Battery Cables for Remote Battery Bank

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System
System Posts: 2,511 admin
in General Solar Power Topics #1
Hello all,

I have a Motorhome presently equipped with 6 Golf Cart Batteries . One Pair is located in the Front engine Compartment, the Other Two Pairs are in a Vented Side Storage Compartment! the Cable Run is aprox 10 Feet and is #6 AWG Stranded Wire! Should I be increasing the size or is this enough to handle the Current Flow? Pairs are joined with#4 AWG Welding Cable!

I'm asking because I Would like to install an Invertor (1750 Watts) fed from the Remote Battery Bank! I had planned to use #4 or #6 to connect since The Invertor will be about 3 to 4 Feet away in another Compartment!

There are 80 Amp Fuses in each Section !

Anyone?
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #2
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    Re: Battery Cables for Remote Battery Bank

    yes, i do believe a change is warranted now. a 1750w inverter could draw as much as 150amps at 12v and #2 would have over a 4% voltage drop based on a 10ft run while #0 would be under a 3% vd. without question the interconnecting battery wires should be between #0 and #00 or better if you can do it. if the inverter is at 24v then go with the #6 you have with #4 interconnecting battery wires for just under a 3% vd. the 80amp fuses will need to be changed too for 150amp at a 12v battery level, but 80amp would suffice at 24v because half of the current would flow. (i'm going on generalities as you will need to follow the recommendations the inverter manufacturer sets forth)
    now if you won't be stressing to the maximum of the inverter very often the smaller wire of the 2 gauges (larger wire gauge number) i mentioned for 12v could be used as your vd precentages will be much lower when drawing 50 or 100amps as compared to the max of 150amps.
    do observe the 50% depth of discharge rule we generally advise so that you will not take away too much from your battery lifespan and make certain to top off your batteries as often as possible also to lengthen your battery lifespan by averting sulphation from taking place.
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #3
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    Re: Battery Cables for Remote Battery Bank

    Previous post revised based on Xantrex ProWatt 1750 Inverter specs (1,500 W continuous). I also revised my calculations from assuming the inverter cables would be run in conduit to being run “free air”.

    Safely and dependable powering a 1,500 W inverter from any battery bank, but especially from a 12 V battery bank, is not a trivial exercise. The NEC doesn’t cover RVs. (Not true... see new post below). But, its guidelines are useful nonetheless. To determine wire size between the batteries and inverter, you need at least three data points: maximum inverter current, proposed cable type, and highest ambient temperature.

    The maximum inverter current is the current drawn by the inverter at full power at its lowest DC operating voltage and including an efficiency factor. Since the 1,500 W inverter’s lowest DC operating voltage is 10 V and it’s 85%-90% efficient, then is rated DC current spec is (1,500 W / 10 V) / 85% = 176.5 A.

    Assuming using THWN-2 cable in free-air (not in conduit) and max ambient temperature of 40 C (0.91 temp correction factor), the NEC would require cable rated at 176.5 A x 125% / 0.91 = 242.4 A. Accordingly, you’ll need at least size 1/0 wire (rated at 260 A at 30 C in free air) and a 250 A breaker rated for continuous DC duty – NOT an AC circuit breaker.

    2/0 cable may well be easier to find than 1/0. 250 A DC breakers can be hard to find and they’re not cheap. Here's a link to a product that might work for you.

    See: http://www.midnitesolar.com/MNDC Instructions.pdf

    The determination of the 1/0 wire and the 250 A breaker appear to be consistent with the ProWatt 1750 manual’s section on “Connecting DC Cables” (page 3-12). In summary, that section recommends 1/0 cable for a wire run of 5 feet each way, and an appropriate fuse rated for 250 A.

    See: http://www.xantrex.com/web/id/15/p/48/pt/10/product.asp
    And: http://www.xantrex.com/web/id/411/docserve.asp

    The manual’s discussion of an AIC (AIR) spec for the fuse/circuit breaker is interesting. CCA and CA specs for GC2 batteries (i.e., T-105’s) are hard to come by. However, Trojan’s AGM GC2 has a CA spec of 1,400 A (higher than the 1,100 A CCA spec), so I’d estimate that a four battery bank arranged in a 2 x 2 configuration (12 V x 450 Ah) would have a CA spec of over 3,000 A.

    The point is that you’ll need a fuse or circuit breaker with a continuous DC rating of 250 A and an AIC (AIR) spec of at least 10K Amps, and perhaps even 20K Amps. The big battery circuit breakers used by Midnite Solar are made by Carlingswitch and are rated at 50K A AIC (AIR) (single pole).

    See: http://magnetic-circuit-breakers.carlingtech.com/magnetic-circuit-breaker__80.asp

    If you’d rather use a fuse instead of the big circuit breaker, here’s a link to an appropriate fuse and fuse holder. A spare fuse might be a good idea.

    See: http://store.solar-electric.com/fb250ampfuwi.html

    I think I got it all right this time… The voltage drop over 10 feet of 1/0 (or 2/0) requires a separate calculation, but it shouldn't be a major concern over the short wire length (10 feet total) and/or under "normal" current values. I recommend against the 80 A fuses.

    You might just want to buy pre-made inverter cables. You may need three cables: One (positive; red) from the batteries to the circuit breaker, one (positive; red) from the CB to the inverter, and one (common; white) from the batteries to the inverter. You may also want to look into buying/using short 2/0 battery cables for all of the battery-to-battery interconnections.

    See: http://www.solarpanelstore.com/solar-power.wire.html

    Installing, wiring, and maintaining your batteries correctly is critical to good performance and long life. Trojan’s battery maintenance guide should be useful. In particular, note the voltages required to properly charge the batteries, and check the diagrams for correct battery wiring.

    http://www.trojanbattery.com/productliterature/pdf/deepcyclemaintenance.pdf

    Finally, it’s unlikely the batteries will ever see these charge voltages from the alternator and/or the charge controller, so you need to invest in a quality multi-stage AC charger for maintaining the batteries when the RV is parked at home.

    See: http://www.xantrex.com/web/id/69/p/241/pt/7/product.asp

    HTH,
    Jim / crewzer
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #4
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    Re: Battery Cables for Remote Battery Bank
    The Invertor will be about 3 to 4 Feet away in another Compartment!

    If you can get the inverter closer, that will :

    1) reduce loss in the wire
    2) reduce the cost of wire

    Have you considered going to a 24V inverter ?
    1750 watts amps draw:
    12V=146A
    24V=73A
    36V=49A
    48V=36A

    Then you need to consider how long this load will be on, more than a couple minutes will completely drain the batteries.
    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

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #5
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    Re: Battery Cables for Remote Battery Bank

    good point on the conduit jim. i ran it as free air at 65 degrees c for a 10ft run. correct me if i'm wrong, but isn't the conduit rating at 90 degrees c? the 80amp fuses i mentioned to him was only a possibility at 24v only and not 12v, just to be clear on that, and that too was not speaking from the manual's recommendations, but only as a generality.
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #6
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    Re: Battery Cables for Remote Battery Bank
    but isn't the conduit rating at 90 degrees c?
    niel,

    I'm not sure I understand your question. :confused: THWN-2 wire such as I suggested is rated for use in an environment of up to 90 C. But, it's free air ampacity is based on a reference 30 C environment. If used in a conduit, its ampacity is reduced based on fill factor.

    For example, #10 THWN-2 at 30 C is rated at 55 A ampacity in free air (NEC Table 310-17), but it's only rated for 40 A ampacity in a conduit with one to three current carrying conductors (NEC Table 360-16).

    HTH,
    Jim / crewzer
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #7
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    Re: Battery Cables for Remote Battery Bank

    you answered me so no need to go further.:D
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #8
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    Re: Battery Cables for Remote Battery Bank

    Correction: RV’s are covered by the NEC in Article 551. The Article applies mostly to safety and AC voltage issues in an RV. “Low voltage” (≤24 V nominal) and automotive vehicle circuits are generally exempted, so that may the source of the “RV’s aren’t covered by the NEC” legend. :blush:


    Note that converters shall be listed for RV use and the installation of the AC side is covered by the NEC {ref section 551.20(B)}. And, inverters shall be listed for RV use and shall be installed in accordance with the terms of the listing (ref section 551.32).

    I don’t believe that this correction changes any of my previous DC calculations or discussions above, as we were discussing a 12 V nominal system. But, these NEC requirements may influence how you decide to proceed with respect to the AC side of your installation

    Ya learn something new every day!

    HTH,
    Jim / crewzer
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