text: calculating max current through wire?
  
  Discussion in 'Electronic Design' started by Michael Noone, Jul 24, 2005.
  
  1. Michael NooneGuest
  
  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?
  
  Thanks!
  
  -MJ Noone
  
  Michael Noone, Jul 24, 2005
  
  2. GarethGuest
  
  Michael Noone wrote:
  > 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?
  >
  > Thanks!
  >
  > -MJ Noone
  
  There is another table here which gives maximum current as well as
  resistance, that may help:
  
  http://www.powerstream.com/Wire_Size.htm
  
  --
  -----------------------------------------------------------------------
  
  Replace privacy.net with: totalise DOT co DOT uk and replace me with
  gareth.harris
  
  Gareth, Jul 24, 2005
  
  On Sun, 24 Jul 2005 15:36:32 -0500, Michael Noone
  <mnoone.uiuc.edu@127.0.0.1> wrote:
  
  >resistance.html - and am trying to figure out the thinnest guage wire I can
  >use.
  
  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)?
  
  >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?
  
  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.
  
  >Thanks!
  >
  >-MJ Noone
  
  -----
  
  4. Michael NooneGuest
  
  news::
  
  > On Sun, 24 Jul 2005 15:36:32 -0500, Michael Noone
  > <mnoone.uiuc.edu@127.0.0.1> wrote:
  >
  >>resistance.html - and am trying to figure out the thinnest guage wire
  >>I can use.
  
  >
  > 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)?
  
  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.
  
  >>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?
  
  >
  > 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.
  
  It will be stretched out in open air.
  
  -Michael
  
  Michael Noone, Jul 24, 2005
  5. Pooh BearGuest
  
  Michael Noone wrote:
  
  > Ben Bradley <> wrote in
  > news::
  >
  > > On Sun, 24 Jul 2005 15:36:32 -0500, Michael Noone
  > > <mnoone.uiuc.edu@127.0.0.1> wrote:
  > >
  > >>resistance.html - and am trying to figure out the thinnest guage wire
  > >>I can use.
  
  > >
  > > 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)?
  
  >
  > 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.
  
  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.
  
  Graham
  
  Pooh Bear, Jul 25, 2005
  6. Poly ChromeGuest
  
  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.
  
  Poly Chrome, Jul 25, 2005
  7. Robert BaerGuest
  
  Michael Noone wrote:
  
  > 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?
  >
  > Thanks!
  >
  > -MJ Noone
  
  This is the rule of thumb that i have used to wind power
  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.
  
  Robert Baer, Jul 25, 2005
  8. Robert BaerGuest
  
  Michael Noone wrote:
  
  > Ben Bradley <> wrote in
  > news::
  >
  >
  >>On Sun, 24 Jul 2005 15:36:32 -0500, Michael Noone
  >><mnoone.uiuc.edu@127.0.0.1> wrote:
  >>
  >>
  >>>resistance.html - and am trying to figure out the thinnest guage wire
  >>>I can use.
  
  >>
  >> 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)?
  
  >
  >
  > 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.
  >
  >
  >>>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?
  
  >>
  >> 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.
  
  >
  >
  > It will be stretched out in open air.
  >
  > -Michael
  
  Why not look at using "flex"; that is, flat flexible "wire"?
  
  Robert Baer, Jul 25, 2005
  9. PeteSGuest
  
  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.
  
  Cheers
  
  PeteS
  
  PeteS, Jul 25, 2005
  10. Terry GivenGuest
  
  Robert Baer wrote:
  > Michael Noone wrote:
  >
  >> 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?
  >>
  >> Thanks!
  >>
  >> -MJ Noone
  
  >
  > This is the rule of thumb that i have used to wind power transformers,
  > 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.
  
  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.
  
  Cheers
  Terry
  
  Terry Given, Jul 25, 2005
  11. AndrewGuest
  
  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.
  
  Andrew, Jul 25, 2005
  12. Michael A. TerrellGuest
  
  Terry Given wrote:
  >
  > #define circular mil dimensional madness
  > Cheers
  > Terry
  
  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
  
  Michael A. Terrell, Jul 25, 2005
  13. Terry GivenGuest
  
  Michael A. Terrell wrote:
  > Terry Given wrote:
  >
  >>Cheers
  >>Terry
  
  >
  >
  >
  > 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.
  >
  
  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.
  
  Cheers
  Terry
  
  Terry Given, Jul 26, 2005
  14. Pooh BearGuest
  
  Terry Given wrote:
  
  > Michael A. Terrell wrote:
  > > Terry Given wrote:
  > >
  > >>#define circular mil dimensional madness
  > >>Cheers
  > >>Terry
  
  > >
  > >
  > > 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.
  
  >
  >
  > 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.
  
  Not to mention units that are unique to the USA, so no-one anywhere else knows
  what you mean.
  
  Graham
  
  Pooh Bear, Jul 26, 2005
  15. Michael A. TerrellGuest
  
  Pooh Bear wrote:
  >
  > Terry Given wrote:
  >
  > > Michael A. Terrell wrote:
  > > > Terry Given wrote:
  > > >
  > > >>#define circular mil dimensional madness
  > > >>Cheers
  > > >>Terry
  > > >
  > > >
  > > > 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.
  
  > >
  > >
  > > 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.
  
  >
  > Not to mention units that are unique to the USA, so no-one anywhere else knows
  > what you mean.
  >
  > Graham
  
  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
  
  Michael A. Terrell, Jul 26, 2005
  16. Terry GivenGuest
  
  Michael A. Terrell wrote:
  > Pooh Bear wrote:
  >
  >>Terry Given wrote:
  >>
  >>
  >>>Michael A. Terrell wrote:
  >>>
  >>>>Terry Given wrote:
  >>>>
  >>>>
  >>>>>Cheers
  >>>>>Terry
  >>>>
  >>>>
  >>>> 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.
  >>>
  >>>
  >>>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.
  
  >>
  >>Not to mention units that are unique to the USA, so no-one anywhere else knows
  >>what you mean.
  >>
  >>Graham
  
  >
  >
  >
  > 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.
  >
  
  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.
  
  Cheers
  Terry
  
  Terry Given, Jul 26, 2005
  17. Paul E. SchoenGuest
  
  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
  effects.
  
  The Excel spreadsheet is on my website at
  http://www.smart.net/~pstech/WireSize.xls. Feel free to make any
  improvements or suggestions. Good luck.
  
  Paul E. Schoen, President
  P S Technology, Inc.
  www.pstech-inc.com
  
  "Robert Baer" <> wrote in message
  newst%Ee.8001\$...
  > Michael Noone wrote:
  >
  > > 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?
  > >
  > > Thanks!
  > >
  > > -MJ Noone
  
  > This is the rule of thumb that i have used to wind power
  > 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.
  
  Paul E. Schoen, Jul 26, 2005
  18. Rich GriseGuest
  
  On Tue, 26 Jul 2005 13:29:08 +1200, Terry Given wrote:
  > Michael A. Terrell wrote:
  >> Pooh Bear wrote:
  >>>Terry Given wrote:
  >>>>Michael A. Terrell wrote:
  >>>>>Terry Given wrote:
  >>>>>
  >>>>>
  >>>>> 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.
  >>>>
  >>>>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.
  >>>
  >>>Not to mention units that are unique to the USA, so no-one anywhere else knows
  >>>what you mean.
  
  >>
  >> 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.
  
  >
  > Yeah, a real smart system. Whats the next wire size larger than 0000 AWG?
  
  That's not wire - that's bar stock. ;-)
  
  Cheers!
  Rich
  
  Rich Grise, Jul 26, 2005
  19. mcGuest
  
  >>>>>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.
  
  Back in the days when you could order "10-inch" and "13-inch" pizzas, we
  used to calculate pizza surface area in circular inches.
  
  mc, Jul 26, 2005
  20. Terry GivenGuest
  
  Paul E. Schoen wrote:
  > 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
  > effects.
  >
  > The Excel spreadsheet is on my website at
  > http://www.smart.net/~pstech/WireSize.xls. Feel free to make any
  > improvements or suggestions. Good luck.
  
  [snip]
  
  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.
  
  Cheers
  Terry
  
  Terry Given, Jul 27, 2005