newbie ??/in-line fuse,ground

System
System Posts: 2,511 admin
in Solar Beginners Corner #1
our system consist of.4 64 watt unisolar panels,2 Trojan 6 volt batteries,specialty concepts charge controller,and I am in the process of ordering a 600 watt true sine inverter, my ? is this.Someone has suggested a ann or anl 100 or 125 in-line fuse.I am having trouble locating one and also 100 or 125 amp seems like a lot.True or false. Also do I need a ground if I mount my panels on the roof. If so where does the connection need to be made
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: newbie ??/in-line fuse,ground

    Using nominal calculations (no adjustments for efficiency losses):

    600 Watts / 12 Volts = 50 Amps.

    So yes, a 100 Amp fuse is too large. It should probably be 60 Amp (the electrical experts here will correct if this is wrong): 50 + 20%

    Also, your 256 Watts of panels would be just enough to charge those Trojans if they are the 210 Amp/hr model: 256 Watts / 12 Volts = ~21 Amps or about 10% of the Amp/hr rating (5%-13% is usually recommended).

    Yes, the mounting hardware for your rooftop solar panels should be Earth grounded.

    Now for the usual questions:
    What is your intended usage? Have you calculated your load requirements with a Kill-A-Watt meter? Have you done everything you can in the conservation line? Don't worry: everyone gets asked the same thing! :D
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,637 admin
    #3
    Re: newbie ??/in-line fuse,ground

    Assumptions:

    600 Watt 12 volt inverter
    85% efficient (inverter is not 100% efficient)
    1.25 safety factor (a wire/fuse should be 1.25x the planned current)
    10.5 volts minimum operating voltage (inverters use more current as voltage falls)

    P=I*V

    I=P/V=P * 1/V * 1/inverter eff * 1/1.25 safety factor
    I=600 watts * 1/10.5v * 1/0.85 * 1/1.25 = 84 amps minimum wire/fuse rating

    Round up to next standard AWG and Fuse rating (100 amp minimum should work fine, even 125 as the next standard should be OK). NAWS (our host) has devices rated (fuses and fuse holders) for this current.

    Normally, you will need a disconnect switch to turn off the inverter when not needed, a circuit breaker or marine battery disconnect should work fine. Problem is that doing this stuff "right" can get expensive (couple hundred dollars or more).

    Regarding Cariboocoot's question about power usage...

    4x 64 watt solar panels driving a battery and inverter does not give that much power... Roughly


    4 x 64 watt panels * 4 hours of sun per day * 50% system efficiency = 512 Watt*Hours per day

    Guessing that you are running a 200 watt load:

    512 Watt*Hours * 1/200 watts = ~2.56 hours per "average" day of solar power

    Is this what you need/expected?

    -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
    Re: newbie ??/in-line fuse,ground

    Thanks, Bill! :D

    That's what I was forgetting. The two things wrong with getting old are that you start to repeat yourself and you think you've said something when you haven't and you start to repeat yourself. :p
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  • System2
    System2 Posts: 6,290 admin
    #5
    Re: newbie ??/in-line fuse,ground

    Fuse issue is resolved.My next hurdle is the ground on my panels. They are on the ground right now but I am planning on mounting them on a metal roof. How do I run a ground wire what size wire, how important is it? As for the disconnect,sounds like a great idea. I am trying to keep the expense down.The inverter Samlex 600 watt continuous,1000 peak. idle power draw .085A I am not sure I would need a disconnect?
    As for use with the system :2 laptops,one with external speakers, cell phone chargers.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,637 admin
    #6
    Re: newbie ??/in-line fuse,ground
    logan wrote: »
    The inverter Samlex 600 watt continuous,1000 peak. idle power draw .085A I am not sure I would need a disconnect?

    If I got the correct unit... The spec:
    Input Current at No Load: 850 milliamps

    So, this is 0.85 amps... Assume 24 hours per day (not turned off/disconnected) at 12 volts:

    0.85 amps * 12 volts * 24 hours = 244.8 Watt*Hours per day

    Assuming an average of 512 Watt*Hours per day of solar power:

    244.8WH/512WH = 0.48 = 48% of daily power would be wasted if inverter is on "standby"...

    This model of inverter does include a DC Power switch (PDF Manual download). However, it is not a "disconnect" but just an inhibit switch that stops the inverter from inverting:
    The on / off switch (2) on the front panel of the inverter is used to switch on and switch off the inverter. This switch operates a low power control circuitry which in turn controls all the high power circuitry.
    • CAUTION! Please note that this switch is not switching the high power battery input circuit. Parts of the DC side circuit will still be alive even when the switch is in the off position. Hence, disconnect the DC and AC sides before working on any circuits connected to the inverter

    So, while this should reduce the "off current" to near zero--it does not open the electrical circuit inside the inverter... So, if you are doing work on the wiring before the inverter--if you mess up (such as reverse +/-), you can still damage the inverter even though it is "off".

    There is also an optional module where you can connect an 18' cable to the inverter and turn it on/off remotely (may be handy for your application)... But in any case, you only want to have the inverter on and running when you are using AC loads--and off at other times to save power.

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