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'[OT] Wire capacity?'
1998\03\09@133339 by

I'm wiring up a small project, and I'm not sure what gauge of wire
to use.  I couldn't find an answer in _The Art Of Electronics_.

I assume that 30 gauge wire is sufficient for carrying digital
signals, since that's what Wire-Wrap used.

Is 30ga sufficient for Vcc and ground wires, which might be carrying
20mA?  How about motor supply wires which need to handle (in my
project) a couple of amps at 6 volts?

Answers to these particular questions appreciated; a formula or
rule of thumb to cover all situations would be even better.

Brian
Brian Scearce <blsBEST.COM> wrote:
>I'm wiring up a small project, and I'm not sure what gauge of wire
>to use.

Brian,

Transformer people use rules of thumb like 500 circular mils/amp.  This
takes into account the fact that the inside of a transformer will get
warmer than a wire in open air.

I think a reasonable estimate for your purposes would be something like
200 cir mils/amp.

For example, 30 gauge wire has a diameter of 10 mils giving an area
of 100 circular mils.  The above rule says you should be able to pass
half an amp and not melt the wire.

Another quick rule of thumb:  Subtracting 6 from the wire gauge
doubles the diameter, increasing the current capacity by 4.

AWG #24 has a diameter of 20 mils and should handle 2 amps.

In most cases it is not melting the wire that is the concern but the
voltage drop.  I would be more inclined to size the wire using that
as the main consideration.

--
Bob Fehrenbach    Wauwatosa, WI     bfehrenbexecpc.com
Transformer people use rules of thumb like 500 circular mils/amp.  This
takes into account the fact that the inside of a transformer will get
warmer than a wire in open air.

I think a reasonable estimate for your purposes would be something like
200 cir mils/amp.

For example, 30 gauge wire has a diameter of 10 mils giving an area
of 100 circular mils.  The above rule says you should be able to pass
half an amp and not melt the wire.

It takes MUCH more current than 500mA to melt a piece of 30g wire-wrap wire.
I just used my bencchtop supply to run 3A through a piece and it didn't even
glow or discolor due to heating.

That doesn't mean the rule of thumb is invalid, though.  I'd expect the
first concern to be "excessive heating", just due to system issues. (and
there's a LOT of wire in a transformer, so even a little bit of heat/cm will
amount up.  Secondly, you'd run into "don't melt the insulation", followed
by "don't melt the solder", with actual wire-melting WAY up there on the
list of concerns.

BillW
William Chops Westfield writes:
> It takes MUCH more current than 500mA to melt a piece of 30g wire-wrap wire.
> I just used my bencchtop supply to run 3A through a piece and it didn't even
> glow or discolor due to heating.
>
> That doesn't mean the rule of thumb is invalid, though.  I'd expect the
> first concern to be "excessive heating", just due to system issues. (and
> there's a LOT of wire in a transformer, so even a little bit of heat/cm will
> amount up.  Secondly, you'd run into "don't melt the insulation", followed
> by "don't melt the solder", with actual wire-melting WAY up there on the
> list of concerns.

According the the book "Reference Data for Engineers; Radio, Electronics,
Computer, and Communications, 7th ed." ISBN 0-672-21563-2 (A copy should
be on every electrical engineer's desk), the fusing current for 30 ga
copper wire is 10.2A (that is, the current at which the wire will melt).
the formula is as given I=K*d^(3/2), where d is the diameter in inches and
K is a constant that depends on the metal, aluminum K=7585, copper K=10244,
and so on.

The most important thing to be converned about is the wire (and junction)
resistance. a piece of another chart for annealed copper follows:
gauge       ohms/1000ft at 20C
12          1.588
14          2.525
18          6.385
26          40.81
30          103.2

So for power supplies, you can use ohms's law to figure out the voltage drop,
but for CMOS, it is very easy to get in trouble, because CMOS can cause
extremely high current spikes, which is why decoupling capacitors are
obligatory.  So the short answer is: for currents that pics can draw, 30ga
wire is fine.  If you need a big slug of current delivered in a short time,
start looking to heavier wires.

Why do I qualify the above paragraph for power supplies?  Remember, a wire
is not just a wire.  It is also an inductor and part of a capacitor, so
you must be aware of that on any line you are doing switching (i.e. introducing
high frequency components.

Matt

-----------------------------
Matt Bennett                |
mjbhazmat.com              |
http://www.hazmat.com/~mjb/ |
From my original post:
>    For example, 30 gauge wire has a diameter of 10 mils giving an area
>    of 100 circular mils.  The above rule says you should be able to pass
>    half an amp and not melt the wire.

Note feeble and inadequate attempt at irony.  The original poster
wanted to know if he could pass 20 ma through 30 ga wires.

William Chops Westfield <billwCISCO.COM> wrote:

>It takes MUCH more current than 500mA to melt a piece of 30g wire-wrap wire.

Agreed.  The 200 cir mils/amp guideline was not an attempt to
estimate a catastrophic level but rather a quick way to determine
a reasonably safe wire size.  In most cases, particularly prototypes etc,
100 cir mils/amp will do fine.

When you ran 3 amps through 30 ga wire (33 cir mils/amp)
did you melt the insulation?    While testing wire wrap prototypes
without my glasses on (a clip lead on the wrong pin) I have had
wires glow for a while before they melted.  I think the power supply
swung up over 5 amps.

--
Bob Fehrenbach    Wauwatosa, WI     bfehrenbexecpc.com
Bob Fehrenbach <bfehrenbEXECPC.COM> wrote:
>    When you ran 3 amps through 30 ga wire (33 cir mils/amp)
>    did you melt the insulation?    While testing wire wrap prototypes
>    without my glasses on (a clip lead on the wrong pin) I have had
>    wires glow for a while before they melted.  I think the power supply
>    swung up over 5 amps.

[Note: these comments aren't particularly directed to Bob.]

This is why bench supplies have adjustable current limiting.  You should
set the current limit to just above the expected power consumption, and
keep an eye on the meter or the CC (constant current) mode light.

For example, if you only expect the circuit to draw 500 mA, don't set the
current limit to 3 A.

I can't believe how many people try to debug prototypes using PC power
supplies and other such junk.  The usual explanation given is "but they're
so cheap".  Which only proves my point.  How many chips (and whole
prototypes) can you afford to damage rather than buying a decent bench
supply?

Every time that CC comes on, the supply has possibly prevented damage.
I've lost count of how many times it's worked for me, but I know that I'd
have blown up a few expensive boards if I didn't have it.

Cheers,
Eric
>    When you ran 3 amps through 30 ga wire (33 cir mils/amp)
>    did you melt the insulation?

(Hmm.  Have to retry on a piece of wire that has insulation.  fiddle,
fiddle.)

No melted insulation.  "Slightly" warm to the touch (at 3A.)

>This is why bench supplies have adjustable current limiting.

Yeah.  I think I bought my benchtop supply primarilly for tweaking assorted
types of rechargable batteries, but it has turned out to be quite a useful
piece of test gear in a number of unexpected areas.

For example, if you only expect the circuit to draw 500 mA, don't
set the current limit to 3 A.

But this is one I hadn't thought of!  Don't forget that any time you go into
current limit, you voltage is going to drop and (probably) do all sorts of
weird things to your digital logic, so you have to start over...

BillW

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