Fuse or circuit breaker size

Tradiscantia
Tradiscantia Solar Expert Posts: 35 ✭✭
in Solar Beginners Corner #1
Planning the wiring from the two 245 watt solar panels to the Tristar 45.

My calculations: add the max current since they'll be wired in parallel 8.18 * 2 = 16,36
multiply by 125% 16.36*1.25= 20.45 This means minimum circuit breaker would be a 21 amp double pole

Here's the problem - a Mr. KnowItAll neighbor, built a nuclear power plant which is his claim to fame, insists to my husband he should use ONLY a 15 amp becuase (a) solar never gives you maximum output and (b) you want to trip the breaker if you get a short. Is he full of S*$#

I argue that since the Tristar 45 can handle 45 amps, we might as well install a circult breaker large enough to handle a third panel if we decide to add it one in the future. . At the least we should not limit the amperage to the charge controller when we want as much current from those panels as we can get.

Trusting one of you experienced guys can make this clear.

BTW, for the lines to the batteries, I not sure how we get the maximum amps. From the batteries to the 800 watt inverter we are planning 80 amp max with a 100 amp breaker or fuse.
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  • Tradiscantia
    Tradiscantia Solar Expert Posts: 35 ✭✭
    #2
    Re: Fuse or circuit breaker size

    Addition, I read some other threads and learned that the breaker or fuse from the controller to the batteries should be 60 amp. So I just need to know about the one from the panels and whether I should stuff wax in my husband's ears when he talks to Mr. KnowItAll.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #3
    Re: Fuse or circuit breaker size

    Between the panels and the charge controller, you need nothing now, if you add a third panel, you'll likely need a fuse or breaker on each one.

    Between the charge controller and the battery, I'd fuse/breaker(and have) for the capacity of the charge controller.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Fuse or circuit breaker size

    Two panels = no fuse or breaker required at all.

    If you're putting in a breaker as a disconnect it needs to be able to handle the Isc of both panels. Since panels are current sources not Voltage sources they can indeed produce their full Imp and will always try to do so. A 15 Amp breaker is likely to trip a lot.

    Mr. I-Built-A-Power-Plant (all by himself? WOW!) doesn't know what he's talking about. The NEC requirement for such a circuit would be Isc * 1.25 * 1.25, round up. Or 30 Amps. (A double pole 15 is not equivalent to 30 Amp, btw).
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  • Tradiscantia
    Tradiscantia Solar Expert Posts: 35 ✭✭
    #5
    Re: Fuse or circuit breaker size

    Thanks Cariboocoot and Photowit. So nice to know I'm actually understanding and intuiting that the guy with the 40ft rig, generator, and a $48 controller is not to be trusted!

    Happy enough to forgo circuit breaker from panel to controller, but why does the Tristar MPPT operator's manual p. 27 write" Circuit breakers or fuses must be installe din both the battery and solar circuits." Is this because the device is usually used for a much greater load?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Fuse or circuit breaker size

    It's because once you go above two parallel panel connections you have to put circuit protection on each. For a 45 Amp controller, it is very easy to have a lot of panel: 45 Amps * 48 Volts = 2160 Watts. That could be made up of all sorts of different configurations. So for the various arrays that might be connected to it "no fuses required" would the exception rather than the rule. Since there's no reason you can not put circuit protection on just two panels the manual goes for the easy explanation: "properly fuse everything".
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #7
    Re: Fuse or circuit breaker size

    As 'Coot was eluding to earlier, if you have a cutoff breaker between the charge controller and the panels you must breaker for the incoming current, so with 3 or more multiple strings or panels, it's typical to find a combiner box near the panels(with fuses or breakers for each) then a single line coming to the charge controller and a breaker for the entire current as a shutoff for the array/panels.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,622 admin
    #8
    Re: Fuse or circuit breaker size

    And remember that all standard fuses and breaker should be up rated by 1/0.80 (or x1.25). The are designed to not trip at 80% of rating and will trip at 100% of rated current (may take hours to trip, but they should eventually trip).

    So, if you have a 45 amp MPPT charge controller then 45a*1.25=56.25amp minimum wiring/breaker (round up to 60 amp next standard size)

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