Nonlinearities in 12-gauge Copper Wire

I should know this, I really should.
12-gauge copper wire is rated for around 20 amps or so if it's buried in the wall of a house -- but how much current would you need to reliably
melt a length of it if it's in free air? How about vaporize?
In case you're wondering I'm working on a heavy-handed example of how _nothing_ in the real world is truly linear. The example I'm using is a copper wire that'll carry 20A for centuries without trouble, yet will melt (or vaporize) given a high enough overload.
Thanks.
--

Tim Wescott
Wescott Design Services
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

12 ga copper: fusing current is given as 235 amps in Reference Data for Radio Engineers.
John
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Tim Wescott wrote:

melt is hard - you'd need to look at the heat convected & radiated away.
vaporise is easier (except what do you mean by vaporise?). I'd just assume and adiabatic process, with heating evenly distributed throughout the material, and do an m*cp calculation for some dT above the melting point. that'll tell you how much energy you need, which can be worked back thru R (varying with temperature) to get I for a given t.
and do you drive it with a current or voltage source? the current source will melt far, far faster due to thermal runaway....

stick it in a bucket of water, and watch it get a lot more nonlinear. You'll have to navier-gate your way around stokes and stokes of nasty equations. Oops, I'm prandtling....
Cheers Terry
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Terry Given wrote:

Good point. And I believe from the names you're throwing around you're only accounting for convection, not the onset of boiling (and is it a sealed vessel, where the boiling point & therefore the heat transfer rate will change with pressure & therefore temperature?)
I was also pondering a section of the same piece titled "common nonlinearities", then I realized that I'm either going to have a very few or a book-length report. Oh well, something to keep me awake at night.
--

Tim Wescott
Wescott Design Services
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Tim Wescott wrote:

oh yes. by comparison, electronics is easy.
a buddy of mine used 5A fuse wire in a bucket of de-ionised water as an ultra-fast 400A fuse, while developing an SCR inverter for his PhD. As soon as the water boiled, *wham* blown fuse. an awful lot cheaper than the real thing.

Cheers Terry
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.