# 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

2. ### GarethGuest

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

http://www.powerstream.com/Wire_Size.htm

--

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. ### Michael NooneGuest

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.

-Michael

5. ### Pooh BearGuest

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

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.

7. ### Robert BaerGuest

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.

8. ### Robert BaerGuest

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

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

10. ### Terry GivenGuest

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

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.

12. ### Michael A. TerrellGuest

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 GivenGuest

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

14. ### Pooh BearGuest

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

Graham

15. ### Michael A. TerrellGuest

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 GivenGuest

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

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

18. ### Rich GriseGuest

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

Cheers!
Rich

19. ### mcGuest

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 GivenGuest

[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