volt easily. It is best to check the instructions for the inverter to size wires & circuit protection. It may suggest 100 Amps (4 AWG will take up to that much). Much depends on the continuous current rating and minimum Voltage. For example a 12 Volt system often cuts off at 10.5 Volts, and at 600 Watts that would be 57 Amps. You may choose to go lower than this current rating, but if the demand goes above that ... well a circuit breaker is a good idea because having to replace fuses from nuisance trips is annoying.

    Normally you would put a fuse or breaker on the output of an inverter, but 99% of the time the inverter will fault from over-current before the fuse blows/breaker trips. 600 Watts @ 120 VAC is 5 Amps, and that's not much of a worry on wiring that's typically 14 AWG and capable of 3X that current continuously. Morningstar, for example, recommends a 3 Amp fuse on their 300 Watt inverter output, but most people just don't bother.
    · Share on Twitter
  • bratan
    bratan Solar Expert Posts: 32 ✭
    #3
    Options
    Re: Fuse size for small solar system

    Wow, thank you so much! Great information! BTW inverter's manual suggest 6AWG wire, but I went ahead with 4 AWG just in case :)
    · Share on Twitter
  • Plowman
    Plowman Solar Expert Posts: 203 ✭✭✭✭✭
    #4
    Options
    Re: Fuse size for small solar system

    I didn't bother to add circuit protection to my inverter's output, for the reasons Cariboocoot outlined. In my experience the inverter will shut off due to overload before there's a problem with too many amps going through the inverter's output wires. I've overloaded my little 150W inverter on a number of occasions and it dutifully shut down.

    I do have a fuse on my battery-to-inverter wires. The 400W inverter I'm currently using has internal blade fuses, but they require soldering to replace (already done it once when I accidentally reversed polarity, would prefer not to do it again). I plan on replacing the fuse with a circuit breaker, will probably go with a Midnite 50A. I'm running 6 AGW from my battery bank to the inverter (my loads are low).

    FYI, your panel to battery ratio is a bit low. 100W panel (assuming 5.3A Imp) for a 125Ah battery = 4.2% charge rate. 5% is generally considered the minimum, 10-13% is best for batteries regularly discharged. I'd add another panel or two.
    · Share on Twitter
  • bratan
    bratan Solar Expert Posts: 32 ✭
    #5
    Options
    Re: Fuse size for small solar system
    Plowman wrote: »
    I didn't bother to add circuit protection to my inverter's output, for the reasons Cariboocoot outlined. In my experience the inverter will shut off due to overload before there's a problem with too many amps going through the inverter's output wires. I've overloaded my little 150W inverter on a number of occasions and it dutifully shut down.

    I do have a fuse on my battery-to-inverter wires. The 400W inverter I'm currently using has internal blade fuses, but they require soldering to replace (already done it once when I accidentally reversed polarity, would prefer not to do it again). I plan on replacing the fuse with a circuit breaker, will probably go with a Midnite 50A. I'm running 6 AGW from my battery bank to the inverter (my loads are low).

    FYI, your panel to battery ratio is a bit low. 100W panel (assuming 5.3A Imp) for a 125Ah battery = 4.2% charge rate. 5% is generally considered the minimum, 10-13% is best for batteries regularly discharged. I'd add another panel or two.
    My inverter also has internal fuses which are hard to replace :)
    You probably right about ratio. It's even worse because shed is in the shaded area, so it only gets direct sunlight until 11 am. But load is not too much. It supposed to power 50watt led flood light for several hours at night... I might get second 100w panel at some point tho, they are not too expensive :)
    · Share on Twitter
  • ILFE
    ILFE Solar Expert Posts: 364 ✭✭
    #6
    Options
    Re: Fuse size for small solar system
    bratan wrote: »
    My inverter also has internal fuses which are hard to replace :)
    You probably right about ratio. It's even worse because shed is in the shaded area, so it only gets direct sunlight until 11 am. But load is not too much. It supposed to power 50watt led flood light for several hours at night... I might get second 100w panel at some point tho, they are not too expensive :)

    Brother, if you are running a 125ah battery, a single 100 watts solar panel, and only get good light until 11am, you are waaaaaaaayyy under panelled. You need enough panels to get your battery a full charge by 11am daily. And, you have to take into account your load. A "second 100w panel at some point" is no where enough, in time or wattage.

    You need, as has been previously stated in this thread, about 10% of your ampere rating in panel amperes, to charge your battery through a typical day. (I am estimating ~5 hours of good light.) That would be 12.5 amperes, or 3 - 100 watts panels. (I am taking in consideration system efficiency - or inefficiency, whichever way you wish to look at it.) If you want that to happen by 11am, just off the top of my head, you will have to double that to 600 watts worth of array.
    Paul
    · Share on Twitter
  • bratan
    bratan Solar Expert Posts: 32 ✭
    #7
    Options
    Re: Fuse size for small solar system

    You probably right, it's an experiment anyway :)
    But keep in mind, I do not plan to drain battery much, so solar will only need to top it off. Also after 11 AM there's still sun, just not always direct (but with leaves falling off more sun will be able to get thru). I was still getting about .5 Amps off it in the total shade...

    I used online battery calculator. If my load is 50W @ 110V (which is about .5 Amp) running for about 3 hours, I will need 3Ah battery. So 125Ah battery should barely discharge at such usage, right? :)
    · Share on Twitter
  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #8
    Options
    Re: Fuse size for small solar system

    Oops--The issue of Amps at 12 VDC vs 120 VAC...
    • 50 watts / 110 VAC = 0.45 amps
    • 50 watts / 12 VDC = 4.2 amps
    And the "real current" (with inverter losses):
    • 50 Watts * 1/0.85 inverter eff * 1/12 volts = 4.9 amps average
    • 4.9 amps * 3 hours = 14.7 AH used (assuming inverter is turned off when not used)
    • 14.7 AH / 125 AH = 0.12 = 12% of battery capacity
    You are using a reasonable amount of battery power.

    If you assume something like 2 hours of (1/2 day of 4 hours full sun equivalent) sun, roughly you would need:
    • 50 Watts * 3 hours * 1/0.52 system eff * 1/2 hours of sun per day = 144 Watts of panels minimum for 9 months of the year (guessing)
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
    · Share on Twitter

Categories