Cables and Currents

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?

Reply to
Chris
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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 to
Jeroen Belleman

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 to
Ralph Mowery

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 to
Al

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 to
Ralph Mowery

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.")

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

They lie, too.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

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 to
Al

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 to
DJ Delorie

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.

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.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

** Cable current ratings are given in amps rms - so the heating effect of the current is what matters, not its peak or average value.

For the same reason, circuit breakers are thermally operated, responding to the rms value of whatever current is flowing in the wire.

The most efficient use of a cable occurs with DC current.

.... Phil

Reply to
Phil Allison

For thing like magnet coils (we do mostly air coils) you can totally run 'em high, we have one instrument, that limits the duty cycle... up to a 20 second period.

For a hunk of copper, there should be some current, that raises the piece 1 deg K/ sec. (Well at least for small changes in T)

A related question, (and currently of more interest to me. NPI) is how much current can a wire carry in vacuum. I've got some graphs on my computer at work, but I'm not sure I believe them.... The wire is phosphor-bronze, this looked good, but they didn't model radiation..?

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I've never heard of this, but do people paint their wires black? Better radiators.

George H.

Reply to
George Herold

Big (non-superconducting) electromagnets are usually water cooled. They have a lot of copper volume to surface area ratio, so get hot.

Most shiny metals have low emissivities at thermal wavelengths. Copper is an almost perfect mirror at thermal IR. So in a hard vacuum, practically the only cooling will be conduction to the end terminations.

Making the wire black (at thermal wavelengths!) would really help. Smashing it into a ribbon would increase the surface area, too.

Painted or insulated wire is better than bare metal, unless you can run literally red hot. Most organics have high emissivity.

Interesting experiment: try bare copper wire vs magnet wire, same sizes, same current, in vacuum. Inferr the temperature from the resistance.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

Thick black copper oxide has an emissivity of about 0.78 in the thermal IR, according to

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

I guess that copper will get hot and tarnish some.

Not to change the subject, but regular office white-out has a very high emissivity. So you can dab it on shiny things, like the metal tops of some FPGAs and such, to read the temps better.

Kapton tape is pretty good.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

Just about any dielectric at least a few mils thick has a thermal emissivity of about 0.95. The rest is Fresnel reflection at the surface.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

Fresnel reflection... I had to look it up. (dielectric mismatch) Is this for IR wavelengths? Certainly off white paint is higher than that in the visible. Maybe I can get the wire with a triple or quadruple build of insulation. That adds... (checks MWS catalog) ~30 mil to the diameter.

George H.

Reply to
George Herold

Emissivity, not reflectance. Blackness is what you want for thermal measurements.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

The more insulation, the cooler the wire! A universe full of plastic conducts heat better than a universe full of vacuum.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

T'other way round. Near room temperature, any thickness of vacuum looks like about 5 mm of air, iirc, which is 0.5 mm of plastic.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

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