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calculating max current through wire?

Discussion in 'Electronic Design' started by Michael Noone, Jul 24, 2005.

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  1. Hi - I'm looking at this page:
    resistance.html - and am trying to figure out the thinnest guage wire I can
    use. Is there a good rule of thumb for this? I was thinking about using
    some 28 AWG wire (as I would like to use some .05" pitch ribbon cable).
    According to that page it's 232ohm/km. I need it to go about 15cm, so
    (232/1000)*.15 = .0348ohms. I'd like it to be able to handle 1.5A max
    current (though I'd be incredibly surprised if current spiked above 1A, and
    normally it should be under 200ma. So, .0348 * 1.5 = 0.0522V drop at peak
    current. Supply voltage is 6V, so .0522/6 = 0.87% of power (.0783W) dropped
    over the wire. Is this OK? Or is this pushing things? Am I reading the
    table correctly and are my calculations correct?


    -MJ Noone
  2. Gareth

    Gareth Guest

    There is another table here which gives maximum current as well as
    resistance, that may help:

  3. Ben Bradley

    Ben Bradley Guest

    Why? Is this a mass-produced product where every fraction of a
    penny (coincidentally made of copper) must be shaved off the
    production cost? Or does the wire have to go through a very small
    area, such as a tube or conduit (this could affect max power it could
    dissipate - see below)?
    It depends on what you want to do with the wire. I presume these
    table assume the wire is stretched out like a cable, thus the heat
    generated would be dissipated over a substantial area or volume. If
    you wrap the wire around a resistor so it has much less area and
    volume to dissipate heat, the power rating will be much lower.
  4. Well - ideally I'd like to use some .05" ribbon cable for this - and all
    the .05" ribbon cable I've seen is 28 or 30 AWG. My choice of .05"
    ribbon cable is due to me wanting as fine as possible of a ribbon cable
    to connect a couple boards. I'm thinking that what I could probabaly do,
    if necessary, is use multiple wires in the ribbon cable for this power
    line . Everything else except for the ground will be very low current -
    it's just this one wire and ground that will be carrying substantial
    amounts of current, so this makes more sense to me than using thicker
    ribbon cable.
    It will be stretched out in open air.

  5. Pooh Bear

    Pooh Bear Guest

    From memory, all the 0.05" pitch ribbon cable I've seen is rated at a
    nominal 1 amp.

    It's essentially nominal since working temperature is actually the
    determining criterion. i.e you have to derate with high ambient temp.

    I see no problem with your case.

  6. Poly Chrome

    Poly Chrome Guest

    Just try it.
    put your finger on the wire to see if it is warm.
    Heat flow out the attachment points is a factor too.
    You may be making fuses.
    Your calculation based upon resistance is only correct for a range of
    current, not near the max at all.
  7. Robert Baer

    Robert Baer Guest

    This is the rule of thumb that i have used to wind power
    transformers, and i have never had any IR problems: Start with the
    maximum current, in milliamperes for the wire in question. and look in a
    wire chart for the size with the closest value cross-sectional area in
    circular mils.
    Example: number 30 wire would be rated for use near 100mA maximum
    current (continuous).
    That same wire seems to be useable asa replacement for a one amp fuse.
    If that follows your tests, then the "fuse rating" would be roughly
    ten times my rule of thumb rating.
  8. Robert Baer

    Robert Baer Guest

    Why not look at using "flex"; that is, flat flexible "wire"?
  9. PeteS

    PeteS Guest

    Flat Flex cable (FFC for the lookup) is rated somewhat lower, but makes
    a fine interconnect. I use it for LCD and touchpanel interconnects,
    amongst other things.

    RS/Farnell carry a fair amount of it, and there's a small outfit in
    Germany that has all sorts of odd lengths/tinning options/number of
    ways - I'll dig out the website later.


  10. Terry Given

    Terry Given Guest

    #define circular mil dimensional madness

    ROT are a terrible way to design anything. As is current density (even
    more so when expressed in amps per circular mil). All that matters is
    temperature rise. Continuous current rating is invariably specified with
    an isolated wire in optimal thermal conditions; winding a whole bunch of
    terns in close proximity (if the feathers dont interfere) changes things
    significantly. See, for example, the various articles written in the
    [cant recall name] PCB design comic, or standards for PCB current
    density, where N adjacent identical tracks are considered the same as
    one track of N* width carrying N* current, due entirely to thermal coupling.

    why not assume adiabatic heating to work out fusing current? the maths
    is trivial. Again, well covered in PCB design comics.

    If the insulation is PVC, and you heat it up, it will outgass chlorine,
    which aint too good for crimped connections.

  11. Andrew

    Andrew Guest

    I think you will be fine as long as you aren't running more than 1A
    CONTINUOUS. But try it for yourself. Ideally if you have a current
    limited voltage supply, short the terminals together with a piece of
    the 28awg cable you want to use, and set the current limit at 1.5A.
    Feel how warm the wire gets. If it gets very hot very quickly, you
    might not want to use it in your design. If it stays merely warm after
    like 10 minutes, you are probably fine. It is all about how hot you
    want the wire to get. If you search google, you might be able to find
    a temperature rise calculator based on wire size, current, and distance.

  12. Search for a copy of the AWG wire table. All will be explained. Its
    so simple that almost anyone can understand it. I did, when I was 10.

    Link to my "Computers for disabled Veterans" project website deleted
    after threats were telephoned to my church.

    Michael A. Terrell
    Central Florida
  13. Terry Given

    Terry Given Guest

    I didnt say I didnt understand it, only that its spectacularly stupid.
    There is a reason MKS is referred to as "rational" units. Circular mils
    are a great example of irrational units.

  14. Pooh Bear

    Pooh Bear Guest

    Not to mention units that are unique to the USA, so no-one anywhere else knows
    what you mean.


  15. The reference was to American Wire Gauge so it doesn't matter if
    people outside the US don't understand circular mils, because they
    won't be using AWG wire anyway.

    Link to my "Computers for disabled Veterans" project website deleted
    after threats were telephoned to my church.

    Michael A. Terrell
    Central Florida
  16. Terry Given

    Terry Given Guest

    Yeah, a real smart system. Whats the next wire size larger than 0000 AWG?

    I stand by my original comments, that circular mils are a stupid,
    arbitrary and useless measure. WTF is wrong with conductor diameter?
    Dude, R = Rho(T)*length/area, baby physics. Circular mils serve only to
    complicate the matter unnecessarily.

    Even the "cookbook" approach becomes more complex, more stupid numbers
    popping up everywhere, the almost complete lack of which is the main
    advantage of the MKS system (hence the term "rational").

    Another great example of stupid numbers is the penchant for using the
    "transformer" equation. 4.44 is a pretty stupid number. E = wNBA is far
    more meaningful.

  17. I have made a crude Excel spreadsheet that calculates current carrying
    capacity for wires as well as bus bar. For wire or bus bar in free air, the
    current carrying capacity is closely related to watts per square unit of
    surface area, so smaller wires can carry more current per cross sectional
    area than larger wires. For multiple conductors in close proximity (such as
    in a wound coil or multi-conductor cable), the capacity is more closely
    related to the cross sectional area, or watts per unit of volume. The
    temperature rating of the insulation also comes into play, so you need to
    determine the ambient temperature and the temperature rise due to power,
    factoring in the temperature coefficient of copper, and also the rate at
    which heat will be conducted or radiated from the wire. I made my wire chart
    based on the NEC ratings of 30 A for #10, 20 A for #12, and 15 A for #14,
    and found a constant 0.0037 A/sq mil of cross section area and 0.240 W/sq in
    of surface area. The surface area calculations correlate fairly closely with
    the published ratings for bus bar, so I am comfortable with those ratings
    for single larger wires.

    Using this chart (which I just updated with approximate data for smaller
    wire sizes), the #28 AWG wire should be able to handle 1.3 amps, and the #30
    AWG should handle 0.9 A. Based on cross sectional area, these ratings would
    be 0.5 and 0.3 A. I normally use this chart for much higher currents and
    very large wires and bus bars. My work with circuit breaker test sets
    involves continuous currents up to 6000 amperes and short pulse currents up
    to 100,000 amperes. At those levels, there are other considerations such as
    skin effect and both mutual and self inductance, as well as mechanical
    constraints to prevent conductors from jumping around due to magnetic

    The Excel spreadsheet is on my website at Feel free to make any
    improvements or suggestions. Good luck.

    Paul E. Schoen, President
    P S Technology, Inc.
  18. Rich Grise

    Rich Grise Guest

    That's not wire - that's bar stock. ;-)

  19. mc

    mc Guest

    Back in the days when you could order "10-inch" and "13-inch" pizzas, we
    used to calculate pizza surface area in circular inches.
  20. Terry Given

    Terry Given Guest


    Hi Paul,

    thats a damn good way of working it out. My only comment would relate to
    the thermal characteristics of the wire insulation, which is probably
    not constant over varying wire sizes. Radox, for example, uses a
    X-linked polymer with a nice low Rtheta, allowing even higher current
    densities (and it wont melt when you run the soldering iron across it,
    nor does it outgas Chlorine when hot).

    Conversely it is highly unlikely that ribbon cable insulation is
    optimised for heat transfer (the insulation can be surprisingly thick),
    so I'd expect it to go horribly wrong with small wire sizes. But it
    certainly gives a reasonable upper limit, which can easily be followed
    up with suck-it-and-see experimentation.

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