Nonlinearities in 12-gauge Copper Wire

[Tim Wescott]

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.

 

[John Larkin]

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.

12 ga copper: fusing current is given as 235 amps in Reference Data
for Radio Engineers.

John

 

[Terry Given]

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?

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....

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.

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

 

[Tim Wescott]

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

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.

 

[Terry Given]

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?)

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.

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.

Cheers
Terry