Electronics Forums > calculating max current through wire?

# calculating max current through wire?

Michael Noone
Guest
Posts: n/a

 07-24-2005, 08:36 PM
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

Gareth
Guest
Posts: n/a

 07-24-2005, 09:12 PM
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

Guest
Posts: n/a

 07-24-2005, 10:00 PM
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

-----

Michael Noone
Guest
Posts: n/a

 07-24-2005, 10:30 PM
Ben Bradley <(E-Mail Removed)> wrote in
news:(E-Mail Removed):

> 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

Pooh Bear
Guest
Posts: n/a

 07-24-2005, 11:32 PM

Michael Noone wrote:

> Ben Bradley <(E-Mail Removed)> wrote in
> news:(E-Mail Removed):
>
> > 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

Poly Chrome
Guest
Posts: n/a

 07-25-2005, 04:20 AM
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.

Robert Baer
Guest
Posts: n/a

 07-25-2005, 06:16 AM
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
Guest
Posts: n/a

 07-25-2005, 06:22 AM
Michael Noone wrote:

> Ben Bradley <(E-Mail Removed)> wrote in
> news:(E-Mail Removed):
>
>
>>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"?

PeteS
Guest
Posts: n/a

 07-25-2005, 07:55 AM
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

Terry Given
Guest
Posts: n/a

 07-25-2005, 09:49 AM
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

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