# 40-Gauge wire current capacity?

## Main Question or Discussion Point

Alright, so looking at how a 40-gauge wire being extremely thin and having high resistance(with respect to it's length), having an extremely short 40-gauge wire(about 1mm), it's possible to maintain 10A or possibly more? I know it's resistance is about 0.002 ohms, which is some what the same resistance as a 1-gauge wire at 10 feet which can easily handle 50A+.

Is it possible to maintain such current without fusing the wire?

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Is it even physically possible to have the length of a 40-gauge(diameter is 0.07m) to 1mm or less like 0.05m or 0.07m as the diameter? I can't imagine a way to do that(maybe leasers?!)

jim hardy
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Baluncore
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Start here. http://en.wikipedia.org/wiki/American_wire_gauge
http://en.wikipedia.org/wiki/Ampacity

For 12V DC circuits, wires often carry 10A per square millimetre.

The temperature of a wire will increase until heat generation equals heat loss. Wire resistance usually rises in proportion to absolute temperature.

Higher wire operating temperatures increase the rate of chemical reaction with the environment. If heat cannot be removed from the filament at the rate it is generated, then the temperature will rise above the melting point of the wire material and so it will fuse.

This in the final analysis has (almost) nothing to do with the gauge or length.... you have the R=0.002 great - how much heat at 10A ? - if the length is really 1 mm - it's terminations can probably dissipate all of the heat and the "system" will reach steady state temp and not fail. However - note that the resistance of the terminations is critical - both for electrical resistance and thermal resistance (conductivity) -
My point being that this is not so much a question about the wire - as it is the system - it is a very simple case - How much heat? (total) - how much can I dissipate. Some LEDs die at 100mw - they are encased in plastic- do you get my point?

jim hardy
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Heat has to conduct from center to periphery of wire, thence to surroundings. There'll be some current that'll cause thermal runaway because resistance goes up something like 0.2% per degree and heat production is I^2R.. If you continue to force current it can be something to watch - from a safe distance.

I am thinking I should give up on this forum.

jim hardy
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it's terminations can probably dissipate all of the heat and the "system" will reach steady state temp and not fail.
I am thinking I should give up on this forum.
Give up ? Why ? I thought your above quote was leading OP into fast fuse designs, like semiconductor fuses.....that's just how they work.
http://www.ferrazshawmutsales.com/pdfs/A50QS.pdf [Broken]

If i've offended you i deeply apologize....... i always enjoy your posts.

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jim hardy
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if the length is really 1 mm - it's terminations can probably dissipate all of the heat and the "system" will reach steady state temp and not fail.

Look at the images here for single element fuse,
http://www1.cooperbussmann.com/library/arcflash/SPDFuseOperation.pdf
figure 1.

I've taken apart ten amp "semiconductor*" fuses of similar construction.
The element is a thin metal foil with narrow sections much like pictured.
The narrow section of the link is what melts.
At normal current the heat generated in the narrow section conducts into the wider sections and makes its way out of the fuse.
At abnormal current the heat can't get out fast enough so it goes into thermal runaway and melts at the narrow point.

The fuses i dissected were ten amp, as i said, and i'd estimate the element's narrowest part around 0.025 inch, 0.6 mm.
Please understand that's an estimate from memories twenty years old.
So i think a very short piece of #40 wire connected to an adequate heat sink as you described, might very well conduct ten amps.

As Windadct said it's a system.

* "Semiconductor" means the fuse is designed for very fast action, to protect semiconductors. It's made of metal, often silver, not silicon like semiconductors.
It takes a fast fuse to protect semiconductors. In 1960's we had a saying - "transistors make really good fuse protectors."

old jim

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