Do I need extra fuses on multiply runs off a busbar?

MontyYoung
MontyYoung Registered Users Posts: 12 ✭✭
in Solar Beginners Corner #1
Here is a quick picture of my small camper setup. Only one charger runs at a time & not both.
Just two busbars running off my 150AH AGM battery, from the busbars there are two feeds that connect to my AC & mppt charger.
All the wiring is 4AWG to avoid voltage drop. The total wire length from battery terminals to chargers is about 1.5M.

So do I need extra fuses between the busbars & chargers? Or is the one enough? My thinking is, it's all like one big wire so no. But I'm not 100% sure if someone could please advise me. 

Thanks
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #2
    It seem to me you should have OCP on the chargers as well. 

    A short across the buss bars, for example would open the battery fuse, but chargers could continue feeding the short absent fuse/breaker protection on those circuits. 

    The chargers should have a max OCP size specification.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #3
    Basically, place fuses or circuit breakers where the red wires (code is something like 18" max to bus bar) leave the bus bar. The idea is to make sure that any short circuit/over current will pop the fuse/breaker before it melts the the wire insulation/starts a fire.

    Nominally, I like to derate the loads or uprate the wiring and OCP by 1.25x (or 1/0.80 = 1.25). For example, a 50 amp DC output battery charger could output 50 amps for a few hours (bringing the battery bank back to >~80% state of charge). NEC is fairly conservative, but for applications like AC inverters and DC battery banks (as well as the AC connection to a battery charger), it is certainly possible to get hours of "100%" current output and the uprating of the wiring will keep things cooler and lessen the possibility of false trips for your fuses/breakers (typically in North America, fuses and breakers rated for NEC will not blow at 80% or less current and will blow at 100%+ current--Although that make take 1/2 an hour to several hours to happen--Fuses and breakers are generally not "that accurate").
    • 50 Amps * 1.25 NEC derating = 62.5 Amps rated branch circuit (round up to next standard fuse/breaker/wire size)
    And you can use this simplified NEC wire rating (real chart is more complicated; temperature, conduit fill; etc.):

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

    Generally oversizing also means heavier wiring and less losses due to wire heating.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    edited August 2019 #4
    Depending on circumstances, it is not unreasonable to have more fuses than less.  Some conductors that can be potentially energized from both ends, need fuses on both.  For example a inverter/charger can send power in as well as out.  In short, every run of wire needs to be fused (Positive side on DC, hot on AC), and fuses for the smallest wire size in the circuit.  Tony
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #5
    I also forgot to add--Even though good quality fuses and holder work well... Circuit breakers combine both a safety function and a manual on/off switch. Frequently, it is neat to have that switch (shutting down system for season, debugging, etc.). And the larger quality fuses/holders are not cheap either.

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