Wire sizing for battery interconnecting cables - 2/0 or 4/0

Hairfarm
Hairfarm Solar Expert Posts: 225 ✭✭✭
in PV Installers Forum - NEC, Wiring, Installation #1
My battery bank will be 12VDC. Six, 6-volt 225ah batteries wired for 12 volts @ 675ah. My inverter is a 2000W Prosine 2.0.

The formula I'm using is 2000w / 12VDC = 166A. 2/0 should be able to handle that. That said, should I use 2/0 or 4/0 for wiring the batteries together? Am I missing something?

FYI, I'll be using 4/0 wire from the battery bank to the inverter.

thanks,
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #2
    For one or two parallel strings, I would use full size cable for batteries. For three our more parallel strings you could reduce by 1/2 cross sectional area (+3 awg gauges smaller).

    Again, my personal opinion.

    Going to fuse each string? (Three or more parallel strings).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Hairfarm
    Hairfarm Solar Expert Posts: 225 ✭✭✭
    #3
    Bill, I'll be using three parallel strings.
    For three our more parallel strings you could reduce by 1/2 cross sectional area (+3 awg gauges smaller).
    Are you saying that 2/0 will be acceptable for three or more parallel strings? Keep in mind you're dealing with first-rate rookie here un-versed in PV vernacular :confused:

    I hadn't considered fusing the battery cables, but it wouldn't be a bad idea.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #4
    Well--If you want to put me on the hot seat... First the inverter calculation:
    • 2,000 Watt continuous * 1/010.5 volts cutoff * 1/0.85 AC Inverter eff * 1.25 NEC derating for wire/breakers = 280 Amp branch circuit rating for full load continuous
    Using the NEC table (without the rest of the NEC temperature, conduit fill, etc. derating calculations):

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

    Between 250 and 400 cmil wire (or paralleled smaller cables).

    If you use marine ratings:

    http://www.boatus.com/boattech/articles/choosing-cables-and-terminals.asp

    1/0 or 2/0 cable minimum...

    So--If you use Marine wiring standards, 2/0 would be OK for main or battery string wiring... You should double check the voltage drop using a voltage drop calculator... Recommend you try for 0.5 volt drop maximum for Inverter and 0.1 to 0.05 for charging.

    http://www.calculator.net/voltage-drop-calculator.html

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Hairfarm
    Hairfarm Solar Expert Posts: 225 ✭✭✭
    #5

    Thanks for your great info Bill. I will keep this in mind when setting up my battery system.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #6
    I think in a previous thread you spoke of this.

    What might be missing is voltage drop over distance. I believe Prosine will have proper sizing in there manual for different distances. at higher amps voltage drop can be substantial particularly for low voltage systems.
    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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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #7
    Hairfarm wrote: »
    ......166A. 2/0 should be able to handle that.....,

    But can a string of 225A batteries (x3) handle that for very long ? 55A per string ? IF it's evenly split amongst the batteries. More than a couple minutes, and the batteries are going to start to sag under a 50a load.

    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 ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

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