Calculating speaker cable impedence?

Discussion in 'Archived Threads 2001-2004' started by Mark Fitzsimmons, Jul 9, 2002.

  1. Mark Fitzsimmons

    Mark Fitzsimmons Supporting Actor

    Aug 18, 2001
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    This seems to be an 'advanced' topic so I hope this is the appropriate section to post in.

    I want to calculate the impedence of my speaker wires. My front stage, (left, center, right) is 12 gauge, 2 conductor cables all 9 feet long. And for my surrounds I'm running 16 gauge, 2 conductor, at 30 feet long per speaker. I'm just curious as to what the difference in impedence of my front and rears is.
  2. Chu Gai

    Chu Gai Lead Actor

    Jun 29, 2001
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    you might find this an interesting read...
    Belden also has a lot of interesting reading material in their 'cable college'.
    impedence of 12 gauge is ~0.048 ohms and if i'm correct, its the same for your front and rears...kind of like a 75 ohm cable, its still 75 ohms if its 2 feet or 200 feet? resistance though is proporional to the length.
  3. Greg Monfort

    Greg Monfort Supporting Actor

    May 30, 2000
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    Unless you're using tube amps or tiny wire for the application which will raise the speaker's Qt, voltage drop is all you're really interested in, and the amount of power likely to be used comes into play here:

    Using the driver's Pe, Re (or total Pe, Re if multiple drivers are used), then with the formula:

    I = sqrt P(Pe)/R(Re)

    you can calculate ~ max current draw capability. If this number is >the amp's rating, then you may clip it on loud passages.

    To determine wire size required, or in your case, what the voltage drop is:

    Circular Mils (CM) = (2*L*I*10.8)/VD, or in your case: VD = (2*L*I*10.8)/CM

    L = length of wire in ft
    I = max current of circuit
    VD = voltage drop

    Some rounded off CM values:

    32ga = 64
    30ga = 100
    28ga = 159
    26ga = 320
    24ga = 404
    22ga = 640
    20ga = 1024
    18ga = 1624
    16ga = 2580
    14ga = 4109
    12ga = 6529
    10ga = 10384
    8ga = 16512


    Pe or short term peak rating = 400W
    Re = 3.4ohms
    wire run (one way) = 20ft
    VD = 1V (this is considered the max acceptable, I usually use 0.3-0.5V)

    Sqrt 400/3.4 = 10.846A

    (2 x 20 x 10.846 x 10.8)/1 = 4685.472CM, or 12ga.

    Anyway, plug in the applicable numbers and as long as the VD is acceptable it doesn't matter if they don't match as you can't hear fractions of a dB difference at transient peak power.


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