Inverter to battery bank cable

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luqman
luqman Registered Users Posts: 14 ✭✭
edited June 2021 in Solar Beginners Corner #1
Hello, If anyone could support me in sizing the cable from the inverter to the battery bank.

My inverter is 5KW and my battery bank is an 840 AH forklift battery at 48V, Also fuse size.

regards
luqman

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,442 admin
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    How long will the battery bus to inverter be?

    A conservative sizing:
    • 5,000 Watts * 1/0.85 inverter eff * 1/42 volts low battery cutoff = 140 Amps max continuous
    • 140 Amps * 1.25 NEC derating for fuses/breakers/wiring if continuous current (vs minutes at 5 kW) = 175 Amp Branch circuit breaker/wiring derated
    Looking at the NEC wiring table--Again Conservative:

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

    Depending on insulation type (temperature rating) ~ 2/0 cable would be nice (and safe).

    Then there is sizing the cable for the cable run length (this website uses "one way" cable run lengths). Suggest a maximum of 2 volts wiring drop for 48 volts from inverter to battery bank/bus. Ask if 2/0, 2 volt drop, 5 feet for 2/0 cable--What is voltage drop:

    https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=6&necconduit=steel&necpf=0.85&material=copper&wiresize=0.2557&resistance=1.2&resistanceunit=okm&voltage=48&phase=dc&noofconductor=1&distance=5&distanceunit=feet&amperes=140&x=57&y=23&ctype=size

    Voltage drop: 0.11
    Voltage drop percentage: 0.23%
    Voltage at the end: 47.89

    2/0 is very low voltage drop @ 140 amps -- So perfectly OK in terms of voltage drop.

    Remember you have some wiring to interconnect batteries or cells to the bus bar. So a bit of extra length there.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • luqman
    luqman Registered Users Posts: 14 ✭✭
    edited June 2021 #3
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    @BB.
    Hi bill, the distance from the battery to the inverter will be about 2-3 feet
  • BB.
    BB. Super Moderators, Administrators Posts: 33,442 admin
    edited June 2021 #4
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    Even better (shorter length). Keep +/- wires bundled together (if they loop separately, they act more like an antenna and can better broadcast electrical noise and "receive" lightning energy).

    Just to give you a "less conservative" wire sizing chart:

    https://www.boatus.com/expert-advice/expert-advice-archive/2020/february/choose-the-correct-wire-size
    Temperature Rating chart

    You could go "smaller" AWG wire size using the boating/marine standards (even down to 2 SAE Gauge) (also based on insulation temperature rating).

    Note, our/my suggested max voltage drop is much less from charger to battery bus... On the order of 0.2 to 0.4 volt max for 48 volt battery bus (0.05-0.10 for 12 volt and 0.1 to 0.2 for 24 volt buses). You want "accurate" battery voltages for accurate/fastest charging.

    And the derating of fuses/breakers... In North America, generally fuses and breakers are rated to "not trip" at 0.80 of rated load, and "eventually trip" at 1.00 of rated load... So, if you have a chance for 5kWatt loads (AC loads, or even AC to DC charging), the 1.25 (or 1/0.80=1.25) NEC derating for continuous current will help prevent "false trips" of breakers and fuses during sustained high currents.

    Regarding fuses and breakers--There are differences between AC rating and DC ratings for switches/fuses/breakers... Generally DC current (over ~12 volts) is much harder to "interrupt" vs AC current (i.e., arcs sustain very nicely with DC currents)--So always make sure the switches/fuses/breakers are rated for the AC vs DC voltages you want to "control".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • luqman
    luqman Registered Users Posts: 14 ✭✭
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    @BB. thank you very much, you have really helped me