This may be a stupid question, but here goes. We all know that cable is graded for its current carrying capabilities according to its cross-sectional area. BUT, could one conceivably pass excessive amounts of current through a cable not rated to carry it by pulsing the current in short bursts at a very low duty cycle?
Cables and Currents
Started by ●March 3, 2017
Reply by ●March 3, 20172017-03-03
On 03/03/17 21:20, Chris wrote:> This may be a stupid question, but here goes. > We all know that cable is graded for its current carrying capabilities > according to its cross-sectional area. BUT, could one conceivably pass > excessive amounts of current through a cable not rated to carry it by > pulsing the current in short bursts at a very low duty cycle? >Not a stupid question at all. Yes, you can exceed the normal rating of the cable while reducing the duty cycle. The rating is usually based on heating due to ohmic loss. The upshot is that you'll have to reduce the duty cycle quadratically: You pass twice the rated current, you should use a duty cycle of 1/4. Also, the pulse length should be short compared to the thermal time constant of the cable. Shorter still if you exceed the cable rating grossly. This being Usenet, yes, I know there are limits to everything. Jeroen Belleman
Reply by ●March 3, 20172017-03-03
In article <o9cj5r$bd5$17@dont-email.me>, cbx@noreply.com says...> > This may be a stupid question, but here goes. > We all know that cable is graded for its current carrying capabilities > according to its cross-sectional area. BUT, could one conceivably pass > excessive amounts of current through a cable not rated to carry it by > pulsing the current in short bursts at a very low duty cycle?In many cases yes. The current rating of cables is often dependent on how hot the wire will get and the insulation heat rating. Sooner or later you will get enough current to melt the insulation, melt the wire, or too much voltage drop. Battery jumper cables for the cars are an example. The wire will not carry large ammounts of current for very long, but long enough to jump off a car with a dead battery.
Reply by ●March 3, 20172017-03-03
On Fri, 03 Mar 2017 16:14:15 -0500, Ralph Mowery wrote:> Battery jumper cables for the cars are an example. The wire will not > carry large ammounts of current for very long, but long enough to jump > off a car with a dead battery.So when you see these jumper cables rated at 400A or 600A or whatever, that's just for 30s or something? I must admit I've never thought about it. Come to think about it, a continuous duty 600A cable would be far thicker than the 1/2" diameter stuff they typically label as 600A jumper cable I would imagine.
Reply by ●March 3, 20172017-03-03
In article <o9cquq$bd5$18@dont-email.me>, iqbalali898@noreply.com says...> > On Fri, 03 Mar 2017 16:14:15 -0500, Ralph Mowery wrote: > > > Battery jumper cables for the cars are an example. The wire will not > > carry large ammounts of current for very long, but long enough to jump > > off a car with a dead battery. > > So when you see these jumper cables rated at 400A or 600A or whatever, > that's just for 30s or something? I must admit I've never thought about > it. Come to think about it, a continuous duty 600A cable would be far > thicker than the 1/2" diameter stuff they typically label as 600A jumper > cable I would imagine.Thats right. The wire to handle the full current of the starting of a car , if it had to carry that much for say an hour or all day would be much larger. I worked as an electrician at a plant that used many circuits of 200 or more amps. Large wire and sometimes 2 wires in parallel to handle that current full time. Unless made of many fine wires like welding cable they would be so big and stiff they would be hard to handle. Even at that they would be very bulky to store in a car. Many of the cables are only 6 or 8 guage. Some are even 10 gauge of copper coated aluminum. While they may be rated for 200 amps to start a car, the # 10 would only be rated for 30 amps in a house if they were all copper. Good enough to turn over a 4 or 6 cyclinder engine for the short time it should take to start it.
Reply by ●March 3, 20172017-03-03
On Fri, 3 Mar 2017 20:20:11 -0000 (UTC), Chris <cbx@noreply.com> wrote:>This may be a stupid question, but here goes. >We all know that cable is graded for its current carrying capabilities >according to its cross-sectional area. BUT, could one conceivably pass >excessive amounts of current through a cable not rated to carry it by >pulsing the current in short bursts at a very low duty cycle?Sure. The cable heats up from the current (current squared, approximately) and has some heat storage capacity. So you can really whack it for a short time, milliseconds to tens of seconds maybe, before the copper gets too hot. Wire can handle a lot of current if you cool it, too. Most power wiring stuff assumes that wires are inside jackets, inside walls maybe, where there's not much cooling. So power wire is conservatively rated for current. Pulse bursts don't increase the long-term RMS current capacity of a wire. They actully reduce it. (Which could restart the argument about "average RMS current.") -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●March 3, 20172017-03-03
On Fri, 3 Mar 2017 22:32:58 -0000 (UTC), Al <iqbalali898@noreply.com> wrote:>On Fri, 03 Mar 2017 16:14:15 -0500, Ralph Mowery wrote: > >> Battery jumper cables for the cars are an example. The wire will not >> carry large ammounts of current for very long, but long enough to jump >> off a car with a dead battery. > >So when you see these jumper cables rated at 400A or 600A or whatever, >that's just for 30s or something? I must admit I've never thought about >it. Come to think about it, a continuous duty 600A cable would be far >thicker than the 1/2" diameter stuff they typically label as 600A jumper >cable I would imagine.They lie, too. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●March 3, 20172017-03-03
On Fri, 03 Mar 2017 15:19:13 -0800, John Larkin wrote:> Pulse bursts don't increase the long-term RMS current capacity of a > wire. They actully reduce it.How so? I look at some high power MOSFETs/"HEXFETSs" you see in TO-220 package IIRC and wonder how those slender leads are supposed to carry the very high currents the devices are often rated for. There must be something going on there - some effect or other of which I'm ignorant - which enables that to happen.
Reply by ●March 3, 20172017-03-03
Al <iqbalali898@noreply.com> writes:> I look at some high power MOSFETs/"HEXFETSs" you see in TO-220 package > IIRC and wonder how those slender leads are supposed to carry the very > high currents the devices are often rated for. There must be something > going on there - some effect or other of which I'm ignorant - which > enables that to happen.I think that's something different - current handling in house wiring is limited by the insulation. When it melts and exposes wiring... fail. Part leads are limited by the melting point of the metal, which is I think steel (vs copper) so the metal part of the conductor can handle a much higher current. Inner-chip wiring is gold? That is an even better conductor, so even more current. Etc.
Reply by ●March 3, 20172017-03-03
On Sat, 4 Mar 2017 01:35:51 -0000 (UTC), Al <iqbalali898@noreply.com> wrote:>On Fri, 03 Mar 2017 15:19:13 -0800, John Larkin wrote: > >> Pulse bursts don't increase the long-term RMS current capacity of a >> wire. They actully reduce it. > >How so?By making peak temperatures higher than what you'd get if the current were steady. Peak temperatures melt things. And heat increases the wire's resistance (well, for most materials) which causes more heat.> >I look at some high power MOSFETs/"HEXFETSs" you see in TO-220 package >IIRC and wonder how those slender leads are supposed to carry the very >high currents the devices are often rated for. There must be something >going on there - some effect or other of which I'm ignorant - which >enables that to happen.IR invented the trick of wildly overstating the current and power capability of small mosfets, like by radically cooling them in exotic boiling fluids. Everybody else had to tell the same lies to be competitive. -- John Larkin Highland Technology, Inc lunatic fringe electronics
