What type of fuse?

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System
System Posts: 2,511 admin
in Solar Beginners Corner #1
I'm getting the pieces together to put together my first solar system. It will be primarily used for emergency back up. The main item using power will be a sump pump. The label is missing off of the pump so I don't know how much power it will pull. It's either a 1/4 or 1/3 hp. I'm guessing it is in the 6-8 amp range. I currently have:

HF 45w panels (3x15w)
Sunforce 60w panels (4x15w)
Xantrex Xpower 1750plus inverter
1/0 wire
Used Trojan battery #409392. It says 75amps for 272 minutes on the end. I can't find it on Trojans website.

So what should I use between the battery and the inverter. I was looking on NAWS website for a class T fuse and the smallest I could find was 110 amps. I'm sure my math is off but I was thinking of using a 90-100 amp fuse.

While I have your attention. The Sunforce panels came with a 7amp charge controller. Is there a chance I could use that for now with my panels? I know my panels will put out over 7amps if they put out the full rated power of the panels. If not would only using 6 panels (90w) get me close enough to not damage the controller?

Thanks
Daren
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #2
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    Re: What type of fuse?

    8 amps @ 120 vac is ~960 watts (Not counting starting amperage!)

    That 8 amps at 12 vdc is ~ 80 amps! (adding that the wattage would dictate the higher current and might read better if said like, that 8amps from the 120vac would translate to ~80 amps at 12vdc as wattage would dictate not including extra for losses. at first i thought you did a math error tony, but inserted this for clarity. niel)

    If you have a 75 ah battery, and wish to discharge no more than 50% that leaves 37.5 ah Drawing 80 amps will give you less than 2 hours until the battery is 50% discharged. That said, an 80 amp draw on a 75 ah battery will probably drop the voltage (especially starting) such that the inverter will low voltage shut down!

    Get a Kill-a watt meter to actually measure the draw on the pump.

    All that said, your 100 watts of PV would be LUCKY to produce 6 amps on a perfect day. ( I wouldn't worry about the 7 amp controller, unless it is very cold where you live) These HF/Northern PV are notorious for putting out less than name plate.

    In the real world if you take that 100 watts divide by two to account for total system loses, then multiply that number by the number of hours of good sun you can expect (seldom more than four, despite ones illusion, on average!) So 100/2*4=200 watt hours out the inverter per day on average. Enough to run that 960 watt pumo for ~15 minutes! You need to do your math and calculate your loads.

    Tony
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #3
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    Re: What type of fuse?

    I would really suggest a Kill-a-Watt meter to measure your AC loads and their Watt*hours / kWH usage of the various devices. Labels are not accurate (tend to over estimate power usage) and it is difficult to estimate kWH usage on cycling appliances (pumps, fridge, etc.).

    Solar RE systems tend to be expensive and you don't want to over or under size the for your loads.

    The size of the wiring to your inverter depends both on the loads you want to power and the maximum size of the inverter. Assuming 85% efficient 1,750 watt inverter on 12 volt battery with 10.5 amp cutoff and 1.25 NEC safety factor:
    • 1,750 watts * 1/10.5 volts * 1/0.85 eff inverter * 1.25 = 245 amp minimum circuit
    As Tony says--that is a lot for a an average single battery.

    Regarding the Harbor Freight panels--They are OK to experiment with--but very few people ever get near the marked rated power from them (at least from what has been posted here).

    Would a 12 VDC marine/boat sump pump work better for you application here? Just the idling/ current of the inverter will use all of the power, and more, that you collect from your solar panels.

    In the end, you really need to start with your loads and other requirements, then design the system. Otherwise, you probably will be very disappointed in solar power.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Steve961
    Steve961 Solar Expert Posts: 93 ✭✭✭✭
    #4
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    Re: What type of fuse?

    Daren:

    When you finally figure out what size fuse you need, you might want to consider one of the Blue Sea Systems terminal fuses. They are reasonably priced, and easy to install by just bolting it directly to the battery. Fuses range in size from 30A to 300A, with 8 different sizes between 30A and 100A.

    For a simple system I think these are a great option.

    http://www.blueheronmarine.com/Blue-Sea-5191-Terminal-Fuse-Block-5551

    5191.jpg
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