What size fuse should I attach to the positive terminals of my AGM battery bank?

Matty
Matty Registered Users Posts: 36 ✭✭
in Solar Beginners Corner #1
I am trying to determine what size fuse I should attach to the positive terminals of the batteries in my AGM battery bank. I am confused as to whether the selection of amperage is related to the gauge of wiring used to connect the batteries, the size of the load that is to be placed on the batteries, the size of the current that would be drawn by inadvertently connecting the positive and negative terminals with a conductive material or a combination of some or all of these factors?
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #2
    The value of the over current protection device (OCP) is directly related to the capacity of the conductors it is protecting in the event of a short circuit event, not the anticipated load. Batteries can supply enormous amounts of current, so the fuse/breaker will limit the current so as not to make the conductors themselves become the fuse. The value of the OCP can naturally be less, but never more than the conductors rating.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
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  • Matty
    Matty Registered Users Posts: 36 ✭✭
    #3
    mcgivor said:
    The value of the over current protection device (OCP) is directly related to the capacity of the conductors it is protecting in the event of a short circuit event, not the anticipated load. Batteries can supply enormous amounts of current, so the fuse/breaker will limit the current so as not to make the conductors themselves become the fuse. The value of the OCP can naturally be less, but never more than the conductors rating.
    How can one know in advance what conductor might cause a short circuit? A shifter used to tighten a terminal could inadvertently be left touching the two terminals. It's not always known when commissioning a solar battery system what is going to be plugged into it in the future. How is the selection of fuse size determined? What is a safe bet size to select?
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  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,113 ✭✭✭✭✭
    #4
    Without protective covers on the terminals there's really nothing to stop a dropped non insulated tool from turning into a red hot puddle of molten metal if accidently dropped across battery terminals. It's a good idea to use insulated tools in this situation.  There's a product called Plastidip, I think, that can be used to insulate the handles of wrenches. As already mentioned by mcgivor the fusing/breaker is sized to protect the conductors its connected to. The size of the conductors in the case of an inverter for example, would be recommended by the inverter mfg.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #5
    The use of tables is generally a good starting point, wether it be IEE regulations for the UK, or NEC for the U.S., they are all very similar, depending on insulation type, ambient temperature and so forth. The values are usually pretty conservative but it is wise to follow guidelines as there is no way of knowing how a short circuit may occur, by shifter I would assume British, that would be a worst case scenario, but this would illiterate what is possible. My advice, would be to follow the guidelines provided within the regulations relevant to the location the installation.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #6
    You are in Australia, and I do not know your electrical codes and wire designations... Here is an example of US codes and how we would size a system.

    Say your maximum load current is planned to be 100 Amps continuous (many minutes to hours of continuous usage).

    In the US, I use a 1.25 derating factor (our fuses/breakers are generally rated to break, eventually, at 100% rated current, and not breaker at 80% of rated current):
    • 100 amps * 1.25 NEC derating = 125 Amp minimum rated branch circuit (wiring and breaker/fuse)
    Now, here is the NEC (US/North American National Electric Code) example (actual code is "more complicated):

    https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

    Wire AWG (American Wire Gauge) ranges from 2 AWG (high temperature rated insulation) to 1/0 (heavier thickness wire, lower rated temperature insulation).

    Or you can use the Marine Recommended ratings (note that SAE gauge is slightly smaller than AWG gauge):

    https://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity

    Note that (very roughly) the Marine rating is 2x that of our NEC current capacity... I highly recommend that you use the conservative NEC ratings and not use the less conservative Marine ratings.

    Also, on that above link, is an example of wire length vs AWG vs current... You want heavy/short wire runs to keep voltage drops low.

    For 12 volt battery systems, I suggest 0.5 volt maximum voltage drop (you may need heavier wire for longer runs--You can "up rate" the fuse/breaker because you use heavier cable if you wish, or not--Your choice).

    Circuit breakers are very nice... Both for protection and to use as on On/Off switch.

    Good quality fuses are not cheap (as you get higher current ratings) and the holders are not cheap either (plus you need to keep a couple spares on hand if you pop a fuse while out camping).

    However, fuses can be nice for tight spaces... BlueSea makes a very nice/small fuse holder:

    https://www.bluesea.com/products/category/16/72/Fuse_Blocks/Terminal_Fuse_Blocks

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #7
    I like the blue seas battery terminal fuses (dual is handy, one for loads  200A , one for charger 60A )


    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
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