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maximum length of wire?

Discussion in 'Electronic Basics' started by [email protected], Nov 15, 2008.

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  1. Guest

    I have some video equipment i need to power off directly from my car's
    battery. I need a wire that is not too big, because i need to solder
    on a special quick-release connector that will be used to easily
    disconnect the cable from my video equipment. The cable requires 2
    wires inside, one for positive and one for negative on the battery.
    The biggest cable i found with 2 conductors, has 2x 22AWG wires
    inside. Some tell me that it is too small and some say it will work no
    problem. I don't want the wires to start melting and cause a fire! The
    video gear will draw no more than 1 amp total, directly off the car's
    12V battery. My question is, what is the maximum length of cable i can
    use to remain safe? Thnks for your advice.
     
  2. whit3rd

    whit3rd Guest

    The fuse limit for typical insulation on 22AWG wires is well over
    2 amps, so PROVIDED WITH A SUITABLE FUSE it's safe to
    connect it to the 12V battery. Your battery does NOT connect
    directly to this wire, it connects THROUGH A FUSE (1.5A
    would be a suitable size).

    The important part here, is that the fuse prevents the wire
    from burning up under ANY fault condition, including higher
    current than your intended device (the video gear) uses.
     
  3. Rich Webb

    Rich Webb Guest

    For some back o' the envelope approximations...

    We'll assume that your equipment load is about 1 A, as stated. The
    battery will really be about 13 V when not loaded but we'll swag the
    equivalent load resistance at 12 ohms.

    AWG 22 cable is about 16 ohms per 1000 feet, so using a 100 ft run, you
    would add 1.6 ohms on "either side" of your existing load. The total
    load, which is now about 15 ohms, will draw about 0.85 A from a 13 V
    battery and you'll have a bit more than 10 V to work with at your load.
    Your equipment may not be happy with that.

    If you follow Tim's recommendation and use heavier gauge cable with
    short transition pieces to the necessary wire size for the connector,
    things look a bit better.

    With regular 14 AWG zip cord, you're now running only about 2.5 ohms per
    1000 feet. That 100 foot run now only adds 1/4 ohm to each leg, so your
    total draw (with that same 13 V battery) is pretty close to 1 A, with
    only about 0.5 V lost in the cabling. Happy equipment!

    As whit3rd mentioned, fuse it. Preferably run from a polarized aux
    connector to a spare fuse on the vehicle's fuse box, rather than hook
    directly to the battery. (Nobody would EVER hook black to positive, red
    to ground, would they?)

    If you ever intend to run from an operating vehicle, be aware that the
    equipment will see some nasty transients. If the stuff is designed to
    run from a car's electrical system, you're good. Otherwise, be aware
    that the magic smoke may come out. See the link for a description of
    some issues http://www.industrologic.com/autotransients.pdf
     
  4. Eeyore

    Eeyore Guest

    Good thing he's in N. America (Canada) or he wouldn't have an earthly clue
    what an AWG is. Does Canada use AWG or have they fully metricated now ?

    Graham
     
  5. Rich Webb

    Rich Webb Guest

    One may note that the OP mentioned that the "cable i found with 2
    conductors, has 2x 22AWG wires" and infer from that statement that he
    indeed has some clue (earthly or not, to be determined).
     
  6. Fred Abse

    Fred Abse Guest

    The OP wrote: "I need a wire ...2x 22AWG wires"

    Says it all.
     
  7. krw

    krw Guest

    Ability to use and ability to think are two different things.
     
  8. gazz

    gazz Guest

    just a quick note, is the equipment designed to run directly off a car
    battery? or are you powering an item that has a 12 volt DC input from a
    power adaptor (wall wart)

    just that video equipment will be pretty sensative to noise and spikes, and
    you get a lot of them on a car's electrical system, also the voltage at the
    battery terminals will vary from 11 volts to 14.8 volts, depending on if the
    engine is running or not.

    i've run my hard drive camcorder fixed to my dashboard whilst driving in my
    motorhome last year, it too wants a 12 volt DC supply at 1 amp max, but no
    way i'd feed it with the power directly from the battery/alternator.

    i was going to get one of the proper 12 volt camera power supplies to run
    it, but it was easier for me to run an extension cable from the living area
    of the motorhome to the dash connected to the main inverter, part of that
    reason was the 12 volt Dc power supply was about 50 quid, the extension
    cable was 2 quid, and i use the mains adaptor and that has the neccisary
    filters to take out any noise on the power.
     
  9. neon

    neon

    1,325
    0
    Oct 21, 2006
    The melting point of AWG GAUGE #22 can be calculated as I=Kd raised to the power of 3/2.
    K is a constant depending on material copper is 10,244 and d=0.0253 even these are aproximation. But the resistance per 1000' is 41.17 ohms. In general the smaller the wire the better clearance of a short. A car battery can supply 500 to 1000 amps into a short if it does not clear you got problems. I sugest #14 stranded house wireing easy to get and easy to inplement. You can add two inline fuses to clear a fault that is a good idea and put the fuses at the battery end not at the other end.
     
  10. Rich Grise

    Rich Grise Guest

    Go to Radio Shack and pick up a 12V lighter adapter.

    Good Luck!
    Rich
     
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