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Max length of #2 Cable

Discussion in 'Home Power and Microgeneration' started by Rob, Oct 1, 2004.

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  1. Rob

    Rob Guest

    Hi all. I operate a CAA roadside assistance truck. (AAA in the
    states).
    Anyway, one of my tasks is to boost cars. The cables in my truck are
    approx.
    15 feet long, they are attached to 2 batts in the rear of the truck,
    these batts are
    charged through an isolator so as to be re-charged from the alt in the
    truck. 25-30 foot
    cables (#2 gauge) would make life better for me. Also, as they are now
    I have to
    back the truck halfway into my garage so as to attach this set up to my
    home
    batts which power my inverter during power outages. (Helps charge the
    home
    system, and adds a little more power to the mix).
    My question is, can I replace my 15 footers with 25-30 feet (approx.
    9-10 meters)
    of #2 wire and still get 250 -300 amps out at the clamp ends. And yes
    before you
    ask, we are talking 12 volts. I'd prefer not to go with #1 wire because
    of the extra
    weight, and physical size. Thanks all.
    Rob
     
  2. Well, since the ampacity of #2 wire is only 100 amps
    http://xtronics.com/reference/wire_gauge-ampacity.htm probably not.
    However, 60 feet should only have about 12 milliohms resistance, which
    will drop a volt and change at 100 amps, so for jumping a car and
    charging it's battery you'll probably be OK. Just let it charge for a
    minute or two before trying to crank it over.
     
  3. Ecnerwal

    Ecnerwal Guest

    2AWG copper is 0.156 milliohms per foot. V=IR.

    You presently have (as I read your post) 2 15 foot cables, for a total
    of 30 feet, for a total of 4.68 milliohms, or 0.00468 ohms. At 300 amps,
    that's 1.4 volts lost in the cables, or roughly 10% loss on a (nominal)
    12V system - at typical 13.5V, you'd still get 12.1 V out the far end at
    300 amps.

    Double the cables to 30 feet each and you double the resistance (9.36
    milliohms), which doubles the voltage drop (2.8V), which doubles the
    voltage loss (20%). When it's actually pulling 300 amps: 13.5-2.8=10.7
    volts, which is probably low enough to cause problems on a 12V system.

    Using 1AWG won't get you back where you started - you'll need 3 sizes
    larger than 2AWG, or 00 (2/0) cable to have twice the copper and half
    the resistance per foot of 2AWG cable, so that twice the length gets you
    the same voltage out at the same amperage. 50-60 feet of 2/0AWG is
    manageable - 100 feet gets a bit annoying to pick up. If you stick to 23
    foot cables for 46 feet total, 1/0AWG cable would be close enough to the
    same as 30 feet of 2AWG, and lighter than 2/0AWG.

    It should be clear that with resistances this small having large effects
    (due to high amperage and low overall system voltage), the importance of
    clean, tight, low resistance connections to the cables and batteries is
    paramount...and you need to keep the road crap out of them so they stay
    good.
     
  4. Ecnerwal

    Ecnerwal Guest

    Well, since the ampacity of #2 wire is only 100 amps That chart does permit 181 amps in the "free air" configuration, which
    jumping certainly is - the wires can cool themselves better than if they
    were stuck inside a conduit ("enclosed").

    At low duty cycles (30%) 100 feet of #2 can be used for welding at 250
    amps (the voltage is higher, which is one reason I didn't mention that
    in my first post); you get different numbers for acceptable amperage
    depending on duty cycle and heat dissipation - running power continously
    inside a conduit leads to lower acceptable numbers, running jumper cable
    for a brief jolt outside in the Canadian winter leads to higher numbers.

    With your chart I now have 3 different resistance values published in
    different places for the same copper AWG numbers, but those are probably
    due to different assumptions about the operating temperature, which
    affects the resistance (0.156, 0.190, 0.201 milliohms per foot for 2AWG).
     
  5. Rob wrote:
    ....
    Would adding a second connector for the cables in the back of your truck
    help?

    Anthony
     
  6. SQLit

    SQLit Guest

    Make sure that you use welding cable or MTW wire. (machine tool wire). There
    are more strands and it will be easier to work with when it is cold. Larger
    cable would be better but it will work.
     
  7. Instead of replacing the cables on the truck with longer ones, why not
    use some #2/0 wire to extend from your home batteries to terminals on
    the front wall of the garage so you can tie on to them with the truck
    when you need the extra capacity?

    That way, the heavy wire is in a fixed installation, and you need
    handle it only once.

    You could use welder jacks, and hide them in an enclosure like a
    mailbox, perhaps.

    Gordon Richmond
     
  8. Walter R.

    Walter R. Guest

    My car broke down recently. The AAA "Battery Van" used a fairly long cable,
    I would say about 30 feet, certainly more than 12 feet. Looked fairly thick
    but readily maneuverable.
     
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